Small-Scale Spatial Dynamics of Phytoplankton in Apalachicola Bay, Florida, a River-Dominated Estuary

This study describes the small-scale (10-1000 m) spatial distribution of phytoplankton in Apalachicola Bay, Florida, a shallow river-dominated estuary with residence times of days to weeks. Chlorophyll a was mapped using a boat equipped with a flow-through sensor array, Dataflow, every 2-4 weeks over a period of 16 months. Patches of high Chl a typically ranged in size from 50 – 5,000 m, and frequently exhibited steep concentration gradients (e.g. 20 µg L^-1 km^-1). The location and magnitude of these high Chl a patches varied over time. River discharge influenced the steepness of salinity gradients (R² = 0.3); when river discharge was below 400 m³s^-1, salinity gradients became more variable. Chl a correlated with salinity outside the river mouth, but was poorly correlated elsewhere in the bay. Chl a increased from the river mouth towards more seaward sites in the bay; the position of this gradient shifted ‘upstream’ along the transect when river discharge declined. These observations highlight the role of the river plume front as a site of phytoplankton biomass accumulation, and its influence on the spatial dynamics of phytoplankton biomass as it responds to changes in river flow.