Table 2. — Range in bottom temperature in Clear Lake, Tex., 1958-66 



- = 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 organic nitrogen in estuarine water (unpublished).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 



