Seasonal variation of phytoplankton nutrient limitation in Lake Erie

Phosphorus (P) has been identified as the primary nutrient limiting phytoplankton biomass in them Laurentian Great Lakes, and thus phytoplankton biomass varies as a function of P loading. While management efforts have reduced point sources of P, a recent rise in non-point source loadings arid the introduction of dreissenid mussels to the Great Lakes are factors suspected to have direct and indirect impacts on nutrient cycling. We re-evaluated nutrient limitation of phytoplankton in the central basin of Lake Erie over spatial (i.e. 3 offshore stations) and temporal (i.e. monthly from June to October) scales. The nutrient limitation of the phytoplankton was evaluated using biomass response measurements to nutrient enrichment bioassays in a complete factorial of P, nitrogen (N), and silicon (Si). Nutrient additions yielded classic growth curves after 7 to 12 d of incubation. Treatments with added P had higher final biomass yields than treatments without P at all stations and during all months. During spring overturn, P+ Si yielded higher phytoplankton biomass than did P additions without Si at 2 of the 3 stations. During the late stratification to autumn overturn (August to October), P+N promoted higher phytoplankton biomass than did P additions without N; this was true at all stations. A quantitative assessment of the bioassays indicated that 63 to 94 % of the biomass yield during spring overturn could be attributed to Si, while 48 to 68 % of the biomass yield during late stratification and autumn overturn could be attributed to N. Water chemistry data collected in 2002 and 2003 predicted similar seasonal trends in nutrient limitation. These results suggest that seasonal variation in phytoplankton nutrient limitation is a probable factor in predicting changes in phytoplankton biomass and taxonomic composition in the central basin of Lake Erie.