Effect of Mississippi River discharge plume on temporal and spatial variability of toxic cyanobacteria in an oligohaline estuary

Globally, estuaries are undergoing profound hydrological alterations due to changes in climate and flood management coincident with the increasing frequency of harmful algal blooms (HABs). In coastal Louisiana, the introduction of nutrient-rich Mississippi River water into Lake Pontchartrain Estuary supports the proliferation of toxic cyanobacterial blooms (CyanoHABs). In 2019, over 28 km 3 of Mississippi River water was diverted into the estuary (volume of 6 km 3 ) to prevent flooding of downstream communities. This study characterized the impact of this river input containing 38,000 + metric tons of bioavailable N and 2,300 + metric tons of bioavailable P on monthly biological measures of the water column including biomass of pelagic cyanobacteria and associated toxin production. The estuary was freshened for months due to the river pulses, which initially limited phytoplankton response due to high total suspended solids. There was a cyanobacterial bloom once the river pulse stopped, related to increased water residence time, clarity, and nutrient availability. Particulate toxin concentration correlated to cyanobacterial biomass but was non-linear. Once bioavailable N decreased, nitrogen-fixing Dolichospermum spp. became dominant. This research on CyanoHABs in Lake Pontchartrain Estuary can help provide a basis to predict the response of estuaries to large nutrient-loading events.