PRIMARY PRODUCTIVITY 389 



uptake lag time. It is possible that a 2-hr incubation time is not 

 sufficient to detect the effects of additions of a limiting nutrient. 



Biological processes are not often linear, and relationships are not 

 generally continuous. Regression analysis assumes that the under- 

 lying relationships between variables are linear and additive. In 

 attempting to analyze the data, however, we found it more expedient 

 to divide the data from each station at 25°C and compute the linear 

 regression equation and measures of correlation before using more 

 compHcated methods. The objectives were, not to develop a model 

 for phytoplankton productivity, but to explain and predict the 

 functioning of the phytoplankton community as it relates to 

 temperature. 



The rate of carbon fixation tends to increase jS units with every 

 degree of increase in temperature up to 25° C and to decrease —(i 

 units with every degree of increase in temperature above 25°C; i.e., it 

 appears that the temperature optimum for the natural 

 phytoplankton community in Lewis Creek Reservoir lies in the 

 vicinity of 25°C, and temperatures in excess of that tend to suppress 

 the rate of carbon fixation. 



By setting Y equal to 0, we can estimate a temperature tolerance 

 range below and above which productivity is suppressed entirely. For 

 Station 1 the estimated temperature tolerance range is 10 to 42°C 

 and for Station 2, 5 to 39° C. This assumption excludes the 

 possibility of a complete change in community structure. It is 

 interesting to note that the estimated temperature tolerance ranges 

 are not the same and that the estimate for the community subjected 

 to consistently higher temperatures at Station 1 is higher. This 

 suggests that organisms at Station 1 are adapted to higher tempera- 

 tures. 



The proportion of variance explained by temperature, r^ , is 

 larger at Station 2; i.e., 51% of the variance at Station 2 between 10 

 and 25°C is explained by temperature and only 13% is explained at 

 Station 1. Thus other factors at Station 1 contribute to the 

 variability of rates of carbon fixation. One possibility is stress to the 

 organism caused by passage through the condensers (Lanza and 

 Cairns, 1972; Gurtz and Weiss, 1974). 



Our work centered only on the phytoplankton of Lewis Creek 

 Reservoir. It became obvious soon after this study commenced that, 

 throughout much of the year, the system is dominated by a large 

 population of benthic algae. Detailed studies on the effects of 

 temperature on benthic productivity and biomass accumulation 

 would be of considerable interest. 



