XFVVS-A 578 1-8(1969) U.S. Fish Wildl. Serv. Spec, Sci. Rep. Fish. 578 Hydrological Conditions in Clear Lake, Texas, 1958-66 SPECIAL SCIENTIFIC REPORT-FISHERIES Na 578 r-M '•■^"xal Laboratory ^ A R Y T APRl UNITED STATES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE BUR E AtTof'cOMMERCiArF IsSiiT SPECIAL SCIENTIFIC REPORT-FISHERIES Robert L. Hacker, Editor Jane S. Hallett, Associate Editor Mary Fukuyama, Assistant Editor Betty M, Hoffman, Assistant Editor PUBLICATION BOARD John A. Guinan James H. Johnson Robert L. Hacker John M. Patton, Jr. John I. Hodges Frank T, Piskur Harvey Hutchings Edward A. Schaefers Leslie W, Scattergood, Chairman Special Scientific Report — Fisheries are preliminary or progress reports and reports on scientific investigations of restricted scope. Established as Special Scientific Reports in 1940, nos. lto67 were issued from that date to 1949, when the new series. Special Scientific Report—Fisheries, with new serial numbering, was started. Special Scientific Report — Fisheries are distributed free to libraries, research institutions. State agencies, and scientists. UNITED STATES DEPARTMENT OF THE INTERIOR U.S. Fish and Wildlife Service BUREAU OF COMMERCIAL FISHERIES Hydrological Conditions in Clear Lake, Texas, 1958-66 By EDWARD J. PULLEN Contribution No. 270, Bureau of Connmercial Fisheries Biological Laboratory, Galveston, Texas 77550 United States Fish and Wildlife Service Special Scientific Report--Fisheries No. 57i Washington, D.C. 20240 January 1969 CONTENTS Page Introduction 1 Study area and sampling procedures 1 Temperature 3 Salinity , . 5 Dissolved organic nitrogen 5 Total phosphorus 5 Oxygen 5 Literature cited 8 Hydrological Conditions in Clear Lake, Texas, 1958-66 By EDWARD J, PULLEN, Fishery Biologist Bureau of Commercial Fisheries Biological Laboratory Galveston, Texas 77550 ABSTRACT Temperature and salinity data were collected in 1958-66, and dissolved oxygen, dissolved organic nitrogen, and total phosphorus analyses were made on samples col- lected in 1964-66 in Clear Lake, a small estuary that flows into upper Galveston Bay. Seasonal trends in bottom water temperature were similar in the different years and were related closely to trends in air temperature. Average water tempera- tures were lowest (about 13° C.) in January and February and highest (about 31° C.) in July. The rate of warming and cooling was directly related to the magnitude of the annual temperature difference between the warm and cool months. Salinity ranged from 0.1 to 23.7 p.p.t. (parts per thousand) and was related in- versely to stream flow and precipitation. Variations in rainfall resulted in fluctua- tions in stream flow and these, inturn,were reflected by variations in salinity. Dissolved organic nitrogen ranged from 23.5 to 171.5 Hg. at./l. (microgram atoms per liter) and averaged 66.4 fjg.at./l. Total phosphorus ranged from 1.7 to 26.3 /ig. at./l. and averaged 8.2 //g. at./l. Phosphorus values increased markedly in July 1966. No correlation existed between nitrogen or phosphorus and stream flow or rainfall. Dissolved oxygen ranged from 3.8 to 19.3 p. p.m. (parts per million). INTRODUCTION The hydrological characteristics of many estuaries bordering the Gulf of Mexico are being altered by water development projects. The effects of these alterations on the aquatic environnnent are difficult to document because water quality data are usually not available for the period preceding the alterations. Clear Lake, a small estuary that flows into upper Galveston Bay, is an area being altered because of a rapidly expanding population. About one-quarter of a million people inhabit the watershed of Clear Lake, and the popula- tion is expected to increase to one nnillion in 10 years (Mock, 1965). Hydrological data now available may be used to establish standards for preserving aquatic life in Clear Lake when we learn the physiological requirements of estuarine species and when we know the in- fluence of the changing environment upon these animals . Personnel of the Bureau of Commercial Fisheries Biological Laboratory, Galveston, Tex., collected hydrological data from Clear Lake from 1958 through 1966. This report (1) summarizes the temperature and salinity data for the 9-year period, (2) summarizes the dissolved oxygen, organic nitrogen, and total phosphorus data collected from 1964 through 1966, and (3) compares these variables with air temperature, stream flow, and local pre- cipitation. STUDY AREA AND SAMPLING PROCEDURES Clear Lake is about 4 km. (2.5 statute miles) long, less than 2 km. (1 mile) wide, and has a surface area of about 405 ha. (1,000 acres). Average water depth, excluding a 2-m. channel dredged through the lake, is about 1 m. at mean low tide. Stations at which measurements were made during at least 1 year of the study are shown in figure 1. The sampling frequency and type of information obtained each year are listed by stations in table 1. Salinity and temperature were measured near the bottom (1) with a portable salinometer having an accuracy of +0.3 p.p.t. (parts per thousand) and ±0.5° C, or (2) by titration of water samples (accuracy, +0.2 p.p.t.) and direct readings with a glass thermometer (accuracy, + 0.5° C). Water samples were taken with a 95" W. GALVESTON BAY Figure 1. —Galveston Bay system and Clear Lake, Tex., showing sampling stations. Table 1.— Station number, frequency of sampling, and type of information obtained from Clear Lake, Tex., 1958-66 Station number Sampling frequency^ Type of information obtained Year Weekly Semi- monthly Monthly Temper- ature Salinity Oxygen Organic nitrogen Total phosphorus 1958... 1959... 1960... 1961... 1962... 1963 . . . 196«;... 1965... 1966... 1, 3-7, 10-11 1, 3-7, 10-11 ^-1, 11 -4-7, 11 4-5, 7, 11 3, 7 3, 6-8 2-3, 8-9 2-3, 8 X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X Sampling frequency varied within some years. Table 2. — Range in bottom temperature in Clear Lake, Tex., 1958-66 Year Month 1958 1959 1960 1961 1962 1963 1964 1965 1966 ° C. ° C. ° C. ° C. ° C. ° C. ° C. ° C. ° C. January 11.3-15.0 4.8-19.8 14.0-18.3 10.5-13.7 6.0-14.5 2.9-15.2 13.0-15.0 11.6 16.0 February. . . 8.0-16.8 8.6-20.9 _ 9.0-17.5 12.0-18.0 5.3-13.4 13.0-19.0 9.5-17.0 10.0-11.0 March 15.2-20.2 15.4-20.9 - 18.2-20.5 14.0-18.5 16.2-21.7 20.4-23.0 11.0-21.0 15.0-27.0 April 19.0-28.7 18.0-22.4 _ 16.7-22.0 16.0-26.0 21.4-26.4 21.0-21.5 19.0-29.2 19.5-28.0 May 2'i.8-32.6 _ 25.4-28.7 24.3-28.0 - 27.0-29.0 28.1-30.5 23.0-30.8 20.5-30.0 June 28.5-33.9 29.8-31.8 27.9-30.0 27.7-28.7 30.0-33.0 31.0 27.0-29.2 28.0-32.0 26.0-36.0 July 28.8-34.4 29.1-33.2 28.8-32.0 24.5-28.5 31.0-33.0 31.0-34.0 29.5-33.6 27.0-33.0 29.0-34.0 August 27.5-34.0 27.0-35.0 28.5-31.6 27.9-29.6 29.0-31.0 30.0-33.0 32.3-33.0 28.0-32.0 24.0-31.0 September. . 24.0-33.0 28.0-29.0 25.7-30.8 25.0-30.8 - 29.0-31.5 26.6-28.0 22.0-33.3 24.0-33.0 October. . . . 19.0-26.8 20.2-21.4 20.0-26.8 18.0-24.4 22.5-23.4 24.0-27.0 22.4-23.0 16.0-26.0 18.0-24.0 November. . . 17.1-23.4 10.9-11.1 18.6-22.0 16.0-19.5 13.0 20.5-22.0 21.2-25.1 18.0-23.0 16.0-21.0 December. . . 9.5-16.0 14.0-17.7 7.9-15.5 10.5-13.0 13.2-13.5 9.0-12.0 11.0-13.0 11.0-19.0 19.0 Annual 8.0-34.4 4.8-35.0 7.9-32.0 9.0-30.8 6.0-33.0 2.9-34.0 11.0-33.6 9.5-33.3 10.0-36.0 - = no data KeiTimerer^ or a triple-bottle sampler. Be- ginning in 1964, dissolved oxygen, in p.p. nn. and dissolved organic nitrogen and total phospho- rus in |Ug.at./l. (microgram atoms per liter) were determined from samples of bottom water. The techniques for salinity, oxygen, and phosphorus determinations were described by Marvin, Zein-Eldin, May, and Lansford ( I960), and organic nitrogen was determined by a Kjeldahl method described by D. C. Willis.^ Air temperature and rainfall data near Clear Lake were obtained from the U.S. Weather Bureau Climatological Data collected at Bay- town, Houston Airport, and Alvin, Tex. (fig. 1); these data were averaged to obtain mean values . Measurements of stream flow for Clear Creek, the major tributary entering Clear Lake, were obtained from the U.S. Geological Survey. TEMPERATURE The method of combining temperature data is based on an analysis of data collected in 1958 when bottom water temperatures were taken weekly at all stations except 2, 8, and 9 (fig. 1). Temperatures taken at eight stations on a given date throughout the year were com- pared by analysis of variance. Since values did not vary significantly between stations (F = 0.37; F.05 for 7 and 376 d.f. = 2.04), data from all stations were combined to compute monthly averages. Table 2 shows the ranges of temperatures from which these average monthly values were computed. ■"■Trade names referred to in this publication do not imply endorsement of the commercial products. 2 The Kjeldahl method was modified by D. C. Willis, University of Tampa, Florida, for the analysis of dissolved organic nitrogen in estuarine water (unpublished). Trends in water temperature were similar between years, although the yearly ranges varied (fig. 2). The seasonal cycles of water temperature and air temperature were related closely. The large and nearly constant dif- ferences between air and water temperatures are probably not real, because average air temperatures were derived from observations over a 24-hour period, whereas average water temperatures were based on measurements made randomly during daylight. Water tem- peratures were lowest (about 13° C.) in Jan- uary and February and began increasing grad- ually in March; they were highest (about 3 1° C.) in July and began declining in August and September. Monthly averages ranged from 8.7° to 32.7° C. The greatest difference in average minimum temperatures between years was about 3° C. (1963 and 1965), and the greatest difference in average maximum temperatures was about 5° C. (1961 and 1963). Year-to-year variations were apparent in the warming and cooling rates of the water in Clear Lake (fig. 3). These rates were de- termined by computing the slopes of the tem- perature curves (fig. 2) during January through July (warming period) and August through December (cooling period) for each year. Waters warmed and cooled rapidly in 1958, 1959, 1962, and 1963 but slowly in 1960, 1961, 1964, 1965, and 1966. The rate of warming and cooling of water each year was directly related to the magnitude of the difference in average temperatures be- tween the summer (June, July, and August) and the preceding winter (December, Janua ry, and F e b r u a r y)- -see figure 3. This relation is illustrated by a comparison of the values for 1960 and 1963. In 1960, when temperatures in- creased 2.3° C. per month in the spring and decreased 3.6° C. per month in the fall, the difference between the warm and cool months was 13.7° C. In 1963, in contrast, when •c 40 20 ni I I ] I I I J ] I I I I I I I I I M I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I J I I I I I I I I I I I I I JMMJSNJMMJSNJMMJSNJMMJSNJMMJSNJMMJSNJMMJSNJMMJSNJMMJSN 1958 1959 I960 1961 1962 1963 1964 1965 1966 Figure 2. — Average monthly water temperatures for Clear Lake, Tex. (1958-66), and monthly average air temperatures recorded by the U.S. Weather Bureau o o CO K8h < q: a 7 < o z < CD 6 o o o 1963 — 1962 • 1959 -^-"-""^ — ^ ^^-^^ 1958 ^^^^^•1964 v- I960 ^^ 1 ^^ 1961 1 1 1966 1 1 1 Y= 1.04 + 0.34X 1 1 14 15 16 17 IB 19 ANNUAL TEMPERATURE VARIATION CO 20 21 Figure 3. — Sum of the cooling and warming rates in relation to the annual temperature variation in Clear Lake, Tex. (1958-66). Table 3. — Range in bottom salinity in Clear Lake, Tex., 1958-66 Year Month 1958 1959 1960 1961 1962 1963 1964 1965 1966 P.p.t. P.p.t. P.p.t. P.p.t. P.p.t. P.p.t. P.p.t. P.p.t. P.p.t. January, . . . 0.1- 8.6 15.2-21.3 _ 0.1- 3.2 14.3-17.5 2.1-13.6 10.6-17.7 17.1 14.4 February... 0.1- 7.3 0.2- 3.2 - 0.2- 2.2 13.8-17.7 5.9-12.2 0.7-14.0 16.4-18.8 2.4- 2.5 March 1.4- 9.9 0.2- 7.8 - 1.2- 5.6 14.9-16.3 7.1-r3.8 10.3-13.0 12.0-17.0 8.0-13.9 April 3.0-13.6 - - 5.0- 9.2 9.7-16.7 13.7-18.6 13.6-15.6 9.9-16.6 0.5-16.0 May 7.9-U.7 - 6.4-13.3 5.6-10.3 15.2 16.3-18.2 0.4-15.2 6.8-16.7 0.3- 4.3 June 5.3- 8.2 1.3- 8.2 0.2-14.6 0.4-10.0 7.3- 8.6 16.2-18.7 14.4-19.8 8.0-13.1 1.2- 5.0 July 5.9- 9.1 0.2- 6.2 2.5- 7.5 0.1- 2.3 6.3-13.6 8.6-16.2 16.5-17.9 8.0- 9.5 7.0-11.5 August A.6-U.4 0.8- 4.2 0.5- 8.1 3.4- 9.6 12.9-15.0 13.7-21.1 16.4-18.2 14.7-19.9 5.3-13.5 September . . 0.1-16.0 8.1 2.9-11.5 0.7-10.1 - 18.7-23.1 19.5-20.5 17.0-21.4 7.5-12.5 October. ... 2.3- 9.2 0.3-12.7 0.9-12.1 1.8-13.6 12.1-14.5 21.0-23.4 18.3-19.6 19.0-21.0 13.4-20.0 November . . . 6.5-11.4 10.0-12.8 4.2- 7.6 0.1-10.5 14.2 16.4-23.7 14.4-21.2 9.8-22.4 11.6-17.4 December.. . 8.6-18.3 0.5-16.4 0.1- 4.9 4.5-10.0 11.5-14.0 10.6-22.9 0.8- 3.3 3.2-19.3 19.2 Annual 0.1-18.3 0.2-21.3 0.1-14.6 0.1-13.6 6.3-17.7 2.1-23.7 0.4-21.2 3.2-22.4 0.3-20.0 = no data temperatures increased 4.4° C. per month in the spring and decreased 4.1° C. per month in the fall, the range between the warm and cool months was 21.9° C. SALINITY Salinities in Clear Lake ranged from 0.1 to 23.7 p.p.t. during the study (table 3). I com- pared the 1958 data by using analysis of vari- ance in the same manner as for temperature. Differences between stations were significant (F = 5.68, F.05 for 7 and 376 d.f. =2.04). Duncan's multiple range test (Steel and Torrie, 1960) was used to determine the salinity zones shown in figure 4. Average monthly salinities were nearly always lower in zone IV than in zone I (fig. 5), and were usually intermediate in zones II and III. Because collections were made only in zones II and III for the entire 9 years, data from these two zones were used to evaluate fluctuations over the 9-year period. Salinity trends for the 9 years are shown in figure 6. Yearly averages were relatively low from 1958 through 1961 (grand average, about 8 p.p.t.), and generally high in 1962 through 1965 (grand average, about 13 p.p.t.). Average salinity in the lake dropped below 10 p.p.t. in 1966. The annual average salinity was highest (15.9 p.p.t.) in 1963 and lowest (4.8 p.p.t.) in 1961. Average monthly stream flow in Clear Creek also is compared with salinity of Clear Lake in figure 6. In general, stream flow was related inversely to salinity during the period of com- parison. The correlation coefficient (r) was -0.57. The correlation between monthly average salinity and local precipitation is not as ap- KILOMETERS STATUTE MILES ZONE is: Figure 4. — Clear Lake, Tex. divided into zones of different salinities. parent (r = -0.41) as when the data are grouped by 4-month intervals. The use of 4-month average salinities and total rainfall during the same periods reduces variation in the data, as indicated by a correlation coefficient of -0.57. When these values were used, trends in the data became apparent (fig. 7). For example, in 1959-61, when rainfall was considerably higher than the 9-year average, salinities m the lake were low; in 1962-65, when rainfall was below the 9-year average, salinities in the lake were correspondingly high. Variations in rainfall from year to year coincided with fluctuations in stream flow and these, in turn, were reflected by variations in the salinity of Clear Lake. DISSOLVED ORGANIC NITROGEN Dissolved organic nitrogen ranged from 23.5 to 171.5 fig.ai./l. (table 4). Monthly averages were computed by combining data from all stations; the lowest and highest average values (33.8 and 171.5 jug.at./l.) were recorded in 1964. Monthly mean values in table 4showthat nitrogen fluctuated erratically and had no ob- vious seasonal pattern. Neither river discharge (r = 0.08) nor rainfall (r = -0.05) was corre- lated with nitrogen. Dissolved organic nitrogen averaged 66.4 jUg.at./l. over the 3 years. TOTAL PHOSPHORUS The concentration of total phosphorus ranged from 1.7 to 26.3 ^g.at./l. (table 5). Monthly mean values fluctuated between 4.7 and 12.1 /ig.at./l. from 1964 through 1965 and reached a minimum of 3.7 /ig.at./l. in March 1966. A gradual increase from April through June 1966 was followed by a marked increase in July. Values remained high for the rest of the year, and the maximum mean was 18.8 ^g. at. /l. in November. Stream flow (r = - 0.24) and rainfall (r =-0,07) were not correlated with phosphorus. The high phosphorus recorded in Clear Lake in the summer and fall of 1966 probably re- sulted from intrusion of water from upper Galveston Bay, where the phosphorus varied from 10.9 to 25.0 ug. at. /I. at that time (Pullen, unpublished data). Dye studies made in a model of the Galveston Bay system show a marked interchange of water between upper Galveston Bay and Clear Lake (U.S. Corps of Engineers, personal communication). Total phosphorus in Clear Lake averaged 8.2 fig. at. /l. for the 3 years. OXYGEN Dissolved oxygen ranged from 3.8 to 19.3 p. p.m. during this study (table 6). Meanmonthly values for 1965 and 1966 indicate a seasonal 20 r Figure 5. — Average monthly salinity by zone in Clear Lake, Tex., 1958. 30r £ 20 ■ CLEAR LAKE ■ ZONE n ZONE m 0'- 16 I I I I I I I I I I I I I I I M M I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I M I I I I I I I I I I I I I I I I I I I I CLEAR CREEK I I 1 1 1 I I I I I Tp I I I I I I I I I I 1 1 I 1 1 1 1 I I I I 1 1 I 1 1 I I 1 1 1 1 1 1 1 I I I 1 1 I M I I I I I I I n I 1 1 iTi 1 1 I I I I I I I I I I I r I I I I I 1 1 M I 1 1 I I I M I I I n JMMJSNJMMJSNJMMJSNJMMJSNJMMJSNJMMJSNJMMJSNJMMJSNJMMJSN 19 7 1958 1959 I960 1961 1962 1963 1964 1965 1966 Figure 6.— Average monthly salinity and stream flow into Clear Lake, Tex. (1958-66). The Clear Creek gaging station was not in operation from May 1960 through May 1963. 30 - ZONE nr - 5* - 20 >- - T/' r TREND S /y ] ' T 1 - NO DATA J^ ■ z /v.- >--t- --/ V ^^^ ~^v. ><. V / ^ 10 -J T T ^ /'' "S / J ^v / - tn y< r CI xr--,^/-" ^ 'V?^ ^■"^ ;._.. -4 f-^ 1 1 1 1 1 r 1 J 1 1 1 1 1 1 1 1 J 1 1 1 1 1 30 p ZONE ir 1 — -T -1 RANGE +-MEAN -r T ^' 1 1 ^20 >- • TREND i-/T 1 T - K NO DATA /k T .'/■^ \ V — ■■ ^K- V \ -. Z / '^N- -^ ^ / \ -^ A / i 10 - 1 r T^ ^ \ J r f/ ^^^^ ^-./J . - (0 ■\ ^/l . s / r J ~ T*< ^'^ 1 I n 1 1 1 ■ 1 1 1 1 ! 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 40i- 1016 < S 1958 o o a. <•) o rr e> <> tc -1 UJ -) UJ n -) UJ Q. < Q < < o < < o < >- n z > a: z > n z < UJ < < UJ < < UJ < 2 in -5 £ yi -^ £ CO -3 1959 I960 1961 1962 a. < <3 < 3 ! 5 ! S c 1963 3 < I I >- I < I 5 ' 1964 z < 5 ( 1965 C3 3 1966 Figure 7. — Cumulative local precipitation and range and average salinity in Clear Lake, Tex., for 4-month periods. Trend lines were plotted by using three-point moving average. Table 4. — Monthly range and mean of dissolved organic nitrogen in Clear Lake, Tex., 1964-66 Month 1964 1965 I"i66 Range Mean Range Mean Range Mean January February March M«?. at. A- - 171.5 90.6 33.8 117.5 69.0 37.3 88.1 73.7 58.0 Mg. at. (^) 50.6- 72.9 52.2-134.0 33.7- 57.5 34.5- 57.5 50.0- 78.9 50.5- 78.4 25.8- 87.0 66.1- 76.6 41.2- 47.5 62.6- 83.1 59.9- 81.3 A- 54.2 56.1 84.3 39.9 58.4 67.4 68.7 56.5 71.3 44.3 70.2 67.4 mJ- at. 28.2- 45.1 44.7-103.5 23.5-121.0 33.5-105.0 40.5-139.0 58.0-132.0 63.5- 74.3 68.5- 97.0 37.0- 72.5 57.5-106.0 A. 40.7 104.3 36.8 70.4 72.9 59.6 70.4 107.2 68.4 78.5 53.5 81.7 April May Jtme July August September October November December Year 33.8-171.5 82.2 25.8-134.0 62.1 23.5-139.0 68.6 no data ■>■ Single observation Table 5. — Monthly range and mean of total phosphorus in Clear Lake, Tex., 1964- Month January. . . February . . March April May June July August September. October. . . November. . December. . Annual. 1964 Range 5.7-12.1 Mean fig, at. /I. 10.1 9.5 8.9 9.4 8.1 10.0 7.3 12.1 5.7 9.0 data ■"■ Single observation 1965 Range Mean fig, at. /l. (^ ) 6 4- 9 5 6 9- 15 7 1 7- 8 3 4 .6- 6 8 4 .9- 8 4 6 .7- 7 4 5 .1- 5 .8 4 .7- 6 .0 6 .1- 7 .2 4 .4- 5 .9 5.0-10.1 1.7-15.7 5.9 8.1 11.1 4.7 5.6 6.9 7.1 5.4 5.4 6.7 5.2 7.5 6.9 1966 Range Mean fig, at. /l. 1.7- 4.7 3.1- 7.5 3.2- 7.4 3.3-10.8 11.6-15.0 10.3-26.3 13.4-16.9 11.2-15.3 15.9-21.4 13.8-17.7 1.7-26.3 12.3 4.9 3.7 4.7 5.7 6.2 13.0 14.8 15.2 13.4 18.8 15.7 9.6 Table 6. — Monthly range and mean of dissolved oxygen in Clear Lake, Tex., 1964-66 Month 1964 1965 1966 Range Mean Range Mean Range Mean January February March P.p (^) (^) (^) m. 19.3 12.5 11.4 9.3 10.0 7.4 9.9 16.3 10.6 8.6 P.p (^) 7.6-10.6 3.8-15.4 8.6- 9.8 5.7- 8.8 5.5- 8.4 {') 5.0- 6.7 {') 5.9- 9.4 9.4-11.7 8.6- 9.6 m. 17.0 9.1 9.3 9.2 7.5 7.0 6.4 5.8 6.6 7.5 10.6 9.1 P.p {') (') 7.6-11.5 5.8- 7.5 4.6- 7.2 6.3-10.1 5.7- 8.8 3.8-12.4 (M m. 12.8 10.3 April May June July August September October November December 8.8 6.9 6.7 8.2 7.2 8.1 10.4 11.3 Annual 7.4-19.3 11.5 3.8-17.0 8.4 3.8-12.8 8.2 no data Single observation trend, but because of wide fluctuations in values within a month and in sampling f r e qu e n c y, these trends are probably not significant. LITERATURE CITED MARVIN, KENNETH T., ZOULA P. ZEIN- ELDIN, BILLIE Z. MAY, and LAURENCE M. LANSFORD. 1960. Chemical analyses of marine and estuarine waters used by the Galveston Biological Laboratory. U.S. Fish Wildl. Serv., Spec. Sci. Rep. Fish. 349, iii + 14 pp. MOCK, CORNELIUS R. 1965. Distribution of bottom sediments in Clear Lake, Texas. In Biological Lab- oratory, Galveston, Tex., fishery re- search for the year ending June 30, 1964, pp. 89-92. U.S. Fish Wildl. Serv., Circ. 230. STEEL, R. G. D., and J. H. TORRIE. 1960. Principles and procedures of sta- tistics with special reference to bio- logical sciences. McGraw-Hill Book Co., New York, 481 pp. MS. #1796 GPO 8 6 3-5dO m "?A™K'|,it;[ff.:.Se'.ah II Mil nil 5 WHSE 01769 As the Nation's principal conservation agency, the Depart- ment of the Interior has basic responsibilities for water, fish, wildlife, mineral, land, park, and recreational re- sources. Indian and Territorial affairs are other major concerns of America's "Department of Natural Resources." The Department works to assure the wisest choice in managing all our resources so each will make its full contribution to a better United States — now and in the future. UNITED STATES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE BUREAU OF COMMERCIAL FISHERIES WASHINGTON, D.C. 20240 POSTAGE AND FEES PAID U.S. DEPARTMENT OF THE INTERIOR THIRD CLASS OFFICIAL BUSINESS Return this sheet to above address, if you do NOT wish to receive this material | | , orif change of address is needed | | (indicate change including ZIP Code).