ASSESSMENT OF PHOSPHORUS AVAILABILITY FOR NATURAL PHYTOPLANKTON POPULATIONS FROM A HYPEREUTROPHIC LAKE

Phosphorus availability for natural phytoplankton populations from a p rimarily nitrogen (N) limited hypereutrophic lake were evaluated using nutrient enrichment bioassays in combination with determinations of b iomass (chlorophyll a), alkaline phosphatase activity (APA) and two fo rms of hot water extractable phosphorus. Chlorophyll a responses indic ated initial weak N limitation followed by N and P co-limitation in Ap ril and phosphorus (P) limitation in August. The P demand of the plank ton was high as indicated by the removal of 80 mug L-1 inorganic P wit hin 2 h following addition. Ambient values of specific APA indicated t hat the plankton were not P limited but became so during the culture. After the apparent removal of all the external soluble reactive P (SRP ), internal supplies of P were depleted during growth. Different phase s of P storage products, as determined by hot water extractable total soluble P and hot water extractable SR.P, were utilized during growth, which indicated the ability of the plankton to utilize easily hydroly zed and more complex forms of stored P. Nutrient enrichment bioassays and hot water extractable P were the best indicators of P availability .