Water Quality and Toxic Cyanobacteria in Oligohaline Estuary Beaches During the Longest Mississippi River Basin Flood Event in 2019

Recent studies have shown that Lake Pontchartrain Estuary in Louisiana experiences frequent harmful cyanobacterial blooms (cyanoHABs). In 2019, the Bonnet Carre Spillway (BCS) that diverts Mississippi River water into the estuary opened twice in the same year for the first time in history to prevent flooding in New Orleans. Short-term water quality monitoring was conducted in shoreline areas with high public use for the presence of cyanoHABs and cyanotoxins to assess the public health risks. Field sampling methods and satellite imagery were used to determine water quality and quantify bloom intensity and toxicity across time and space. Long-term BCS opening created a fresh (salinity < 0.2) and nutrient-rich estuary that supported several cyanoHABs in warmer months during and after the second BCS closure. Cyanobacterial biomass ranged from 35 to 4972 & mu;g PC L-1, while toxin microcystin ranged from undetected to 8.41 & mu;g MC L-1. The highest biomass and toxin were detected on June 25 at the north shore, station LP8, Mandeville Beach, dominated by Microcystis and Dolichospermum species. CyanoHABs occurred mostly in the northern part of the estuary, where tributary discharge is also a strong influence. Some of these blooms exited the estuary and were transported to the Gulf of Mexico following passage through Lake Borgne and then Mississippi Sound. Modifications in the timing and duration of river diversion operations can create prolonged cyanobacterial blooms that can cause environmental and public health risks, especially in warmer months, and this may intensify due to a changing climate.

Automated summary

Recent studies have found frequent harmful cyanobacterial blooms in Lake Pontchartrain Estuary, Louisiana. The opening of the Bonnet Carre Spillway (BCS) twice in 2019 created a fresh and nutrient-rich estuary, supporting cyanoHABs during and after closure. Biomass ranged from 35 to 4972 μg PC L-1 and toxin microcystin ranged from undetected to 8.41 μg MC L-1. CyanoHABs mainly occurred in the northern part of the estuary, influenced by tributary discharge. Some blooms were transported to the Gulf of Mexico. Changes in river diversion operations may intensify cyanobacterial blooms and pose environmental and public health risks, especially due to climate change.