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 (Popt) divided by the maximum phosphate uptake velocity (Vmax), 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, TAPAB; and Chl-specific maximal phosphate uptake velocity, Vbmax) 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, SPb) failed to show short-term variations in algal nutritional status. Correlations were found between PDI and TAPAB, PDI and TT, as well as PDI and turnover time of surplus P(TSP). PDI ranges previously suggested to indicate different degrees of algal P deficiency were found to describe P deficiency adequately in Lake Erken.