Seasonal dynamics of micro-, nano-, and picoplankton in the tidal freshwater Potomac River in and around Gunston Cove (Virginia)

To understand the seasonal variation in phytoplankton size classes and the relative importance of microplankton, nannoplankton and picoplankton to total phytoplankton productivity, 14C photosynthetic rates and chl a of size-fractionated samples were investigated throughout the year 1998 in the tidal freshwater Potomac river and its embayment Gunston Cove. Size-fractionated chlorophyll a gradually increased in early spring and reached maximum in summer, generally following seasonality. Maximum microplankton surface chl a value in the cove was around 80 μg/L, while depth-integrated chl a value peaked at 95 μg/L. Nannoplankton surface chl a was highest at around 100 μg/L, while depth-integrated chl a value peaked at 65 μg/L in the Cove. Picoplankton surface and depth-integrated chl a remained below 15 μg/L. Lower chl a values were observed in the Potomac River. Generally the chlorophyll a peak occurred during early and mid-summer. Overall, nannoplankton were dominant in spring, micro- and nannoplankton were co-dominant in summer and picoplankton of minor importance all year.; Microplankton and nannoplankton shared productivity dominance in Gunston Cove and Potomac River, while the picoplankton contribution was minor to negligible. Size fractionated photosynthetic rate per unit chlorophyll (P B max) increased gradually in early spring and reached their maximum during August and September. However nannoplankton photosynthetic rate per unit chlorophyll values also exhibited a spring maximum.; Phytoplankton maximum density was observed during summer with the highest numbers at Gunston Cove. Cyanobacteria cell densities were elevated during spring and summer in Gunston Cove and Potomac River, while diatoms were important during spring and fall. Chlorophytes and cryptophytes also contributed to algal density. Biovolume graphs depicted irregular crests and troughs in early spring, suggesting less stability at both stations. In Gunston Cove diatoms were the major contributor to biovolume during spring and early summer while cyanobacteria dominated biovolume during late summer. Diatoms dominated river biovolume throughout the year except during summer when cyanobacteria were dominated.