Environmental forcing of phytoplankton floral composition, biomass, and primary productivity in Chesapeake Bay, USA

We examined environmental forcing of floral composition, biomass as chlorophyll a (Chl a), and primary productivity (PP) of phytoplankton in Chesapeake Bay for six years (1995e2000). Our goal was to describe regional, seasonal, and interannual variability in the context of environmental forcing of phytoplankton dynamics. Floral composition was reconstructed from pigment concentrations obtained using high performance liquid chromatography (HPLC), and expressed as the proportion of Chl a comprised by major taxonomic groups of phytoplankton (f_chl-ataxa). PP was measured using 14C methods in simulated in situ incubations. Diatoms dominated the annual cycle. Seasonality explained most of the variance of the four major groups present in the Bay (diatoms, cryptophytes, dinoflagellates, and cyanobacteria). A significant, positive influence of Susquehanna River flow (SRF) on f_chl-adiatom in spring and summer, specific to Bay region, emerged from an analysis of interannual deviations from long-term averages (LTAs) based on principal components (PCA) and multiple linear regression (MLR) analyses. Optimal water-column photosynthesis, Popt B (mg C mg Chl a 1 h 1), was negatively correlated with f_chl-adiatom in spring, and positively correlated with f_chl-adiatom in summer. These patterns were consistent with the co-occurrence of high f_chl-adiatom and high average cell size in spring, and high f_chl-adiatom and alleviation of nutrient limitation of Popt B in summer. We conclude that the LTA seasonal floral composition of Chesapeake Bay is disrupted by the influence of interannual variability of SRF, particularly in spring and summer. Floral composition, Chl a, and PP respond predictably to environmental forcing associated with variability of SRF and attendant nutrient loading, affecting the function and fate of phytoplankton not conveyed by bulk measures of biomass and productivity alone.