Influence of Colorado River discharge variability on phytoplankton communities in Matagorda Bay, Texas

Phytoplankton are important primary producers in estuaries and are also indicators of environmental changes, such as freshwater inflow or nutrient loadings. Some phytoplankton are also harmful algal bloom (HAB) species, negatively impacting estuaries with toxin production or by generating hypoxia during bloom termination. Blooms of the HAB Dinophysis sp. have negatively affected the Matagorda Bay (Texas) ecosystem on multiple occasions since 2008. Estuaries of the Texas coast, such as Matagorda Bay, are vulnerable to long-term decreases in freshwater inflow due to increasing human freshwater needs as well as climate change. It is critical to understand how phytoplankton communities, including HABs, respond to freshwater inflow variability to estuaries in order to project how future inflow changes may affect estuaries. Over a 24-month period, a suite of environmental parameters along with phytoplankton community composition and biovolume was measured to determine the influence of freshwater inflow from the Colorado River on these parameters in Matagorda Bay. Spatiotemporal distribution of nutrients, chlorophyll and phytoplankton were influenced by riverine inflow. In particular, all of these factors decreased moving away from the river mouth, indicating a strong influence of the river on their distributions. Dinophysis sp. was observed in January, February and April 2020 during a period of cool temperatures and higher salinities, and its biovolume was positively correlated with nitrate+nitrite and the ratio of silicate to dissolved inorganic nitrogen. Other non-HAB phytoplankton taxa had distinct responses to environmental variability. Future work should consider HAB taxa on an individual basis, as well as consider more frequent sampling and toxin analysis. The unique environment, as well as phytoplankton communities and responses of Matagorda Bay underscore the necessity of investigating changes over time on an individual estuary basis, and for potential blooms, on a species-specific basis.