Phytoplankton Populations in Lake Pontchartrain

Learn about some of the environmental consequences of introducing large quantities of nutrient-rich freshwater into a brackish system and how some of the organisms there react. Changes like this can ripple through a food web, so to best understand the implications, we will focus on some of the smallest residents in Lake Pontchartrain – phytoplankton.

2019 was a historic and devastating year along the Mississippi River. Baton Rouge spent a record 211 days at flood stage from January 6 – August 4; the former record dates back to the Great Flood of 1927, when the river was at flood stage for 135 days. Levees were breached on the Arkansas, Missouri and upper Mississippi rivers. By summer, eleven states requested federal disaster assistance. The economic impact of the floods and recovery is several billion dollars.

New Orleans has a contradictory relationship with the Mississippi River: it relies on the river for navigation, transportation, fisheries and even its drinking water. However, too much water in the Mississippi River can and has devastated the city. Therefore, river levels are monitored quite closely and when the river approaches flood stage (17 feet as set by the US Army Corps of Engineers), they open a river diversion structure called the Bonnet Carré Spillway. Located about 25 miles upstream from New Orleans, the Bonnet Carré Spillway diverts some of the water from the swollen Mississippi River into nearby Lake Pontchartrain.

This diversion of river water is bound to have environmental consequences as the two systems are different. The Mississippi River is carrying freshwater from 31 states and 2 Canadian provinces. Many of the states that it travels through support large amounts of agriculture, so the river ends up carrying nutrients like nitrogen and phosphorus (from fertilizer applications) along with sediment. It is a freshwater system. Lake Pontchartrain has a range of habitats from freshwater to brackish systems. It is a 630 square mile estuary that connects to the Gulf of Mexico at its eastern end and hosts a number of recreational and commercial fisheries.

When conditions are right the populations of phytoplankton can explode. Population explosions, also called blooms, can be harmful to other animals, humans and the environment. These blooms are referred to as Harmful Algal Blooms (HABs). A bloom in the cyanobacteria population can more specifically be called a cyanoHAB.