Impacts of the 2019 Mississippi River Flooding on Estuarine Phytoplankton Communities

Freshening of Louisiana estuaries is becoming an increasingly important topic as the state aims to combat coastal land loss through proposed sediment diversions and hydrologic restoration activities. While these activities are expected to benefit coastal Louisiana, the increased delivery of freshwater is also likely to have significant impacts on estuarine ecosystems. Phytoplankton communities, in particular, are expected to shift as nutrient, turbidity, and salinity regimes change with increased riverine input. The historic 2019 Mississippi River flood event has been the longest on record (123 days) and offers an opportunity to study this phenomenon. This flood event has had measurable impacts on numerous Louisiana fisheries, especially in the Teche-Vermilion and Atchafalaya Basins of south Louisiana. These basins are heavily influenced by the Atchafalaya River, which carries approximately 30% of the Mississippi River’s discharge into a series of estuarine bays. Using data collected from non-flood years (2016–2017), we aim to examine the impacts this flood has had on phytoplankton communities in these bays during the summer and fall of 2019. Preliminary data suggest that mean salinity at the nearshore sites is lower during the flood year (~1.2 ppt) compared to non-flood years (~3.4 ppt). Additionally, overall phytoplankton biomass is significantly lower in 2019 (~8.7 μg/l) compared non-flood years (~16.2 μg/l). Data will continue to be collected through fall 2019 to monitor the longer-term impact of the flood waters. Samples will also be analyzed for phytoplankton community composition and particulate microcystin, a hepatotoxin previously detected in low concentration in these bays. This toxin is becoming more of an ecosystem and human health issue in estuaries throughout the Gulf of Mexico. This research will provide valuable insight for future management of the ecologically and economically important estuaries of coastal Louisiana.