Abstract
Freshwater discharge and related physical factors control the temporal and spatial distribution of phytoplankton in estuaries. Variability in abundance of these primary producers, in turn, can affect the distribution of fish and other higher trophic level consumers and impact eutrophication. The goal of this project was to determine the specific physical controls of phytoplankton abundance in a subtropical estuary, the Caloosahatchee River Estuary (CRE), in southwest Florida, USA. To accomplish this goal, we sampled and measured vertical profiles of Chlorophyll a (Chl a) concentration, and physical and geochemical parameters at night along the CRE monthly for 16 months. The distribution of phytoplankton concentrations was controlled by convergences of subtidal baroclinic currents, both in classical ETM formations, and in fronts generated by bathymetric constrictions of the CRE. These processes focused and trapped the phytoplankton, and potentially created extended residence times locally, allowing blooms to generate. Seasonal variations in freshwater flow mediated the location where fronts and phytoplankton concentrations were strongest. Dry season peak concentrations were found near a bathymetric constriction upstream. Wet season peak concentrations were found near a bathymetric constriction downstream, except during peak discharge months when most phytoplankton and suspended sediments were flushed completely out of the estuary. In the CRE, phytoplankton trapping occurred around horizontal constrictions that created fronts, however this mechanism for phytoplankton trapping may be also manifest in estuaries with vertical constrictions such as sills and shoals. This physical mechanism for trapping primary producers should be considered as a potential underlying factor when studying the spatial and temporal distribution of higher trophic levels and eutrophication in estuaries.
