Nutrient status of phytoplankton across a trophic gradient in Lake Erie: evidence from new fluorescence methods

Abstract

Variable fluorescence of chlorophyll a was measured by pulse amplitude modulated fluorometry to determine its relationship with measures of nutrient status and phytoplankton community structure in Lake Erie. In 2005, nitrogen (N) deficiency was most common in May, phosphorus (P) deficiency was most common in June, and neither were common in September. The maximum quantum yield (Fv/Fm) measured by pulse amplitude modulated fluorometry was lower in May and June than in September. The observed range of Fv/Fmincluded many values lower than previously reported in the lower Laurentian Great Lakes, while Fv/Fmvalues showed strong inverse correlations with indicators of N and (or) P deficiency. Community structure was also associated with nutrient status. Cyanobacteria were common at sites displaying N deficiency, while flagellates dominated P-deficient sites in all basins. N deficiency is surprising in a lake with generally high nitrate levels, but was supported by N debt, particulate C:N ratios and depressed Fv/Fm. Further work to characterize and compare results obtained with different variable fluorescence methodologies is desirable, but the present results support the belief that Fv/Fmcan characterize nutrient deficiency of phytoplankton community in this large lake.