LIMNOLOGY AND POLLUTION IN LAKE VALENCIA, VENEZUELA by C. Kwei Lin Great Lakes and Marine Waters Center International Programs Report Number 1 The University of Michigan Ann Arbor, Michigan 1982 CONTENTS Acknowledgments iv List of Figures v List of Tables ix Introduction • 1 General Project Plan 3 Materials and Methods , 4 Morphology and Bath3niietry „ 12 Light 20 Water Current and Velocity 26 Temperature 29 Dissolved Oxygen 32 pH and Alkalinity 44 Specific Conductance 46 Total Dissolved Solids 46 Sulfur 49 Phosphorus 51 Nitrogen 62 Trace Metals 73 Chlorophyll a. , 77 Primary Productivity and Respiration 83 Phy toplankton 86 Zooplankton. 95 Coliform Bacteria 101 Pollutant Inputs from Three Major Tributaries 104 Macrophytes and Marsh System 116 Discussions and Conclusions 121 References 127 111 ACKNOWLEDGMENTS This project was initiated and administered by the director of Direccion de Investigacion del Ambiente (DISCA, now D.I. A.), Ing. Gustavo Parra Pardi, whose great enthusiasm and full support were indispensable for completion of the investigation throughout the project period in 1978. Ing. Luisa Damia deserves my greatest appreciation for her patience in coordinating the cruises and organizing raw data. Marcos Mota Carpio, Jose Luis Franco, Luis Carrillo Nuiiez, and Zaida Duran worked long hours during the cruises, and are gratefully acknowledged. I am also thankful to the following laboratory personnel who participated in supervising, processing, or analyzing samples for chemical and biological parameters: Chemists: Leopoldo Blumenkranz G. Maria Moron de Ramirez Vicenta Salazar As is Alfonzo Mercedes Garcia Biologists: Elizabeth Bilbao de Marquez Zaida Duran I am also indebted to many others who assisted on the paper work or the physical operations devoted to this project, particularly to T. Ladewski and^ P. Morissey for their assistance in data analysis, and to S. Schneider for editorial service . IV LIST OF FIGURES Figure Page 1. Monitoring stations for physical, chemical, and biological parameters. Circled stations indicate the locations where samples were taken for laboratory analysis of chemicals, phytoplankton, and zooplankton. Distance between two adjacent stations is approximately 3 km 5 2. Samipling scheme, volume, and treatment of water samples for water quality analysis 11 3. Watershed boundary of Lake Valencia tributaries 14 4. Bathymetric map of water column in Lake Valencia (contour in 5-m depth intervals) 15 5. Lake surface area and volumes corresponding to depth intervals 17 6. Water levels recorded between 1962 and 1976 18 7. Water level fluctuation recorded in 1978 19 8. Incident solar radiation on lake surface recorded on a clear day in February, 1978 21 9. Depths (m) of light penetration in water column at stations along a transect on May 25j 1978 22 10. Depth contour (cm) of Secchi disc transparency during four cruise periods in 1978. . 23 11. Depth contour (m) of 1% light penetration during six cruise periods in 1978 24 12. Correlation between depths (m) of 1% light penetration and Secchi disc transparency 25 13. Current direction and velocity in surface water (0.5 m) during four cruise periods in 1978 27 14. Variations in direction and velocity of water current recorded at various depths at station 20 on three dates in 1978 28 15. Annual isothermal variation (**C) of water temperature recorded from station 20 during 1978 30 16. Vertical temperature profile recorded at station 31 during six cruise periods 31 V 17. Depth-time isopleths of daily temperature variation at station 20 on February 21, 1978 33 18. Depth-time isopleths of dissolved oxygen concentration (rag/L) at station 20 during 1978 34 19. Vertical distribution of dissolved oxygen at station 31 recorded on six dates in 1978 36 20. Depth-time isopleths of dissolved oxygen concentration (mg/L) during day hours on February 21, 1978 37 21. Depth-time isopleths of dissolved oxygen concentrations (mg/L) on September 11, 1978 38 22. Changes of dissolved oxygen concentration throughout the water column at station 20 during 12-hr of daytime on five dates in 1978 39 23. Horizontal distributions of dissolved oxygen concentration at five different depths during April 24-28, 1978 41 24. Vertical gradient of dissolved oxygen concentration (mg/L) in north-south transect during six cruise periods in 1978 42 25. Vertical gradient of dissolved oxygen concentration (mg/L) in east-west transect during six cruise periods in 1978 43 26. Seasonal variations in pH, conductivity, and concentration of sulfate total dissolved solids in surface water at station 31.. 45 27. Distribution of specific conductance (ymho/cm @ 25**C) recorded during three cruise periods in 1978 47 28. Distribution of sulfate concentration (ppm) recorded during two cruise periods in 1978 50 29. Annual variation in total phosphorus, total dissolved phosphor- us, and inorganic soluble orthophosphate at station 20, 1980. Values are presented by surface area 53 30. Vertical distributions of total dissolved phosphorus, total phosphorus, and orthophosphate at station 20 during six cruises in 1979-80 54 31. Horizontal distribution of total phosphorus during six cruise periods 56 32. Time-depth distribution of total phosphorus concentrations, in 20 pg/L gradient contour, during 1980 57 VI 33. Horizontal distribution of total dissolved phosphorus during six cruise periods , . . . 59 34. Time-depth distribution of total dissolved phosphorus, in 20 yg/L gradient contour, during 1980 35. Horizontal distribution of orthophosphate during six cruise periods 37. Annual variation in total K nitrogen, NH3 - N, and NC3 + NO2 concentrations at station 20 38. Vertical distributions of total K nitrogen, NH3 - N, and NO3 + NO2 at station 20 during six cruise period 39. Horizontal variation in total R nitrogen at selected stations during six cruise periods 40. Time-depth distribution of total K nitrogen at station 20 in 1980 46. Depth-time isopleths of chlorophyll a concentration at station 31 in 1978 "" 48. Vertical variation in chlorophyll £ concentration at station 20 on February 22, 1978 50. Vertical distribution of respiration in the euphotic zone at station 20 on five dates in 1978 60 61 36. Time-depth distribution of orthophosphate concentration during 1980 ^3 64 periods 66 67 68 41. Horizontal variation in NO3 - NO2 concentration at selected stations during six cruise periods 70 42. Vertical variation in NO3 - NO2 concentration at station 20 during 1980 71 43. Horizontal variation in NH3 - N concentration at selected stations during six cruise periods 72 44. Vertical variation in NH3 - N concentration at station 20 during 1980 74 45. Vertical distribution of iron, nickel, lead, zinc, copper, mercury, calcium, and magnesium concentrations 75 79 47. Lakewide distribution of chlorophyll _a (mg/m^) during six cruise periods in 1978 3q 82 49. Vertical distribution of primary productivity in the euphotic zone at station 20 on five dates in 1978 34 85 Vll 51. Seasonal variation in total cell numbers of phytoplankton and Microcystis at station 31 in 1978 87 52. Vertical distribution of total phytoplankton during six cruise periods in 1978 88 53. Vertical distribution of Microcystis during six cruise periods in 1978 90 54. Horizontal distribution of predominant genera of three m.ajor phytoplankton classes during six cruise periods in 1978 91 55. Variation in zooplankton composition during October 1979 98 56. Horizontal variation in total zooplankton numibers during October 1979 99 57. Vertical distribution of major zooplankton taxa at station 31 during October 1979. 100 58. Density of coliform bacteria at sample stations in May, July, and October 1980. Blank circles and squares indicate total coliform, and dotted S3niibols indicate fecal coliform 103 59. Distribution of major emerged macrophytes on the lake shore.... 119 Vlll LIST OF TABLES l£^. Page 1. Cruise schedules and activities 4 2. Morphometry ^ 13 3. Major tributaries and their lengths and areas.. 13 4. Areas (A) and volumes (V) in 5-m depth intervals 16 5. Mercury content in muscle and viscera of Tilapia mossambica -79 6. Species composition and population density of the zooplankton community in October, 1979 95 7. Density of total and fecal coliform (MPN/100 mL) sampled at 12 stations on three dates in 1980 102 8. Typical contaminants discharged by agricultural industries in the Lake Valencia basin 207 9. Chemical and physical parameters of discharges of three major tributaries in the Lake Valencia basin , 108 10. Annual flows and discharges of contaminants from three major tributaries in the Lake Valencia basin m IX INTRODUCTION Lake Valencia is the largest natural freshwater lake in Venezuela. The lake is located in one of the most populated areas in the country and its watershed possesses approximately 8% of the total population, with a density of 350 persons/km^. Increasing population in the watershed, coupled with rapid expansion of industry and agriculture, has caused gross environmental contamination in Lake Valencia. Once known for its great beauty and potential natural resources. Lake Valencia water quality is currently undergoing a rapid deterioration. The eutrophication of Lake Valencia has long been accelerated by the pronounced natural desiccation and greatly enhanced by modern human activity in the lake basin. The desiccation phenomenon and its effect on lake hydrological balance was first reported by von Humbolt (1856) and more recently by Bockh (1956), Cartaya and Montano (1969), and MARNR (1980). Although several investigations have been devoted to pollution problems in the lake (INOS 1971, Torrealba and Cardenas 1972, AVIS 1973, and Fuchs and Mosqueda 1975), little effort has been made to obtain more basic and comprehensive limnological information. As most available documents on eutrophication and pollution in the freshwater environment were obtained from the northern temperate region, the problems and consequences associated with those processes in tropical lakes are relatively unfamiliar. Therefore, investigation on the fundamental limnological features in Lake Valencia would generate valuable scientific information which, in turn, may provide a practical guideline for improving water quality and managing the resources of this aquatic ecosystem. Several specific objectives were set in the present project: 1. To measure the velocity and direction of water movement. This infor- mation is essential in order to understand the mixing process of the water mass and dispersion patterns of contaminants from point sources. 2. To measure the light penetration and transparency in the lake water. The light regime in the water column is of primary im.portance for determining the depth of the euphotic zone which governs phytoplankton production. 3. To analyze major nutrients, trace metals, and organic contaminants in lake water. 4. To document the community composition and succession of planktonic organisms and their environmental factors. 5. To survey the submerged and emerged macrophytes. 6. To survey the pollutant inputs of three major tributary inflows: Rio los Guayos, Rio Guey, and Cano Central. GENERAL PROJECT FLAN To meet the project objectives, six cruises were conducted approximately bimonthly from January to December 1978. The exact schedule and activities for each cruise are listed in Table 1. All the cruises were carried out with a 24-ft Rotork boat, A normal cruise required five persons on shipboard to under- take all research activities. The general survey was conducted at 40 stations distributed evenly on a grid of 3 km between neighbor stations (Fig. 1). Several field parameters were measured at each of the 40 stations, and 17 of those stations were selected for chemical and biological sampling. Those samples were delivered to either the field laboratory in Valencia or the central laboratory in El Hatillo. Each cruise included four types of investigations: (1) monitoring and sampling for physical-chemical properties and plankton, (2) measuring primary productivity of phytoplankton, (3) monitoring dissolved oxygen, current, and diurnal cycle of temperature, (4) monitoring pollutant dispersion in Rio Los Guayos and Rio Guey . Measurement of primary productivity of phytoplankton was carried out in situ at three fixed stations (17, 20, and 23), located in the western, central, and eastern parts of the lake respectively. The diurnal changes of pH, dissolved oxygen, temperature, and current were monitored throughout the water column during 12-24 hours at the central location of the lake (station 20). The last part of each cruise involved the monitoring of the dispersion of the two major tributary inflows: Rio Los Guayos in the west part and Rio Guey in the east part of the lake proper. Measurements of temperature, light, current, dissolved oxygen, and chemical samplings were made at nine stations adjacent to each river mouth covering an area of 9 km^ on a 3 km x 3 km grid. TABLE 1. Cruise schedules and activities. Cruise No. Date Activity 1 Jan. 11-14 Routine 40 stations Jan. 25-27 Primary productivity, station 20 Pollution dispersion 2 Feb. 7-10 Routine 40 stations Feb. 21-23 Primary productivity, station 20 Pollution dispersion 3 Apr. 23-28 Routine 40 stations May 23-25 Primary productivity, station 20 4 Jul. 17-26 Routine 40 stations Primary productivity, station 20 Pollution dispersion 5 Sep. 29-Oct. 6 Routine 40 stations Primary Productivity, station 20 Pollution dispersion 6 Nov. 21-24 Routine 40 stations Primary productivity, station 20 Pollution dispersion MATERIALS AND METHODS Physical Parameters 1. Water movement - The velocity and direction of current flows were measured with a Braystoke current flow meter with 5" diameter im- peller. At each station the measurement was made at 5-m intervals starting from the surface. At station 20 the measurement was made at 3-hr intervals throughout the en- tire depth over 12 hours. Current velocity is expressed in cm/sec and direction in degrees of compass. o < ZZ. ■ - p UJ •J en _J . o 2 LU _l ^ • o. < s _J ^ < CO o CO 4-> c ^ o c •H CO ■U f— 1 CO (X 4-i o CO 4-} >. T3 ^ (D a r-i U •< U CO •H f— I u CO o • e • cc QJ B ^ J= ^ 0) U u en 0) Mh e O >! CO t-H }^ CO 0) CO •H 4-J a CO CO >. a r-t I-H •H CO CO X O c o •H CO ^ 00 cu O >> a 1^-1 U CO o o •H 4J CO ^ CO •H XJ o CO c Xi C CO CO o 1-H •H •> 4J r— 1 u CO CO o 4-) U MH CO •H g c 4J 0) QJ C ^ ^ 0) CJ CO CJ iJ CO •> •'~n i-H o; T3 CO Jh CO a ^ CO 4-> rC 0) a r-i c a 0) vm g (D o CO :^ •4-1 CO CO cu ja C »H o OJ CD •H ^ o 4-1 :^ c CO CO 4_) CO 4J CO C CO O H oo- H Q c 4-1 •H CO i~l U • o o c ■U ,—1 o •H 4-1 c cu ^ o ^ c s 4J CO pH CD O- • U o I-H CD o U N • • H O TD -73 M c C fe . H CO 2. Light penetration and transparency - Photosynthetic light regime was measured using LAMBDA underwater quantum meter (Lincoln, Nebraska) at 0, 0.5, 2, and 10 meters at each station. Light level is expressed in y Ein m"^ sec""-^, and penetration was calculated as percent of surface light intensity. Transparency was determined at all stations with a standard black and white Secchi disc. 3. Hydrolab parameters - Other limnological measurements made in situ were depth, temperature, pH, specific conductance, and dissolved oxygen, using a Hydrolab Surveyor Model 6D Water Quality Analyzer (Hydrolab®, Austin, Texas). Measurements were made at a series of depth intervals throughout the water column at all stations. Diurnal variations of those parameters were recorded at station 20. B. Biological Methods 1. Aquatic macrophytes - Aquatic macrophytes growing along the coastal region of the lake and islands were sampled for species identification and their distributions were recorded on a map. Submerged plants were collected by hand or hooks. The distribution of emerged vegetation was surveyed from the lake by a small boat and from land on foot. Several collections were made to determine the vegetative and reproductive growth cycle throughout the year. Specimens collected were pressed dry for species identification. 2. Phytoplankton - Phytoplankton samples were collected at selected stations (0, 1, 2, 5, 8, 12A, 17, 18, 21, 23, 24, 25, 31, 33, 35, 39, and 40). At each station, a composed sample was taken from surface (0.5 m) , mid, and maximum depths of the water column using a centrifugal pump. At station 31, a series of profile samples was taken from 0.5, 5, 10, 20, and 30 m. The 250-mL phytoplankton sample taken from each station was immediately preserved with 5 mL of Lugol solution and subsequently concentrated to 25 mL by settling. The phytoplankton taxa were identified at the generic level and all numbers of each taxonomic entity were counted using an inverted microscope. 3. Chlorophyll a. - A 250 to 500 mL phytoplankton sample, taken from each station and from various depths at station 31, was filtered through a GFC filter. The chlorophyll a. was extracted with 10 mL of 90% acetone in an opaque vial in ambient cold for a minimum of 24 hours. The chlorophyll a concentration was determined by spectrophotometric method (Strickland and Parsons 1969). 4. Zooplankton ~ Twenty liters of sample composed equally of surface, mid, and bottom lake water were filtered through a #25 plankton net (mesh size 65 ym) . The sample was concentrated to 300 mL and preserved with 5% formalin. Species of major genera were identified and numbers of each taxon were enumerated. 7 5. Primary productivity of phytoplankton - Phytoplankton productivity was measured several timies at four locations in 1978, using the light-dark bottle dissolved oxygen exchange method. Water samples taken from surface, 0.5, 2, 3, 5, and 10-m depths were enclosed in 300-mL Winkler bottles (two light and two dark bottles for each depth). Those bottles were suspended for 4 hours (1000-1400 hours) in the water column in respect to the original sampling depths. The dissolved oxygen concentrations in those incubation bottles were measured at the beginning and the end of the incubation period. The productivity was calculated by the following formulae: net productivity =03-0]^ gross productivity =03-02 where cj = initial concentration of dissolved oxygen C2 = final concentration of dissolved oxygen in dark bottle. C3 = final concentration of dissolved oxygen in light bottle. To determine the specific productivity, phytoplankton standing crop in each incubation bottle was filtered to determine chlorophyll a^ concentration. 6. Coliform bacteria - Sampling locations. To determine the sources and distribution of total and fecal coliform in the lake, 8 twelve sample stations were chosen. Six were near the outfalls of major tributaries and the remaining six stations represent open lake water. Those locations are designed as A, B, F, H, I, P, 5, 6, 12A, 31, 33, and 39. Stations A and B are located at the mouths of Rio Los Guayos and Cano Central, respectively. Stations 5 and 6 are adjacent to B and A on the open lake side. Another heavily polluted area is the western end of the lake in the vicinity of Rio Guey outfall where stations H and 33 are located. Stations F, I, P, and 39 are situated near the shore of plantations representing non-point source discharges, and occasional discharges from cattle and pig farms. Station P is located in the vicinity of Puenta Palmita, which is the most popular bathing area for local residents. Station 12 represents the least contaminated area, remote from any point and non-point sources of domestic and industrial waste.. Station 31 represents the major mass of lake water because it is the deepest and most turbulent area of the lake. Sample logistics. Bacterial samples were taken from the 12 stations in the morning and completed within 2 hours. At each station, triplicate samples were taken from surface water and put in pre-sterilized bacterial sample bottles. Those samples were transported to the field laboratory in Valencia immediately after collection. Laboratory procedures for determination of total and fecal coliform density in the samples were according to the multiple-tub fermentation procedures to obtain the Most Probable Number (MPN) index. The details of the method are in Standard Methods (APHA 1975). Chemical Procedures 1, Water samples were collected at 17 selected stations (Fig. 1) using a centrifugal pump. The water sample taken from each station was composed of an equal mixture of surface, mid, and bottom water, except for station 31 where water samples were taken separately from five depths at 0.5, 5, 10, 20, and 30 m. The sampling scheme, volume, and preservation for water chemistry are shown in Figure 2. Chemical parameters analyzed in the laboratory were chloride, heavy metals (As, Cd, Cu, Fe, Pb, Mn, Zn, Hg, and Cr), nitrogen (ammonium, nitrite, nitrate, and total Kjeldahl), pH, phosphorus (soluble reactive othophosphate, total dissolved phosphate, and total phosphate), solids (total, total dissolved), and sulfate. Analytical procedures used for quantitative determination of the chemical parameters were based on Standard Methods (APHA 1975). Trace metals were analyzed with a Beckman atomic absorption spectrophotometer, phos- phorus and nitrogen with a Technicon autoanalyzer . Sulfate was determined by the turbidimetric method and solids by the gravimetric method. 10 E O O fate, ssolved Calcium esium o o o o Central Lab Sul Chic Total D Solids, Magn cr UJ en UJ tr CL UJ :e «J O > UJ UJ X o CO o < (n •H CO >^ CO c CO CO =J cr u CO o CO 0) a B CO CO M 0) +J CO 15 c e CO ^-1 T5 C CO e I— I O > e 0) u CO s CO M 11 MORPHOLOGY AND BATHYMETRY Lake Valencia occupies a drainage basin of 3,000 km^ in northern central Venezuela. Geologically, the lake basin is situated on metamorphic basal rocks and was formed by a relatively narrow depression (a graben) between two fault scarps running WSW - ENE (Peeters 1968). The depression has been gradually filled with Pleistocene and Holocene sediments. Accumulation of the sediments blocked the drainage outlet of the Rio Paito to the Orinoco basin when the lake elevation dropped to 429 m above sea level around the year 1727. Since then the lake has become a closed system. The modern lake morphometry is listed in Table 2. The surface area of the lake is 356 km^, approximately 12% of the water shed area (Fig. 3). There are 16 tributaries distributed throughout the watershed (Table 3), but the lengths of most tributaries are less than 30 km and originated from steep Andean mountains in the north. Most of those rivers carry a considerable amount of silts from surface runoff during the rainy season and have little flowing water during the dry season. Only those rivers such as Los Guayos, Caiio Central, and Rio Guey, which drain domestic and industrial waste water, have continuous flow throughout the year. The most recent bathymetric map (Fig. 4) was made in 1963 and the depth contour and shoreline have since altered considerably. The maximum depth has been reduced from 40 m to 37 m, and Champego Island is now connected to adjacent land. The lake was relatively shallow at the western portion and the drop in water level has created an extensive soft, muddy shore. The deeper part (35 m) of the lake extends between Punta Palmita and Macapo-Yuma shore. Table 4 shows 12 TABLE 2. Morphometry. Altitude 405 m (asl) Maximum length (L) 30 km Maximum width (b) 17.8 km Mean width (b) 12.7 km Area (A) 356 km^ Volume (V) 6,740 X 10^ m^ Maximum depth (Zm) 37 m Mean depth (z) 18 m TABLE 3. Major tributaries and their lengths and areas, Tributaries Aragua Turmero Guigue Paya Guayabita Limon Maracay Guey Tocoron Noguera Guacara Guaica Los Guayos San Joaquin Mariara Cura Length (km) 58 .7 41 .2 22 .5 2L .25 17, .7 21, .2 27. .7 14, ,5 23. ,5 28. ,0 30. .5 7, 5 31. 5 10. 5 10. 14. Area (km^) 360 .0 245 .1 119 .3 77, .0 54, .1 78, .6 126. .6 33. .0 140, .1 118. ,0 111. ,2 15. ,1 102. 9 17. 7 14. 5 41. 6 13 CO •H u CO D X) C cd o C r-l CO > CO CO C O 0) 4: CO u Q) U CO 3 M 14 CO > U B I in u O c o a o c i-H CT3 > 0) CO C e i-H O u a; 4-1 CO 15 CO B 0) CO O M 15 TABLE 4. Areas (A) and volumes (V) in 5-m depth intervals, Depth (m) A (km-^ 356 5 286 10 243 15 215 20 177 25 132 30 99 35 A5 A% Vz (lO^m^) Va (lO^m^) 100 80 1,602 1,602 68 1,321 2,923 60 1,144 4,067 49 978 5,045 37 769 5,814 27 575 6,389 13 351 6,740 the water volumes at 5-m depth intervals, and the decrease of surface area may be estimated by the loss of corresponding water volume as indicated in Figure 5. Decrease of water level has been a historical and most serious problem in Lake Valencia. As a rough estimate, the average annual water loss has been 60 X 10^ m^ in the past 170 years (Apmann 1973). The recent accelerated desiccation has most probably been due to the destruction of forests, the slash and burn land cleaning, and agricultural and industrial consumptions of both surface and underground water in the lake basin. Figure 6 shows the decreasing trend of lake elevation from 406 m in 1962 to 403 m in 1976. The water loss was particularly pronounced between 1971 and 1976 during which the lake level dropped 2 m. The annual fluctuation between 1978 and 1979 is shown in Figure 7. In this yearly cycle the water level dropped 50 cm from the end of rainy season in October 1978 to February 1979, but a large rise occurred starting in May 1979. The rise of water level in 1979 was accelerated by the diversion of Rio Cabrales which added 4-5 m-^/sec of water from sources outside the watershed. 16 AREA(Km2) Q,^ 50 10 150 200 250 300 350 1000 2000 3000 4000 5000 6000 7000 volume: (10® m^) FIG. 5. Lake surface area and volumes corresponding to depth intervali 17 (|9Aan oas aAoqv sja^aiAi) N0llVA3n3 3>1V"I c CO CM c 0) 0) :5 4-1 Xi T3 OJ U O U 0) a; > CO 12 M 18 {^^) Noiivnionid n3A3"i 3>ivn 00 r^ ON s C •H no 0) Ul 13 ^ O U CD U -3 C O •H 4J CO 4J Q 00 u 1^ CD ^^m r-H o; z: > (V r-i ^-1 o; u /) CO ::2 r^ d M fe 19 LIGHT The solar radiation on Lake Valencia's surface on an average day is 12 hours with a maximum intensity of approximately 2,000 +^ 500 VEm m ^ sec ^ near mid-day. Figure 8 shows the distribution of light intensity throughout a typical clear day in February. Day length and light intensity in this low latitude (10'' N) are relatively constant throughout the year. Typical light attenuation in the water column at several transect stations is shown in Figure 9. The depth of light penetration in Lake Valencia is extremely shallow. As shown by Secchi disc transparency (Fig. 10), the visibility ranged from 23 cm to 284 cm, with the majority of values between 100 and 150 cm. The greatest transparency occurred in January and the lowest in July. Lakewide , the central area between Punta Palmita and Isla Otama had greater transparency than other areas of the lake. There were two areas where water visibility was persistently poor, with values less than 50 cm during most cruises. The poor transparency in those areas was mainly caused by the heavy loading of suspended matter from Rio Los Guayos and Cano Central in the southwest and Rio Guey in the northeast. The turbidity, resulting from sediment resuspension, also contributed to low trans- parency in the shallow waters (<2 m) of the western basin. The variation of transparency in open water near the center of the lake was largely due to the phytoplankton blooms. Attenuation of photosynthetic light intensity as measured by quantum is highly correlated to Secchi disc transparency values (Fig. 11), and the calculated depth of 1% light penetration (Fig. 12) reached 5 m in the central basin. The major portion of the lake received this light at 3 m. In situ 20 0655 0855 HOO 1305 1510 1715 1830 DAY TIME (Hour) FIG. 8. Incident solar radiation on lake sjurface recorded on a clear day in February, 1978. 21 o o o _ e? o _ V vv'Ni \ V \ 00 ^ 0* \ h- ■" V 21 c , ^v jHoincDi^cxxT* \<3 ^.-rrrororororo — > i-.X \ o • ^ D o < v-\\ \ -X^ • •■V ^ \ ■ x^ \ % — \ "^ — \ ^ - \ \ \ "■" \ 1 1 • 1 1 in — d oj ro ^ {^) Hld3a If) to 00 ON CN C o u 0) CO c CD U CO) c o CO c o CO u CO C B o rH O U U U CD C O CD U u C 00 CO a- Q 22 00 CO T3 O •H U 0) w •H }^ U a o M-l c J-l D >^ o c (D U CO Cu CO c CO }^ u CO u CJ CO >-4 o o ■u c o a Q 23 JANUARY 1978 JULY 1978 FIG. 11. in 1978, Depth contour (m) of 1% light penetration during six cruise periods 24 1.0 I J L JAN. 12-14, 1978 J L 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 0.5 0.7 0.8 0.9 1.0 LI MAR. 25 -28, 1978 0.8 0.9 1.0 .1 1.3 SECCHI DISC DEPTH (m) FIG. 12. Correlation between depths (id) of 1% light penetration and Secchi disc transparency. 25 measurements of oxygen production and consumption indicate that 1% light level is a critical compensation zone where photosynthetic and respiratory activities are nearly equal. WATER CURRENT AND VELOCITY Current direction and velocity of water mass in Lake Valencia are illustrated for four cruise periods by Figure 13. The maximum velocity observed during those cruise periods frequently reached 50 cm/sec or greater; occasionally up to 100 cm/sec was recorded. The current directions were extremely variable and durations of any given direction were normally no more than a few hours. Large numbers of islands located in the southern half of the lake complicated the current flow pattern as the velocity and direction of surface current exhibited great regional differences. The greatest surface currents often occurred in the central, eastern, and western basins when the wind blew directly from the opposite end of the lake. Several observations were made for the duration of current throughout the water column, over a period of 12 hours or longer, at station 20 near the geographical center of the lake (Fig. 14). Those observations indicate that the movement of water mass was extremely responsive to wind stresses over the lake surface. On February 21, a gust of wind (20 kph) occurred during the night. The velocity of the water column was measured at 0650 hr on the following day. At most depths, the current velocity reached 40 cm/sec. At depths greater than 20 m, the velocity was reduced to 25 cm/sec. The surface current direction was east-north-east and shifted to east-south-east in the lower half of the water mass. The current velocity normally dampened within a few hours, from 40 cm/sec 26 E o O CVJ >- 1- O q 00 -J N UJ > cn 2 2 O i o LlI cr O or X 1 cn 3 UJ UJ O Q Q U- E o O > h- o 3 00 LU r^ > en «*-«. ^ URRENT IRECTION EPTH(m 1 O 2 < u c^ c^ -^ 00 CO O •H u a w •H D U >-< o o 3 ^ LU UJ O Q Q (O u h- •• C 00 a; 2 *'^ OJ 5-1 H o ^ 1 J-4 X in CVJ D O %^ (L q: • LU CL 00 O Q Q < r— ( 27 UJ O O o (M o o m in \ \, > >~^ ^ 4r3 ^ 5=z 1 ^ o CO CO {^) Hld3a A o ro CD O 2? •^ Q in (r p •" q: 5 I- o V) o Q 9} —I e is LU > o y o £ O 00 ^ o . CM h- Z O 22 2?0 CO LU o - h " q: r- — ^ =^ i^ ^ i cj en 2 Q cj^cr)a)OCM^u)ooOcj^ LU > E g o CVJ cm" 2 o z CM 1 O CM g2 CM CJ II U i CD LJ UJ Q i i {^) Hld3a u o I— I 0) p> C CD C O U 0) U •H . T3 CX) C 0^ CO C o •H CO U 0) CO iJ •H CO U TJ CO > (D CD • ^ r-H C • O M o fe CM 28 to a standstill in 4 hours on Feb. 21, and from 60 cm/sec to flow in 5 hours on May 1 . Tlie wind data, collected from a northwestern lake shore weather station (Base Sucre) by the Venezuelan Air Force, show most of the maximum wind velocity was recorded in the afternoon and evening. Frequent changes in wind direction throughout the day caused the velocity and direction of water current to be extremely irregular and unpredictable. Langmuir circulation appeared to be a common water movement pattern in surface water as indicated by frequent occurrence of streaks. These streaks, ranging from 0.5 to 2 m wide, contained mostly phytoplankton, aquatic macrophytes, and invertebrate residues. TEMPERATURE The annual thermal variation in Lake Valencia is shown in Figure 15. The maximum temperature (30**C) occurred in surface water during June and the minimum temperature occurred in subsurface water during January and April. In general, the low temperature (26°C) of the water mass prevailed in the early part of the annual cycle and was relatively uniform throughout most of the water column. The water temperature increased progressively from April to July and it reached 27*'C in the deep water toward the end of the year. Although the vertical thermal differential in the lake was small, approx- imately 5C°, the formation of transient thermal stratifications occasionally occurred during June and October (Fig. 16). Thorough mixing of the water col- umn, as indicated by the isothermal conditions of the water mass, occurred in December. Pronounced diurnal variations in water temperature occurred mostly at the surface. For instance, on February 21, 1978, the surface water temperature 29 GO X UJ o CM C O •H U CO 4-J CO B O u CO B u 0) 4-J o CO CO C C • < C30 ON m (tu )Hld3a o 5-1 30 TEMPERATURE (°C) 25 26 27 28 29 30 31 STATION 31 Jan. 12/1/78 Feb. 9/2/78 Apr. 28/4/78 July. 18-21/7/78 Sept. 26/9/78--2-6/I0/78 Nov. 21-24/11/78 FIG. 16. Vertical temperature profile recorded at station 31 during six cruise periods. 31 in the morning was less than IS'^C and it rose to 28°C in late afternoon (Fig. 17), The heat that accumulated in the surface water was quickly dissipated during the night. The temperature of the water mass below 3 m remained stable at 25.5**C. The cool water mass in the subsurface upwelled during the night, which may possibly have resulted from surface cooling at night. DISSOLVED OXYGEN Annual Variation The annual variation of DO in the water column near the geographical center of the lake is extremely dynamic (Fig. 18). Oxygen stratification, from supersaturated to anoxic conditions, prevailed during most of the year. Supersaturation frequently occurred near the surface (<2 m) , and severe oxygen depletion deficit (<50% saturation) constantly occurred below 10 m depth. During December-March the lake water was well mixed as indicated by the isothermal condition. The oxygenation reached the maximum depth of the lake. Although the mixing process effectively replenished the DO in the anoxic deep water, the oxygen deficit in the entire water column became pronounced. On January 22, 1978, the DO level in the surface water was depleted as low as 3 ppm. This event of extreme DO deficit, coupled with extensive H2S released from premixing in the anoxic zone, caused a massive fish mortality. The strong oxygen stratification began in early April, and a rapid DO depletion followed during May and June. The extensive anoxic depth occurred below 20 m in mid- April and surfaced to 8 m in October. The largest DO gradient in the water column was observed between 5 to 15 m, particularly in May and October during which the DO decreased from 9 ppm at the surface to 6 ppm at 5 m and to ppm at 10 m. Such a large change in DO gradient appeared to be related to a large 32 6 OJ ro ^ m (UJ) Hld3a CD ro 33 Oi 00 0^ P u O 1— I o CO CO CO o CO •H CD g I 4-» CD Q to lO o lO CJ -T- o ro (uu) Hld3a 00 o M 34 phytoplankton biomass buildup in the euphotic zone as a result of calm weather. The DO profiles measured during six cruise periods are shown in Figure 19. The greatest fluctuation of DO content in the water column occurred between January and February, increasing from 5 to 12 ppm at the surface 5m. A similar distribution pattern, typical clinograde, was observed during April, July, September, and November. Daily Variation The DO concentration exhibited in diurnal variation was often episodic and the amplitude of variation depends primarily on rates of photosynthetic produc- tion, respiratory consumption, and mixing depth. Figures 20 and 21 illustrate two specific dates showing two different DO variation patterns in the water column during the day. On February 21 a relatively uniform DO distribution was recorded from the surface (6.0 ppm) to the bottom (5.5 ppm) in the morning, and became highly stratified in the late afternoon. The greatest DO reduction occurred between 3 and 5 m, with DO reducing from 9 to 6 ppm at 7 pm. The photosynthetic oxygen production was 3 g 02/tn^/day in the surface and the compensation depth was at 3 m; at that depth no apparent increase in net DO production occurred. A rapid destratif ication of DO followed during the evening. On September 11, when the anoxic layer was extensively developed (below 15 m depth), the DO fluctuated between 8 and 10 ppm in the top 5 m and drastically reduced to between 10 and 15 m. The mixing event had rarely extended to the pyncnocline (anoxic zone). The budget of DO in the water column was measured in several cruises (Fig. 22). On January 25 and March 23, there were net gains of DO throughout the water column, but during the remaining dates various degrees of DO deficit 35 DISSOLVED OXYGEN (mg/L) 4 5 6 7 8 9 . 10 II 12 13 14 STATION 31 Jan. 12/1/78 Feb. 9/2/78 Apr. 28/4/78 Jul. 18-21/7/78 Sept. 26/9-2-6/10/78 Nov. 21-24/11/78 FIG. 19. Vertical distribution of dissolved oxygen at station 31 recorded on six dates in 1978. 36 (iu)Hld3a 37 700 6.0 6.5 TIME (Hr) 100 1500 7.0 8.0 1900 2300 9.0 8.5 7.5 FIG. 21. Depth-time isopleths of dissolved oxygen concentrations (mg/L) on September 11, 1978. 38 00 c •H u o o c o CO CO C e rH O u a; ■u CO o o c o CO u c CD U a o u c OX) 00 >% r^ O r-H 0) 'H > (^) Hid3a O (D CO iJ CO CO •H T3 0) M-i > O -H CO O) c 00 o c CO 0) ^ B 4-» >^ • CO CM M • ^ e> I M CN 39 prevailed in the subsurface water. As indicated in Figure 18, the lake remained totally oxygenated during the early part of the year and the pyncnocline devel- oped from May throughout the rest of the year. The oxygen depletion in subsur- face water during the anoxic period was most probably enhanced by the greater organic loading from surface runoff during the rainy season (from May to November), and the greater phytoplankton standing crop (shown by chlorophyll level in Fig. 33) reduced the light penetration and contributed dead biomass in the aphotic zone. Lakewide DO Distribution The horizontal distribution of DO in the lake water varies a great deal depending on the depth of the water column, the land use of the shoreline, the influence of tributary inflow, the photosynthetic production, the respiratory consumption of oxygen, and the mixing effectiveness by wind action. Figure 23 shows the lakewide DO distribution at depths of 0.5, 5, 10, 20, and 30 m during the April cruise. The maximum DO value in the surface water (0.5 m) was 10 ppm near the west and south portions, and gradually decreased to 7.5 ppm near the center of the lake. The DO concentration was above saturation level in most of the surface water except nearshore of the northeastern region where DO sagged to 4 ppm (50%). It is expected that the DO content progressively decreased in the deeper water, and a major portion of the zone below 20 m was free of oxygen during this period. The horizontal and vertical DO distributions along the north-south transect during six cruises in 1978 are shown in Figure 24, and the east-west transect in Figure 25* During the January-February period the entire lake was oxygenated, with the maximum DO of 13 ppm and minimum at 1 ppm. The low values were 40 DO (ppm) 0.5 m APR.25-28J978 DO (ppm) 5m APR. 25-28 J978 DO (ppm) 10 m APR.24-28J978 DO (ppm) 20 m iiil ppm APR.24-28J978 0.2\ ^ K DO (^ppm) 30 m' ppm APR. 24-28, 1978 FIG. 23. Horizontal distributions of dissolved oxygen concentration at five different depths during April 24-28, 1978. 41 5 10 I t 20 UJ o 25 30 35 5 10 5 15 & 20 o STATION 40 37 30 20 II -1 Ui + t vj:^'.^/ ■ "w — \ *^ -"T^ ■V -^7 — \. • • / — \ y^^ / DO TRANSECT — ^ Vj /. / JAN.II-I4.I978 STATION 40 37 30 20 II 25 30 35 DO TRANSECT APR. 25-28,1978 STATION 40 37 30 20 II 5 10 1 '5 X o 25 30 35 5 10 i 15 X &20 o 25 30 35 STATION 40 37 30 20 II I DO TRANSECT FEB. 7-I0J978 STATION 40 37 30 20 II DO TRANSECT JULY 18-21.1978 STATION 40 37 30 20 11 DO TRANSECT NOV. 21 -24,1978 FIG. 24. Vertical gradient of dissolved oxygen concentration (mg/L) in north-south transect during six cruise periods in 1978. 42 5 10 1 15 X fc 20 25 30 35 STATION 25 24 23 22 21 20 19 18 17 16 • V "^7 — \ • • • • • • • • 7 • • / — \ * * • • • • ""X / DO TRANSECT \^ , / \/ - JAN.II-I3,l978 Xr-^ -.3—^ / -^~>__y STATION 25 24 23 22 21 20 19 18 17 16 35 L- 5 I ^15 X U20 25 30 35 L- STATION 25 24 23 22 21 20 19 18 17 16 DO TRANSECT JULYI8-2IJ978 STATION 25 24 23 22 21 20 19 FIG. 25. Vertical gradient of dissolved oxygen concentration (mg/L) in east- west transect during six cruise periods in 1978. 43 recorded from the water mass near the bottom (20-25 m) at stations 22 and 23 in January and stations 16 and 17 in February. Those stations were adjacent to either end of the east-west transect receiving the Outfalls of the Rio Los Guayos and Rio Guey, respectively. The continuous deoxygenation trend which developed on the eastern shore encroached into the entire lower strata of the lake basin. The persistent long-term oxygen depletion in subsurface Lake Valencia presents a very serious water quality problem. Under unaerobic conditions, the production of H2S and methane became prevalent, and regeneration of nutrients was enhanced. Several major factors responsible for the oxygen depletion in Lake Valencia are the external input of organic matter through tributaries, the respiratory consumption of planktonic organisms (phytoplankton, zooplankton, and bacteria), and sediment oxygen demand. In order to accurately assess the oxygen budget in the lake, these major processes involved in oxygen loss must be investigated, PH AND ALKALINITY Several measurements of alkalinity made in Lake Valencia during 1978 ranged between 300 and 350 mg/L CaC03. The seasonal and spacial variations in alkalinity were relatively small (Fig. 26). The pH value in lake water ranged from 7.4 to 9.2, with high values in the surface and low ones near the bottom. Seasonal pH variation in surface water was minimal (Fig. 26). However, the pH in the bottom water was lowered to 7.6 and 7.4 in April and November, respectively. 44 sanos a3Anossia nvioi '3ivdnns CvJ 00 X Q. o o o o o o (UJD/0qUUT7) AllAllDnaNOO CO O CO CD p> o CO CO CO 4-J O 4-» CD 4J CU CO C o •H 4J CO >-4 c CD O C O u XI c CO > •H 4-J O D no C o u CO CO C o a •H O 4-J "H CO 4J •H CO ^ 4J CO CO > CO CO C O CO CO CO CD 15 a; CJ • CO ^ C+-I CN u o • CO 45 SPECIFIC CONDUCTANCE Specific conductance in Lake Valencia ranged from 1,200 to 2,300 ymho/cm, with mean value around 1,900 ymho/cm. The seasonal fluctuation in the surface water of the main water mass remained relatively constant, with high values (>2,000) recorded during February and September (Fig. 26). Vertical variation at most stations also showed a great uniformity, with differences less than 200 ymho/cm, or 10%, between surface and bottom at the central deep station. There were no persistent seasonal and horizontal variations in the surface water (Fig. 27). During April 24-29, the maximum value (2,000 ymho/cm) was observed in the northwest region of the lake, and gradually lowered to 1,200 ymho/cm in the southern and eastern portions. During the September-October cruise period, the value in most parts of the lake was 2,000 ymho/cm or greater. The specific conductance of the common bicarbonate type freshwater lake is closely proportional to concentrations of total dissolved solids, approximated by the equation: Sp. Conductance (ymhos/cm 25°C) x A = total dissolved solids. A is the coefficient obtained experimentally. The factor A is generally between 0.55 and 0.75. However, the analyses of Lake Valencia data on specific conductance and total dissolved solids show little correlation. TOTAL DISSOLVED SOLIDS The lakewide distribution of total dissolved solids (TDS) in 1978 ranged 46 CO T> O •H u Ca > Na Anions: SO4 > CO3 > Cl The measurements made on November 22 show that Mg:Ca ranged from 2-10. The most uncommon feature of the chemical constituents in Lake Valencia is the unusually high sulfate concentration, comprising approximately 30 to 50% of dissolved solids . From a health standpoint the TDS concentration in Lake Valencia has already reached a harmful level of 2,000 mg/L (National Academy of Science 1973). It was reported that when the sum of the Mg and SO4 content exceeds 1,000 mg/L, or TDS content is above 2,000 mg/L, the water has a laxative effect on most persons. A similar effect also occurs in domestic animals if the drinking water contains TDS above 1,000 mg/L. It is important to note that water which contains a high concentration of TDS may conceivably have toxic effects on many individual organisms. Water containing a high TDS concentration is objectionable in industrial uses because it may cause formation of boiler scale, accelerate corrosion, and interfere with the clearness and color of finished 48 products (McKee and Wolf 1963). The rapid increase in TDS over time is one of the most indicative features of deterioration of water quality in Lake Valencia. From a simple input-output model, Apmann (1973) estimated that the TDS concentration was 348 mg/L in 1727 and rose to 1,332 in 1970. The most rapid increase occurred after 1960 and the value observed in 1978 rose above 2,000 mg/L, which was greater than that predicted by Apmann' s model (1,700 mg/L in 1980). The difference may result from the desiccation of the lake, which was not included in Apmann* s model. The major external sources of TDS inputs to Lake Valencia are tributary inflows, weathering of soil and rock in the lake basin, and atmospheric precipitation and fallout. Examination of the existing chemical data on Lake Valencia shows that the rapid increase in SO4 concentration contributed the single largest component in TDS's loading budget during the past few years. Desiccation in this closed lake was undoubtedly an important causal factor in the rapid increase in TDS concentration. SULFUR Two major forms of sulfur present in Lake Valencia are sulfate and sulfide. The sulfate concentration reported by INOS (1971) was less than 400 ppm. In this study in 1978, the value ranged from 400 to 1,500 ppm. Figure 26 shows the concentration measured for surface water in the center of the lake, with high concentrations (>1,000 ppm) occurring during the January-February period and decreasing gradually throughout the remaining periods. However, the lakewide horizontal variation was rather large (Fig. 28). The major factors causing large seasonal and regional changes may be the degree of sulfur reduction and 49 SULFATE (mg/L) JULY 17-21, 1978 > 900 ppm 800-900 ppm n 700-800ppm 600-700ppm SULFATE (mg/L) OCT 2-6, 1978 700-800ppm 600-700ppm FIG, 28. Distribution of sulfate concentration (ppm) recorded during two cruise periods in 1978. 50 stagnation in the subsurface water where anoxic situations prevailed frequently during the year. Planktonic organisms may also influence the sulfate concentration in the lake due to the mineralization of organic cellular sulfur content. There are many sources of sulfate input to Lake Valencia. In addition to natural weathering and erosion of sulfur-containing soil and rocks in the watershed, human activities (through sewage and industrial discharges) probably contribute a major input of sulfate to the lake. The preliminary data on sulfate input from tributaries show an extremely high value. For example, the sulfate concentration in Rio Guey during March through May ranged from 60 to 100 ppm, with an estimated total daily input of 6,000-10,000 Kg. Sulfide (H2S) is expected to be a dominant form of sulfur in the anoxic zone. Although we did not measure the actual H2S concentration in the lake water sample, we did notice the powerful rotten egg odor in water devoid of dissolved oxygen. In the February cruise, the obnoxious sulfide odor was pronounced even in the surface water. Presumably the complete mixing during that period brought up the sulfide from deep water. A massive fish kill was also observed at that time of the year. Sulfides are known to be extremely toxic to aquatic organisms. A concentration of total sulfides greater than 0.002 mg/L is unsafe for all aquatic organisms, including fish (National Academy of Science 1973). The extent to which sulfides cause the fish mortality in Lake Valencia remains an important feature for investigation. PHOSPHORUS The phosphorus was measured in three forms: total phosphorus (TP) that includes dissolved and particulate, organic and inorganic; total dissolved 51 phosphorus (TDP) that includes organic and inorganic forms filterable with 0.45 ym pores; and inorganic orthophosphate (PO4-P) that is the simplest and most available form in the lake water. The annual fluctuation of phosphorus contents in those three forms, as integrated throughout the depth at station 20, is presented in Figure 29. During the early part of the year, the concentrations were lowest at 3,000 mg/m^, 200 mg/m^, and 1,500 mg/m^ for TD, TDP, and PO4-P, respectively. Those concentrations increased steadily until they reached their maxima in October at 5,500 mg/m^, 4,500 mg/m^, and 4,000 mg/m^, respectively. Comparisons between the TP and TDP show that the TP content contained 60-90% TDP. In other words, the particulate P, including organic and inorganic forms, constituted less than 50% of the TP. The vertical variation of TP, TDP, and PO4-P concentrations in the deepest part of the lake during the annual six cruises are shown in Figure 30. In gen- eral, the three forms of phosphorus followed close patterns in their vertical distribution, and in most cases their concentrations were lower toward the surface and greater in the deeper water column. Such a pattern was particularly pronounced during the July-October period. For example, during September- October concentrations of TP, TDP, and PO4-P were 129, 29, and 13 yg/L in the surface water, compared to 289, 285, and 283 yg/L at the 30-m depth, respectively. During this period, while the TP concentration was relatively comparable with the rest of the year (100-150 yg/D , the concentrations of TDP and PO4-P (<30 yg/L) were much lower than other periods (>50 yg/L). The most homogeneous concentrations of those phosphorus forms in the water column occurred in the period between November and March. In general, the TP content in non-polluted natural waters in the world 52 CO JC =3 Q_ o s^ X> o (I) SI > o w o o w n C/J b CL "d "d o -f-* -+-' V-^ o o Q_ h- t— O II 1 B H 1 1 1 1 1- o CO < 0009 OOOS OOOt OOOe^ 0003 OOOL ^LU/DUU D t— 1 O CO o •H C CO t>0 ^ o c •H TD C CO •« CO • a CO M (U O u JZ CO a CO (U o a 42 CO CX ^ u TD o 0) CO > rH >% O ^ CO CO HD •H a f— 4 • CO o 4J 00 o 4J i-H CO CO a Q) c U c CO < 42 Pu CO • o 0^ £ CN CL O • ^ O *J M U ^ o 53 mg/m* 300 Oct. 23-31, 1979 mg/m' 300 Mar. 3-9, 1980 300 ipg/'^J 300 July 7-9, 1980 200 300 Sep.25 - Oct.2, 1980 mg/m* 100 T 300 May 19-20, 1980 Nov.25 - Dec.5, 1980 D = Total Dissolved Phosphorous o = Total Phosphorous - = PO4-P FIG. 30. Vertical distributions of total dissolved phosphorus, total phos- phorus, and orthophosphate at station 20 during six cruises in 1979-80. 54 extends over a wide range from <1 yg/L to MOO mg/L, but the majority of them are between 10 and 50 vg/L. Separation of TP into inorganic and organic fractions in a large number of lakes in the temperate region indicate that a great proportion of the TP is in the organic phase (>85%) (Wetzel 1975). Of the total organic P, about 70% or more is in the particulate organic fraction, and the remainder is present as dissolved and colloidal forms. In a detailed treatment on phosphorus and productivity of lakes, Vollen- weider (1968) demonstrated that the amount of TP generally increases with the algal productivity. Lakes with TP ranging from 30-100 yg/L are considered eutrophic, and those greater than 100 yg/L are hypereutrophic . Total Phosphorus . Figure 31 shows the lakewide distribution of TP con- centration in Lake Valencia. With few exceptions, the total P concentration is well above 100 yg/L in most areas during most times of the year. Persistently higher concentrations were found near the western and eastern ends of the lake. During the annual cycle, the lower TP concentrations occurred in October 1979 with lakewide concentrations ranging from 28 to 228 yg/L; the majority of values during the rest of the year were between 100 and 200 yg/L. At the western end of the lake, total P content ranged from 250 to 1,500 yg/L. The annual cycle of TP concentration varied considerably with time and depth (Fig. 32). It clearly shows that the TP concentration was relatively homogeneously distributed throughout the water column during the period from January through March, with the majority of concentrations ranging from 100-200 yg/L. It also varied very slightly around 120 yg/L in the surface 5 m during the annual course. However, a marked gradient, increased from 80 yg/L at 10 m to a concentration greater than 200 yg/L at 30 m, prevailed from June throughout the end of the year. 55 Oct. 23-31, 1979 July 7-9. 1980 Mar. 3-9. 1980 Sep.25 - Oct.2, 1980 May 19-20. 1980 Nov.25 - Dec.5. 1980 o = 100mg/m'' Total Phosphorous FIG. 31, periods . Horizontal distribution of total phosphorus during six cruise 56 u o o c o u c •H CO U DO DO ;3L o o CO U 4J c zOi E D O O _> O (/) (A) D ■♦- O o CM O (lu) M+daa U D O c o o c (U •H CO 60 ;3. o CO u o CO o x: a. T3 > r— I O CO CO CO o O •H CO •H CU 'V I 0) a H O 00 ON • •— ) CO 60 C o ^ M D {*4 13 60 Oct. 23-31, 1979 July 7-9. 1980 Mar. 3-9, 1980 Sep.25 - Oct.2, 1980 May 19-20, 1980 Nov.25 - Dec.5, 1980 ° = 100mg/m' PO^-P FIG. 35. Horizontal distribution of orthophosphate during six cruise periods, 61 during the period from April throughout November, and it reached the maximum concentration of 280 yg/L at 30 m in October (Fig. 36). This pronounced buildup of PO4-P superimposed on the anoxic event in the lake water presents a classic example demonstration of the mobilization of PO4-P in an anoxic hypolimnion. As the oxygen in the hypolimnion is removed by active decomposition of organic matter, which in turn releases soluble phosphate, the reductions of iron and manganese are expected to increase markedly. With the reduction of ferric hydroxides and complexes, ferrous ions and absorbed phosphates are mobilized and appear in the water. The existence of hydrogen sulfide in the anoxic water column may further enhance the PO4-P accumulation in deep water. It has been shown that in very productive lakes, where the decomposition of organic matter produces anoxic conditions and hydrogen sulfide in the hypolimnion, ferrous sulfide (FeS) precipitation occurs. If FeS precipitation is sufficiently large to remove large proportions of the iron, phosphorus may be kept and accumulated in solution. NITROGEN Four major chemical groups of nitrogen compounds were analyzed for Lake Valencia water. They are total ammonia (NH3), nitrite (NO2), nitrate (NO3), and total Kjeldahl nitrogen (TKN). Nitrogen compounds may vary their chemical forms throughout the water column depending on the degree of prevailing oxidation and reduction process at a given specific depth. To accurately estimate the total quantity of the various nitrogen compounds from surface to the bottom of the lake, values integrated over the depth for TKN, NH3, and NO3 + NO2 are shown in Figure 37. Content of TKN was relatively stable in comparison with either total (lu) mdea O 00 c •H u c o CO u c cu u o u CO co o o }^ o o c o I B on M 63 - Q S O <1 l i i i I i I — h mi I I I il l l M il Ml I I I I gCHXl fOLA uj/6uu HIM I — h lom o CO O •H U CO CO 4-J C 60 o ■u •H C h l:^ r— J CO 4J O -^ 4J c •H n q o U CO •H z: >-• CO > T-4 CO o C C CO > CN fX4 71 Oct. 23-31. 1979 July 7-9, 1980 Mar. 3-9. 1980 Sep.25 - Oct.2, 1980 May 19-20. 1980 Nov.25 - Dec.5. 1980 o = 100mg/m' NH3-N FIG. 43, Horizontal variation in NH3 - N concentration at selected stations during six cruise periods. 72 annual cycle is shown in Figure 44. Very low NH3 (<100 ]ig/L) appeared in the surface 10 m from January to October. A pronounced vertical concentration gradient began to establish below the 10 m depth after June and reached the greatest level at 1,700 yg/L near 20 m in October. The development of the NH3 gradient was closely related to the oxygen depletion pattern in the hypolimnion. TRACE METALS The concentrations of several heavy metals were analyzed at several depths at station 20 (Fig. 45). During January, when the lake water mass was well mixed, a greater degree of homogeneity in vertical concentrations occurred. Iiron. Iron concentration was 50 yg/L from the surface to 20 m and 200 yg/L at 30 m. During the lake turnover period (December to March) the lake water was thoroughly oxidized. It is expected that the ferric ion would be the predomi- nant form that in turn precipitates to the sediment as a hydoxide, Fe(0H)3. In the anoxic water during the remainder of the year the presence of H2S probably removed iron in the form of ferrous sulfide (FeS), which resulted in the production of black mineral muds. Ni£kel. Nickel concentration was 100 yg/L throughout the water column. High concentrations of nickel were reported to be toxic to plant life O500 yg/L) and to fish reproduction (>700 yg/L). However, concentrations at or below 100 yg/L have no apparent harmful effects on aquatic organisms. Le_ad. Lead concentration ranged between 40 and 50 yg/L, which is the permissible level for domestic water supply in the USA. It was shown that 30 yg/L concentration caused reproductive impairment in Daphnia magna (Biesinger and Christensen 1972). 73 •o E CM O (tu) mdaa o 00 ON oo o O CM CO 4J CO CO C O CO U u C cu o c o u I en PC c o CO CO O U (1) > 74 CONCENTRATION (ppb) 100 200 50 150 40 80 10 30 50 5 10 J 15 S] 20 25 30 — ( MM 1 ' • 1 > > i 1 i 1 " 1 1 1 1 \ (» \ — ( \ • » ) \ :..\ Ni [pb Zn • JAN 10-14,1978 5 10 I 15 X O 25 30 CONCENTRATION (ppb) 10 30 50 10 30 50 70 M/M 7 M M «k l~T 10 30 I I I Zn JULY 18-21,1978 CONCENTRATION (ppm) 30 50 70 400 4 40 480 520 560 600 I Ivf I FIG. 45. Vertical distribution of iron, nickel, lead, zinc, copper, mercury, calcium, and magnesium concentrations. 75 zinc . Zinc concentration ranged from 20 to 40 yg/L and from 10 to 35 Pg/L in July 1979, Those concentrations were relatively low compared to the mean concentration (64 pg/L) in U.S. waterways. Numerous bioassays have been conducted to test the toxic effect on all levels of aquatic organisms. Zinc is also an essential trace metal required by all kinds or organisms (Bowen 1966, Underwood 1971). Minimum concentration required for optimal algal growth was reported at 50 yg Zn/L (Eyster £t jd. 1958). Zinc concentrations toxic to human are quite high (>5 mg/L). The test for chronic effects of zinc on fathead minnow reproduction showed that 30 yg/L had no effect, whereas 180 g/L caused a 93% reduction in fecundity. Copper . Copper concentration was between 20 and 50 yg/L at station 20 in Lake Valencia. The vertical profile shows that the highest concentration was at 30 m. All organisms require minute quantities of copper for growth. Free copper ions at concentrations greater than 50 yg/L have an adverse effect on phytoplankton species. The copper toxicity to algae depends on the temperature and alkalinity of the water. Deleterious effects of free copper concentrations on fish and zooplankton were greater than 18 yg/L in soft water (<30 mg/L CaC03) and 30 yg/L in hard water (>200 mg/L CaCOs) (Mount 1968). Mercury . Mercury concentration from the surface to 30 m ranged between 15 and 70 yg/L at station 20 in July 1978. The toxicity of mercury has been known to man for a long time. Although it may exist in inorganic (Hg^ and Hg'**^) and organic forms in the environment, the organic methyl mercury (HgCH^) is 50 times more toxic to organisms than inorganic forms (Schroeder 1974). Algae and aquatic plants accumulate mercury by surface adsorption, and fish take it up both directly from the water and from food (Hannerz 1968). The accumulation rate was shown to be fast while the elimination rate was slow, leading to 76 concentration factors of 3,000 fold and higher. Concentration factors by fish in excess of 10,000 times that in the surrounding water have been reported (McKim 1974), and 1 yg/L of mercury showed a distinct effect on marine organisms. Two chronic toxicity tests on Daphnia magna showed that mercury as mercuric chloride and methylmercuric chloride caused significant reproductive impairment at concentration of 2.7 and 0.04 yg/L mercury, respectively (Biesinger 1974). To detect the mercury level in fish in Lake Valencia, samples of Tilapia mossambica , the most abundant species in the lake, were taken for analysis. Table 5 shows the mercury content in fish muscle and viscera. Mercury content in fish muscle and viscera ranged from 20 to 70 yg/kg of fish in wet weight. The levels of mercury in those fish samples were relatively low, compared to the safety levels (500 yg/kg) in livestock recommended for human consumption. The accumulation factor from lake water appears to be extremely small. One explanation would be that the mercury in Lake Valencia water reacted with the prevalent hydrogen sulfide, forming extremely insoluble precipitates which make it unavailable to organisms for uptake. CHLOROPHYLL a The chlorophyll a^ concentration fluctuated widely over depth, time, and location. Figure 46 shows that at the beginning of 1978, at station 20, the chlorophyll concentration was low (10-20 mg/m"-^) and relatively homogeneous throughout the water column. As indicated by the depth profiles of temperature and dissolved oxygen, the water mass was well mixed during the January-February period. Chlorophyll a concentration in the upper 10 m increased progressively 77 TABLE 5. Mercury content in muscle and viscera of Tilapia mossambica , sampled at Turmero rivermouth and Macapo in Lake Valencia. Sample No. Muscle Viscera Turmero 2 0.02 0.04 Turmero 4 0.03 0.03 Turmero 6 0.03 0.07 Turmero 8 0.03 0.04 Turmero 10 0.04 0.05 Turmero 12 0.03 0.07 Macapo 5 0.02 0.05 Macapo 6 0.06 — Macapo 7 0.02 0.07 Macapo 9 0.07 Macapo 11 0.02 0.02 Macapo 13 0.02 0.02 Macapo 15 0.04 and the vertical gradient became pronounced toward the latter part of the year (Fig. 46). The maximum concentration in the surface water (5 m) occurred in October with value greater than 50 mg/m^ and less than 10 mg/m^ at 25 m. Chlorophyll level remained relatively low and constant at approximate 10 mg/m^ in the deep water below 20 m. As this depth was far below the euphotic zone (<5 m) , chlorophyll was not expected to carry out photosynthetic activity. But the chlorophyll appeared to be healthy and functional as indicated by a high chlorophyll/phaeophytin ratio. The lakewide chlorophyll distribution during six cruise periods showed large regional variation throughout the year (Fig. 47). In general, high concentrations occurred in the southwestern portion of the lake, and this pattern was particularly pronounced from July to November. During the January- April period, a major portion of the lake contained chlorophyll at 10-20 mg/m^ 78 m Hld3GI 00 c o CO u 0) c o CO u •u c a; o c o o a. o u o CO o CO 8 I a Q M Ex, 79 > o Q. I O o 3 E O ro I O (VJ O A q: o 3 511 E E o o ro o O CM E s E ro E lO E •% ^ v^ V. ^ o -J f E r a* E >• o 8 s O u. fO V S 1 €0 A H 00 ON CO o u (U D M O •H CO C ° ^ -^^ -^ _j o» o» o» rj E E E ^ o a O Q_ r. CM ro O • • flC O O 3 X o _ o — (Vl E O to col o u o 1— I a o c o •H •H CO M PC4 80 with considerable shifting, and maximum concentration (>30 mg/m^) occurred only in a small area. During the July-October period the chlorophyll concentration increased considerably, with values in the major portion of the lake greater than 20 mg/m^. Annual maximum chlorophyll production occurred in November, during which approximately half of the lake contained >50 mg/m^ chlorophyll. The most productive region was located in the extreme southwestern sector where the maximum chlorophyll content was 176 mg/m^ (station 5). The most obvious factors that cause the spatial heterogeneity of chlorophyll distribution in lakes are variation in nutrient concentrations, herbivore grazing pressure, water movement pattern, and sinking rate. However, the nutrient levels (phosphorus and nitrogen) in Lake Valencia were sufficiently high to support optimal chlorophyll production within the limited euphotic zone. As the predominant phytoplankton species in Lake Valencia was Microcystis aeruginosa > the heterogeneity caused by zooplankton grazing and the sinking process is expected to be minimal. Wind action appeared to be the most significant factor in mixing and redistributing the surface-inhabiting Microcystis aeruginosa . The chlorophyll heterogeneity also occurred in the euphotic zone at all locations over a short time. Figure 48 shows the changing chlorophyll concen- tration in surface water (5 m) at station 20 on a clear, calm February 22, 1978. At 0700 hr, the depth profile of chlorophyll in the euphotic zone was 10-11 mg/m. As the day progressed, the chlorophyll content in the immediate surface water decreased and the maximum value (16 mg/m^ at 1900 hr) occurred at 3 m. 81 X UJ O OsJ ^ CD (^) Hld3a 00 00 ON CM U CO :^ u a; c o o CM c o CO u CO C o CO U C CO u > CO 82 PRIMARY PRODUCTIVITY AND RESPIRATION The gross productivity measured by oxygen production at station 20 on five dates in 1978 is shown in Figure 49. Maximum productivity mostly occurred in the surface 1 m, ranging from 2-5 g 02/ni3/day. On July 27 and November 29, the productivity at the immediate surface (10 cm) was less than that at 0.5 m, presumably inhibited by excessive solar radiation that caused chlorophyll damage. Comparison of the productivity integrated throughout upper 10 m per m^ per day (Z P in Fig. 49) shows that the highest production (16 g C2/m2/day) occurred on November 29, and the lowest (1.42 g 02/ni2/day) on February 21. The specific productivity based on unit weight of chlorophyll £ shows that the maximum value was 184 mg 02/nig Chi £/day at station 17 at 10 cm on July 27 and the value decreased to 25 mg 02/mg Chi £/day at 5 m; and the gross productivity in unit surface area at stations 17, 20, and 23 were 13.07, 13.30, and 13«55 g 02/m2/day, respectively. The vertical variation in specific productivity was strikingly different among those stations. The respiratory oxygen consumption measured at station 20 on five discrete dates is shown in Figure 50. Respiration rate at the surface was less than 0.5 g 02/m3/day and the consumption increased toward the aphotic zone (>5 m) in most cases. Total DO consumption rates integrated over depth at station 20 were 0.94, 4.47, 16.11, 4.55, and 2.77 g 02/ni2/day on February 21, May 25, July 27, October 5, and November 29, respectively. Net oxygen productivity in the top 50 m water from photosynthesis and respiration at station 20 during those five dates were 0.77, 5.56, -2.81, 5.25, and 13,23 g 02/m3/day, respectively. Oxygen contributed by phytoplankton production is the largest source of dissolved oxygen in Lake Valencia. A rough estimate based on the average 83 GROSS PRODUCTIVITY (gOa/mVdy) -2-1 I 2 3 4 IP=I.42 STATION 20 FEB 21,1978 GROSS PRODUCTIVITY (g02/mVdy) 12 3 4 5 X o 4 T T 1 — r STA' 10 Y.P= 10.03 ION 20 MAY 25,1978 GROSS PRODUCTIVITY (gOz/m^/dy) GROSS PRODUCTIVITY (gOa/mVdy) 01 2345601 2345 IP= 13.30 STATION 20 JULY 27, 1978 GROSS PRODUCTIVITY (gOs/m^/dy) 1 2 3 4 5 6 e 9 _ 3 2 X o 4 - 5 - — 1 1 •^L 1 — y/ IP=I6 V STATION 20 NOV 29,1978 3 - 10 FIG. 49. Vertical distribution of primary productivity in the euphotic zone at station 20 on five dates in 1978. 84 J 2 I I- uj ^ o 4 5 10 RESPIRATION (gOz/mVdy) -2 -I 12 3 1 IR = 0.95 STATION 20 U FEB. 21, 1978 X RESPIRATION (gOa/m^/dy) 1 1 2 3 4 5 6 1 11 1 1 1 1 1 i 2 — \ X O 3 4 ■"" \ Ir = 16.11 5 m — \ STATION 20 JULY 27,1978 I i ^ X t 3 tu o 5 - RESPIRATION (gOa/mVdy) I 2 3 4 5 T IR= 2.77 STATION 20 NOV. 29, 1978 10 RESPIRATION (gOz/mVdy) 12 3 4 5 - 2 i[L ILlI '=^4 T I R = 4.47 STATION 20 MAY 25, 1978 RESPIRATION (gOg/rnVdy) 12 3 4 5 T T T IR= 4.55 STATION 20 OCT. 5,1978 FIG. 50. Vertical distribution of respiration in the euphotic zone at station 20 on five dates in 1978. 85 observed photosynthetic rates at stations 17, 20, and 23 in five discrete measurements showed that the gross oxygen production in the top 10 m over the whole lake ranged from 1.5 x 10^ to 5.7 x 10^ g O^/day, with an annual average of 2 X 10^ g 02/day. Oxygen generated in the eutrophic zone diffuses downward by a mixing process to replenish DO in the deeper water. In comparison, the oxygen influx from external aeration into Lake Valencia is relatively small. Apmann (1973) estimated that the transfer rates from air to water over Lake Valencia were 10-30 x 10^ g/yr assuming a reaeration coefficient of 0.1 g/m^/hr under various hydrodynamic conditions. PHYTOPLANKTON Phytoplankton taxa and their quantitative distribution in Lake Valencia during six cruise periods in 1978 were identified to a total of 37 genera belonging to Chlorophyta, Cyanophyta, Chrysophyta, and Pyrrhophyta. Population density of the phytoplankton community among lake-wide stations during the study period ranged from 28,210 to 572,880 cells/mL. As those samples were composed of surface, mid, and lower depth at each station, the actual density is expected to be larger in the surface and smaller in the bottom water. The seasonal variation of total phytoplankton and Microcystis cell numbers, shown in Figure 51, indicates that the maximum standing crop occurred in January and the minimum in October. During most of the year. Microcystis occupied more than 90% of the total cell counts, and the minority species were in significant proportion only in January. Figure 52 illustrates the vertical distribution of total phytoplankton at station 31 on six discrete sampling dates. In January and February large populations (28-45 x 10^ cells/mL) occurred in the surface 86 CP lO ro (\j e^^/gOi X snn30i > CO o •H U u o •H H- O CO o c o u ^ c CO i—i o ■u 3 -3 a M-^ o CO }^ a; >T < § 2 C r— 1 r-i 0) u rH CO m u (jj O 4J Li_ c •H C o •H 4-) CO 2 CO 00 < -D a^ I—I f—i CO C G O -H CO CO .— 1 CD en CO C O • 'H •— f 4-1 un CO J-) • CO O M +J 1X4 CO 87 UJ Q TOTAL PHYTOPLANKTON ( CELLS x 10^/ m^ ) 100 200 300 400 100 200 300 5 h 10 15 - 20 - 25 - 30 - ^ 10 e - 15 X P4 20 25 30 5 10 15 20 25 30 / APR. OCT. JULY NOV. FIG. 52. Vertical distribution of total phytoplankton during six cruise periods in 1978. 88 (1 m) and decreased drastically at/below 5 m. But the variation was relatively small in other sampling periods. As Microcystis bloomed persistently, its vertical distribution dominated the community vertical distribution pattern (Fig. 53). In January, Ankistrodesmus and Nitzschia populations reached 36,022 and 28,210 cells/mL in the surface water, respectively, and constituted the second and third largest phytoplankton populations. Lake-wide quantitative distribution of predominant phytoplankton genera during six cruises in 1978 is shown in Figure 54. The major species of Cyanophyta were Microcystis , Anabaena, Oscillatoria , and Spirulina . Among those entities. Microcystis and Oscillatoria were particularly abundant. As Microcystis is well adapted to flotation the size, shape, and patchiness of its colonies are dependent on water movement and wind action. This blue- green alga bloomed during most of the year, with population density ranging from 20,000 to 500,000 cells/mL. Oscillatoria was most abundant during February, with a maximum population density of 10^ cells/mL, and had a relatively low profile (<10^ cells/mL) during the rest of the year. Like Oscillatoria , the two genera of nitrogen fixer, Anabaena and Anabaenopsis , never developed a population competitive with that of Microcystis . Chlorophyta, although numerically much smaller than Cyanophyta, consisted of diverse taxa. In January and February there were 10-12 genera present, and the composition became more complex during the remainder of the year. There were 20 genera in September. Among those entities, Ankistrodesmus and Cosmarium were most abundant in January, but became sporadic during the rest of the year. No other genera of green algae developed any significant population size. 89 -10 £ 20 25 30 5 10 15 20 25 30 FIG. 53. in 1978. MICROCYSTIS (CELLS x l06/m3) 100 200 300 400 100 200 300 400 JAN. APR. OCT. FEB. JULY NOV. Vertical distribution of Microcystis during six cruise periods 90 373,000 1 '69,000 461,000 467,000 198,000 1 260,000 JAN.I0-I4,I978 CYANOPHYTA Microcystis b Anabaena c Oscillatoria d Spirulina 252,000 1 50.000 cells ml FEB.7-9.1978 a Microcystis b Oscillatoria c Anabaena FEB. 7-9,1978 CHLOROPHYTA a Ankistrodesnnus b Oocyst is c Closteriopsis d Closterium e Closteridium 20,000 cells ml JAN. 10-14, 1978 CHRYSOPHYTA a Nitzschio b Cyclotella c Diatomella FEB. 7-9,1978 FIG. 54. Horizontal distribution of predominant genera of three major phytoplankton classes during six cruise periods in 1978. 91 75,000 cells APR 25-28,1978 CYANOPHYTA Q Microcystis b Oscillatoria c Anabaena d Lyngbya e Anabaenopsis JULY 18-21,1978 CYANOPHYTA a Microcystis b Oscillatoria c Anobaeno 3.5; mesotrophic, 2.5-3.5; and eutrophic, <2.5. The values calculated for each station during all cruise periods ranged from 0.4 to 1.7. Those extremely low diversity indexes undoubtedly indicate that Lake Valencia is in a highly eutrophic state. 94 ZOOPLANKTON Zooplankton samples taken during 1978 cruises were mostly used for preliminary taxonomical identification. The zooplankton results presented in this report are samples taken during October 1979 cruises. A total of 18 taxa were recorded for the zooplankton community (Table 6), including three species of copepods, two species of cladocerans, six species of rotifers, and seven species of ostracods. Among them, the rotifers were most abundant, particularly Branchionus calyciflorus and B^. havanaensis whose populations consistently constituted 50% or greater of the zooplankton community (Fig. 55). The total zooplankton individuals ranged from 100 to 1,926 organisms/L among 23 sample locations, and a population density greater than 1,000 org/L occurred in the shallow water along the southern and northern shores (Fig. 56). It is noteworthy that the greatest densities were found at stations 5 and 33 with 1,837 and 1,926 org/L, respectively. Those two stations are respectively situated near the mouths of Cano Central and Rio Guey, which are major point sources of contamination in Lake Valencia. The zooplankton community in those regions was dominated by rotifer populations, which comprised more than 80% of the community. Another interesting aspect is that many taxa of ostracoda exist, although in low number. It is speculated that those ostracods found in plankton samples were actually benthic dwellers stirred up along with sediment resuspension at those shallow stations. Figure 57 shows the vertical distribution of predominant zooplankton entities at station 31 in October 1979. The largest population density (313 org/L) occurred at the surface and the lowest (24) near the bottom. 95 I I I I I I I O CO I 00 es I I I I I I I I I I o o (i) JO o u u o on I CO I CM CO I cvi I r^ I I I I t I I I I O r^ icomiiioj iiliiiiocM rH CO . o Ivrr^coii^a- 1111111003 C O CO lONvOCOIlOO COI|||||COO> a o o N I CM CNJ 0) o ON vo m I I I I I I I O CT\ I I I I I I I O O CO c c o 13 O c CC c o CO O Pu e o u CO cu •H U CU a en PQ I CM CM O H o a. T^ T^ o CO CO vO I 00 vO I 00 I c^ 00 f-i r^ 00 I m CO m CO CO ON f-i in T-i 00 CO r-. o I CM ■<}• CM o a> r^ r*". ON CO o \ \o ft s CO O •H T-\ > •H ^4 CO CO rH 0) o xi a § CO CO CO 3 3 3 CO a C c rH o o o (H •H •H •H (U ^ JS JS ■U O o CJ CO ca CO CO U (-4 u u o PQ PQ PQ i^ 00 I I I I f I 00 r- CO I I I I I I CO 00 I CM I -^t I I I v£> CO CM CM CM I I I I I CO I CO 00 I I I I I I O O O rH CN CO o o 96 I 00 00 00 cr> % cd o •H iH > •H M cd CO rH O o ,£: a a CO CO CO CO 3 3 3 CO C C fl r-( O o o iH •H ■H •H a> ^ ,J= ^ u O O o CO CO CO CO Vj M U U (U PQ iM PQ fc^ o H -W *J 4J 4J <3 H O H 97 n Rotifers H Copepoda ■ Cladocera n Ostracoda FIG. 55. Variation in zooplankton composition during October 1979, 98 = 1000 Organisms /Liter Diameter = I cm FIG, 56. Horizontal variation in total zooplankton numbers during October 1979. 99 jrfUMUMdbUMrirfiMMki^AMhti** "!;.:.:.;.I.;. ; . ! .;............i. i .M.;. . . . . 1 1 i jj, i jjjj, ii . J . O tn '(/) c a> o c o > o ''''■''•■-''•'•■•'•'-■•'-■•'•'-'-'■•'•'-■■'•■•'-"""■•'-''• i 1 " " U "1 ^ c g o o (U CD O CD (/> K c o 3 o o O 14— o 8 fO o II 2 o £ o o o h- (/) < -J h- c C/) o o o CD a; o u O C •H O CO CO X CO c o CO t— I a o o N u o CO e c o I I CI o u CJ Xi •H 4-i CO •H TD r— I CO U CD > M ir> o (LU) Hld3a o ro 100 Branchionus havanaensis , the most abundant species, showed the most distinct density gradient in its vertical distribution. COLIFCRM BACTERIA Table 7 shows the densities of total and fecal coliform at 12 sampling stations during three different periods in 1980. Highest densities (MPN/100 m) were found persistently at stations A and B adjacent to the outfalls of Rio Los Guayos and Cano Central. At station A, the total coliform bacteria ranged from 1.4 X 10^ to 8.4 x 10^ MPN/100 mL, and fecal coliform from 9.3 x 10^ to 3.7 x 10". Those high bacteria densities declined drastically in a short distance toward the open lake. For example, in May the density of total coliform at station 5 was reduced to 1.2 x 10^ MPN/100 mL, less than 20% of that of station B at a distance of 2 km. Similar reductions also occurred during other sampling periods and other locations (Fig. 58). The second most contaminated area in the lake was at the western end near the outfall of Rio Guey, represented by stations H and 33. The total coliform index at stations H and 33 ranged from 5.3 x 10^ to 9.3 x 10^ and from 4.6 x 10^ to 7.6 X 10 . On most occasions, the fecal coliform densities were the major component of the total coliform in this area. In the areas where there was no apparent adjacent contamination source, the bacteria density was relatively low. At stations F, P, 12, and 39, located on the fringe of farm land, the total coliform index was often less than 500 MPN/100 mL. The density at open lake station 31, representing the major water mass of the lake, was less than 1,300 MPN/100 mL for total coliform, and fecal colifoim was persistently less than 300 MPN/100 mL. 101 o 00 CO CD u CO 13 (y a; u c o CO c o CO CO CN CO O PL4 u •\ V V •V V o vD o o 00 o o V o o o o o o o r-H LO en en 00 00 s£> vO <^ 00 1— I B CO CO CO U o O o o o o o o o o o o o O 00 o o o o en o o o o o o VO o 00 en o m en en o en •\ •« *» V •* V V •\ V o o o* vX> m o o o o 00 o v£) o en m o B o o 2: B u o •-5 O H en r-H en en en 00 in 0^ •\ •s •\ •» •s V •» •\ 0% •\ •s o\ 00 en vO 1—1 CN en vO 00 vl" vO en VD vO 00 o a CO o c CO CO o CO c o CO 0) Pt4 ON cn f^ en en cn cn «\ •» V «« •\ M V V V •» V en m r^ % JZ JC v.. h- o c/^ o o CO C O CO cu 4J o CJ ciJ S a; u 0) o c o CO •H Q 119 The submerged macrophyte species composition is dominated by Chara sp. and Potamogeton sp, Chara sp. One or more species of Chara (Characeae) occurs sparsely in the shallow water, often found less than 1/2 m deep in the sandy or gravel bed. They commonly occur around the south shore and around the islands, where the water quality is relatively good. The plant appears in vivid green and each plant grows in tufts of half a dozen to a dozen shoots which converge at the base. Shoots are unbranched, 5- 10 cm high. No reproductive structure was observed during the survey. Potamogeton latifolius (Robbins) Morong. This is the most abundant submerged plant found in Lake Valencia. It grows almost continuously around the lakeshore and islands, except where the water has high turbidity and/or there are badly polluted river mouths. The most dense growth is found along the south shore of the lake and around the islands, but there is little growth at the east and west ends of the lake. The plant is distributed in the littoral zone from 0.5 to 4 m deep, and the length of the plant varies considerably with the depth of the water column in which it grows. In shallow water, the plants average 20-40 cm in length, while in deep zones they reach over a meter in length. The predominant flowering season is not certain and only a small number of the plants bore flowers during the period of this survey. A filamentous alga (Chlorophyceae) , its species yet to be identified, is commonly associated with the upper shoots of Potamogeton . This association is 120 extremely productive as indicated by the abundant tiny air bubbles evolving from the vegetation beds. In addition to submerged species, a floating species, Eichhornia crassipes (Mart.) Solms., was observed to grow near the river mouth, and individual plants were occasionally dispersed to the open lake. A dense growth occurred on the north side of the Rio Guey mouth. The environmental conditions that prevent this plant from becoming wide spread are not known. DISCUSSIONS AND CONCLUSIONS The physical features of Lake Valencia have been undergoing a rapid change due to chronic desiccation which results in the rapid reduction of surface area, volume, and depth. The lake has dwindled to an enclosed stagnant system that accumulates and condenses incoming soluble and insoluble substances. Results of the present study indicate that the depth of the water column has decreased to a level sufficiently shallow to allow frequent circulation to the maximum depth of the water column. As the wind action in the Lake Valencia basin is extremely variable in direction and velocity, the movement of water masses driven by wind stress shows little persistent pattern. The results also indicate that the water movement is so episodic and ephemeral that it does not appear to exert an effective vertical mixing mechanism, as indicated by dissolved oxygen during most of the year. It was difficult to obtain a simultaneous lakewide water movement pattern as the measurements were made using one current meter over several days during each cruise. More comprehensive features of the lakewide water movement pattern in Lake Valencia may be obtained by simultaneous measurements using a number of current meters at several stations in the lake, 121 or by continuous monitoring of water movement at several depths near the center of the lake. The light regime in Lake Valencia plays a very important role in governing the phytoplankton ecology. Despite the high intensity of incident light in the tropical climate, the attenuation of light penetration in Lake Valencia is extremely great and the maximum depth of Secchi disc transparency is less than 2 m. Accordingly, the maximum depth of the euphotic zone extends to approximately 5 m, and on a volume basis only 25% of the lake water is autotrophic and 75% heterotrophic. In reality, the proportion of the auto- trophic and heterotrophic zones is much smaller as the average transparency in most areas is much less than 2 m during most of the year. Accumulation of phytoplankton biomass on the surface water is apparently the major cause of the shallow euphotic zone. The dynamics of dissolved oxygen in Lake Valencia present the complex limnological processes in a single parameter, and the severe oxygen depletion imposes a most serious managerial problem. The major source of dissolved oxygen in the lake is phytoplankton photosynthesis. Input through atmospheric diffusion is expected to be insignificant because the surface lake water is frequently supersaturated with dissolved oxygen that prevents efficient air diffusion into the lake water. On the other hand, the dissolved oxygen is consumed by several major components in the lake system, i.e. respiration of phytoplankton, zooplankton, and fish; sediment oxygen demands; chemical oxygen demands; and biochem^ical oxygen demands of the large quantity of organic materials from tributary inputs. Those processes result in a negative dissolved oxygen budget in Lake Valencia and create a severe oxygen depletion in the aphotic zone where an anoxic state 122 prevails in a large proportion of the water column. Predictably, the continuous external loading of organic substances from tributaries will demand a greater quantity of the dissolved oxygen produced by phytoplankton and drive the anoxic layer upward to the surface. Further understanding of the dynamic role of dissolved oxygen in Lake Valencia will require systems analysis and development of a numerical mass balance model. This type of model is essential for any managerial attempt to improve oxygen tension in the lake. Water chemistry in Lake Valencia is probably far more comxplex than any general survey can comprehend. The exceedingly high concentrations of total dissolved solids (-2,000' mg/L) with sulfate as a major constituent (500-1,000 mg/L) represent an unusual case in freshwater lakes of this size. Unlike most natural lakes with relatively long'-term stable chemical characteristics which are governed largely by the geological features of the lake basin, the major chemical constituents in Lake Valencia are anthropogenic. The composition and quantity of those chemical constituents are constantly changing with increasing population and land use patterns in the lake basin. The concentration of total dissolved solids has risen progressively since the lake became a closed body of water: it has been increasing exponentially during the last two decades. According to a m^ass balance predictive equation (Apmann 1973), the total dissolved solids will rise to 6,000 by the year 2020. During recent years, the increase of total dissolved solids has paralleled a rise in sulfate concentration. Although sulfate is generally considered a conservative ion in the aquatic environment, the effects of exceedingly high concentrations on aquatic biota are not well known. Certainly, sulfate is the most available form to be reduced to sulfide in the 123 prevailing anaerobic subsurface water in Lake Valencia. The formation of sulfide in lake water may have a significant impact on the chemistry of heavy metals as the sulfide of heavy m.etals forms notoriously insoluble precipitates. Theoretical treatment indicates that the insolubility of ferrous sulfide in the sediment prevents release of certain metals such as copper, silver, lead, and cadmium to lake water (Hutchinson 1957). The oxidation of hydrogen sulfide during the mixing period may also enhance the oxygen depletion in the lake water. The nutrient data presented in this report indicate that the levels of phosphorus and nitrogen were in excessive supply for phytoplankton growth. A large proportion of those nutrients was in the organic or particulate forms, presumably retained mostly by the phytoplankton cells. The internal loading of nutrient through organismal cycling and sediment regeneration is likely to be an important source of nutrient supply in Lake Valencia. Under the prevailing anoxic conditions in the sediment and deep water, a substantial quantity of nutrients might be regenerated and released to the euphotic zone. Therefore, the analysis of sediments is an important tool for the study of the chemical and biological functions of the overlying water body. Transfer of dissolved matter between sediments and overlying water is of considerable importance in the control of the chemical processes taking place in the sediments and interstitial and basin waters. Some of the primary concerns are those of the role of sediments in the cycling of nutrients and as a sink of oxygen. These processes are governed by complex biochemical mechanisms. One of these processes is sediment oxygen demand (SOD). In Lake Valencia, incorporating SOD into a quantitative model will be indispensable to predict mass balance of dissolved oxygen for lake restoration. 124 Source of phosphorus (P) and nitrogen (n) released from the sediment to the overlying waters is expected to be of critical importance for successful prediction of the effects on lake trophic status resulting from the removal of nutrients by treatment. The external loadings through tributary discharges constitute the largest pollutant input to Lake Valencia. As those loadings contain mostly untreated domestic and industrial discharges and silts from surface runoffs, the lake proper adjacent to the tributary outfalls acts as a treatment which may be conveniently termed a self-purification system for the pollutant. Several processes are involved in the system: sedimentation of solid material, decomposition of organic substances, and release of inorganic nutrients. As a result, those areas of the lake show a large sedimentation rate, severe oxygen depletion, and high nutrient concentrations. Thus, the chemical constituents of the pollutants, the physical features of the treatment area, and the biological responses are primary factors involved in the self-purification capacity. Our previous investigation indicates that the river mouth of Cano Central exhibits a great capacity of self-purification. Such a process is particularly important in this area because it receives the largest quantity of domestic discharge to Lake Valencia. Physically, the extensive shallow contour provides efficient aeration which results in the active aerobic decomposition of organic materials. The nutrients released from organic forms stimulate massive phytoplankton blooms. Population density of the zooplankton community has been found greater here than in other parts of the lake, particularly the density of rotifers which are believed to be bacteria feeders. Ostracods are also commonly found in this area. The pollutant discharge, dispersion pattern, and heterotrophic and autotrophic processes in 125 the Cano Central region should be another focus for future research. Results of this investigation indicates that excessive nutrient loading and severe oxygen depletion present the most urgent issues in Lake Valencia, To estimate nutrient and oxygen budgets in the lake water, we roust consider both external and internal loadings. The external sources of nutrients and biochemical oxygen demand are primarily contributed by tributary discharges which are being thoroughly investigated. The internal loading of nutrient and oxygen involves uptake and release by various constituents in the water column as well as in the sediment. However, the magnitude of uptake and release of nutrients and oxygen through internal compartments of the lake is virtually unknown in Lake Valencia. The dynamics of nutrient and oxygen concentrations are governed by physical, chemical, and biological processes, and to understand the complexity of those processes requires system analysis and numerical modeling. 126 REFERENCES American Public Health Association (APHA). 1975. Standard methods for exam- ination of water and wastewater. American Water Works Association. New York. 13th ed. Apmann,, R. P. 1973. Report on investigation of the effect of agricultural wastes on the contamination of Lake Valencia, Venezuela. 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