Effects of nutrient input on phytoplankton productivity and community structure in the Grand Bay estuary in Mississippi

The estuarine system at Grand Bay National Estuarine Research Reserve in Mississippi is a near pristine wetland home to a diversity of flora and fauna. While seasonal fluctuations in water quality are well understood, less is known about the coupled dynamics of water quality and phytoplankton production. Light availability and nutrient levels are key factors regulating phytoplankton. Previous studies have revealed Grand Bay to primarily be limited by nitrogen rather than phosphorus or light. Since then, extended phosphate inputs from the neighboring Mississippi Phosphates fertilizer plant have occurred provoking the question: will the phosphate inputs affect the growth and structure of the phytoplankton communities? This study is investigating the effects of inputs of an array of nutrients (ammonium, nitrate, silicon, and phosphate) on phytoplankton growth, community structure, and production over an annual cycle. Phytoplankton production is being monitored by accumulation of biomass (chlorophyll a concentration) and C¹⁴ incorporation. We are also evaluating changes in the phytoplankton community composition using Flowcam imaging over the course of the incubation. Currently the summer months have shown nitrogen limitation as previously observed, with little difference between nitrate and ammonium additions. Flowcam images have revealed increases in ciliate abundance in all treatments. C¹⁴ experiments show significant decreases in efficiency for all treatments compared to the initial condition, however there is no significant variation among treatments. The results of this study will provide a strong foundation in understanding the nature of phytoplankton response to various nutrient inputs in Grand Bay.