PRIMARY PRODUCTIVITY: ANALYSIS 

 OF VARIANCE IN A THERMALLY 

 ENRICHED AQUATIC SYSTEM 



MARGARET O. WELCH and C. H. WARD 



Departments of Biology and Environmental Science and Engineering, 



Rice University, Houston, Texas 



ABSTRACT 



Primary production experiments were performed in a 400-ha cooling reservoir 

 for a 530-MW electric generating plant in eastern Texas over a 15-month period. 

 Annual primary productivity of the heated and ambient surface waters and the 

 nutrient enrichment response were investigated in situ by ' C methods. On the 

 basis of analysis of variance techniques, primary productivity was significantly 

 higher at the heated station on 18 of 26 sampling dates, but the magnitude of 

 the difference was not correlated with the difference in water temperature at the 

 stations. Nutrient enrichment had no effect on primary productivity, but 

 temperature did. Regression analysis indicated that the temperature optimum 

 for the natural population of phytoplankton was ~25 C. The temperature 

 tolerance range for the phytoplankton community subjected to higher tempera- 

 tures was higher than for the community at ambient temperatures. 



An understanding of the biology of high-temperature aquatic systems 

 is necessary for predicting and managing the effects of man-induced 

 thermal additions. The extensive literature (e.g., Patrick, 1969; Fogg, 

 1975) documenting the thermal tolerance range and the thermal 

 optimum of many species of freshwater algae enables us to generalize 

 that more species of the Cyanophyta are thermsil tolerant and grow 

 well above 35°C, that the Chlorophyta tend to grow best up to 35°C, 

 and that the diatoms, the Bacillariophyta, succeed best below 30°C. 

 Most investigators agree that the thermal optimum for all species in 

 the laboratory is generally higher than that in natural aquatic 

 systems. Castenholz and Wickstrom (1975) and Patrick (1969) 

 pointed out that most studies deal with the effects of thermal 



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