OPTIMUM TEMPERATURE FOR CARBON ASSIMILATION IN KENTUCKY LAKE FOLLOWS SEASONAL CHANGE IN AMBIENT-TEMPERATURE

Long-term (six years) field-descriptive data from Ledbetter Bay, Kentu cky Lake, Kentucky showed that phytoplankton carbon assimilation (mg C l(-1) h(-1)) was dependent upon chlorophyll concentration (r(2) = 0.6 6, p = 0.0001) and temperature (r(2) = 0.11, p = 0.02), while assimila tion number (mg C h(-1) mg chlorophyll(-1)) was not dependent upon tem perature. When assimilation number by natural phytoplankton assemblage s collected eight times throughout 1995 was measured in the laboratory using an experimental-manipulative approach at 5, 10, 15, 20, 25, and 30 degrees C, the optimum temperature for assimilation number showed a significant positive relationship with ambient temperature. The opti mum temperature shifted from a low of 11 degrees C for the winter comm unity to 30 degrees C for the summer community. The following second o rder equation (r(2) = 0.91) adequately described the relationship betw een optimum temperature for assimilation number (OptT) and ambient tem perature (AmbT); OptT = 13.75 + [(5.067 x 10(-3)) (AmbT)] + [(1.55 x 1 0(-2)) (AmbT)(2)]. The assimilation number of the winter phytoplankton measured at 5 degrees C was almost 165 % greater than carbon assimila tion of the summer phytoplankton measured at 5 degrees C, while the as similation number of the winter phytoplankton measured at 30 degrees C was about 75 % less than the summer phytoplankton measured at 30 degr ees C. These data suggest that adaptations which allow for greater car bon assimilation during one seasonal extreme preclude equivalent or gr eater carbon assimilation at the opposite seasonal extreme. It appears that the same phytoplankton community cannot maintain optimal carbon assimilation rates at both cold and warm temperatures. and that phytop lankton communities adapt to seasonal change in temperature. These dat a demonstrate that the optimum temperature of carbon assimilation for freshwater phytoplankton can vary with seasonal change in ambient temp erature.