Phytoplankton and nutrient dynamics in a tidally dominated eutrophic estuary: daily variability and controls on bloom formation

To better understand nutrient dynamics and factors that promote the initiation of algal blooms, the Lafayette River, a tidal subestuary of Chesapeake Bay that experiences seasonal algal blooms, was sampled daily for a period of 54 d in the fall of 2005. Three phytoplankton blooms (chl a concentrations exceeding twice the average of monthly measurements from 2000 to 2009) occurred during this period: a mixed bloom of Akashiwo sanguinea and Gymnodinium sp., a mono specific Skeletonema costatum bloom, and a monospecific Gymnodinium sp. bloom. Over the sampling period, nutrient concentrations increased following precipitation events and were elevated between bloom periods but low during blooms. All measured forms of nitrogen (N) were positively correlated with dinoflagellate abundance with a lag time of 3 to 5 d, suggesting a possible triggering effect, although not by any single form of N. Concentrations of NO2- reached 10 mu M between September and October, indicative of incomplete nitrification. Over a 24 h period, nutrient concentrations and chl a biomass varied by an order of magnitude (0.1 to 1 mu M N and 4.5 to 45 mu g chl a l(-1), respectively) and were strongly linked to the tidal phase. In the highly eutrophic Lafayette River, when nutrient concentrations are high, phytoplankton blooms appear to be controlled by spring-neap tidal modulation and wind-driven mixing; however, picoplankton abundance does not appear to be linked to the spring-neap tidal cycle.