Flood-driven increases in phytoplankton biomass and cyanobacteria abundance in the western Atchafalaya-Vermilion Bay System, Louisiana

The 2019 Mississippi River flood impacted estuaries and coastal economies across the Gulf Coast, USA. This flood resulted in blooms of toxic cyanobacteria in eastern Louisiana and impacted fisheries region wide. The Atchafalaya-Vermilion Bay System (AVBS) is located along Louisiana’s central coast and receives nearly one-third of the Mississippi River’s flow through the Atchafalaya River distributary, in addition to localized freshwater inputs. To understand how this extreme flood affected the AVBS, we compared environmental, phytoplankton biomass, and community composition data collected during and after the 2019 flood to historical data from previous years (2016, 2017). Compared to historical data, 2019 was characterized by higher concentrations of silica and nitrate and lower salinities. Following the flood period, increased phytoplankton biomass and relative abundance of potential toxin-producing cyanobacteria were observed at sites farthest from the Atchafalaya River outlet. We found no differences in community composition at sites closer to the river mouth due to hydrologically induced disturbance. Particulate and dissolved microcystins were detected throughout the system. These results suggest that, even in a system that receives regular freshwater input, the extreme 2019 flood resulted in significant alteration of phytoplankton biomass and community composition in the western AVBS.