Influence of freshwater flow, ocean exchange, and seasonal cycles on phytoplankton – nutrient dynamics in a temporarily open estuary

Abstract

Mecox Bay is a shallow, closed embayment, located on the south shore of Long Island, NY, which has an inlet that is periodically opened by natural and anthropogenic processes, allowing for tidal exchange with the coastal Atlantic Ocean. The aim of this study was to characterize the phytoplankton community and water chemistry of Mecox Bay, while assessing the impact of inlet openings and freshwater flow on these characteristics. Results indicated that groundwater and tributaries entering Mecox were an important source of nitrogen (N) to the bay, being enriched in dissolved inorganic nitrogen (DIN = 140 ± 20 μM). Inlet openings yielded increased salinity and chlorophyll a levels, but decreased depths and diatom densities in Mecox Bay. Other parameters, such as nutrient concentrations (N, P), pico- and nanophytoplankton densities, and the nutrient limitation of phytoplankton communities were more strongly influenced by seasonal changes associated with temperature. Dissolved inorganic nitrogen-to-phosphorus ratios decreased from above 100 during winter to below one during summer, as concentrations of DIN and freshwater flow rates in Mecox Bay declined and DIP levels concurrently increased. In concert with changes in the nutrient regime, field experiments indicated that growth rates of the phytoplankton community in Mecox Bay shifted from P-limited during winter and spring to N-limited during summer and fall. Finally, the abundance of phytoplankton (mean annual chlorophyll a concentration ∼10 μg L −1) and the temporarily closed nature of Mecox Bay may both contribute toward enhanced secondary productivity of resident shellfish populations.