NUTRIENT LIMITATION IN A HYPEREUTROPHIC FLORIDA LAKE

Nutrient enrichment bioassays (NB) with natural phytoplankton communit ies were used in Lake Apopka, a hypereutrophic lake in Florida, to det ermine growth-limiting nutrients in 20 monthly experiments conducted f rom December 1989 through June 1991. A 2 x 2 factorial design with nit rogen (N) and phosphorus (P) was employed in experiments conducted und er laboratory conditions. In 19 of the 20 experiments, N was either th e primary limiting nutrient or co-limiting with P. P was the primary l imiting nutrient in only one experiment. We conclude that N is the pri mary limiting nutrient because the water of Lake Apopka generally cont ains large supplies of P in the form of polyphosphate that can be util ized for phytoplankton growth when the water is enriched with N. Becau se the lake has high standing crops of phytoplankton (average chloroph yll-a of 100 mug/L) and total P concentrations (200 mug P/L), it also must be concluded that N limitation of phytoplankton production in Lak e Apopka is the result of excessive P loading to the lake.