EVALUATION OF PHOSPHORUS DEFICIENCY INDICATORS FOR SUMMER PHYTOPLANKTON IN LAKE ERKEN

Short-term changes in nutrient status of summer phytoplankton were studied in Lake Erken in 1988. The P deficiency index (PDI), which is defined as the rate of light-saturated photosynthesis (P(opt)) divided by the maximum phosphate uptake velocity (V(max)), was a superior indicator of P deficiency, despite the fact that dramatic shifts in nutrient status did not occur during the study period. Indicators containing physiological rates (orthophosphate turnover time, TT; Chl-specific total alkaline phosphatase activity, TAPA(B); and Chl-specific maximal phosphate uptake velocity, V(B)max) were also sensitive enough to show changes in the P status of algae. In contrast, indicators based on the chemical composition of the particulate matter (PN: PP, Chl-specific surplus P content, SP(B)) failed to show short-term variations in algal nutritional status. Correlations were found between PDI and TAPA(B), PDI and TT, as well as PDI and turnover time of surplus P (T(SP)). PDI ranges previously suggested to indicate different degrees of algal P deficiency were found to describe P deficiency adequately in Lake Erken.