Comparison of phytoplankton biomass and community composition in three Texas estuaries differing in freshwater inflow regime

Abstract

Phytoplankton are at the base of the marine food web and are sensitive indicators of environmental change, such as changes to freshwater inflow in estuaries. Because freshwater inflows to Texas estuaries are projected to decrease in the coming decades as a result of increasing human freshwater demands and climate change, it is critical to understand ecosystemlevel responses to freshwater inflow variability. This study examined phytoplankton community composition and biomass, along with relationships to environmental variables in three Texas estuaries with differing freshwater inflow regimes. The estuaries were San Antonio Bay (highest inflow levels), Nueces-Corpus Christi Bay (intermediate inflow levels), and Baffin Bay (lowest inflow levels). The goal was to understand the impacts of freshwater inflow variability and magnitude on phytoplankton dynamics. Baffin Bay had the highest phytoplankton biovolume, followed by NC and SA. Higher frequency and magnitude inflows in SA corresponded to overall lower biovolume and shifts of biovolume maxima downstream, while infrequent inflows in BB were followed by a sustained diatom bloom. Inflows to NC affected only the upper estuary, while the lower estuary appeared to be more influenced by ocean exchange. Large, fastergrowing taxa, such as Rhizosolenia, were observed in greatest abundances in Baffin Bay, while slower-growing dinoflagellates dominated in NC and SA. Findings of lower biovolume and higher proportion of slow growing taxa in the high inflow estuary, SA, are contrary to our understanding based on the available literature and underscore the importance of understanding system-specific drivers for effective coastal management.