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.