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 



