Effect of the Herbicide Atrazine on Phytoplankton, Water Quality, and Ecosystem Functions in Louisiana Estuaries

Abstract

Pesticides are used primarily for agricultural purposes in the US and while these chemicals provide many benefits, the inherent toxicity of the compounds pose a substantial risk to the environment. These chemicals may enter water bodies in areas with a high proportion of agricultural land use through surface run off, ground water discharge, and erosion, and negatively impact non-target aquatic organisms. As a result, Louisiana’s estuaries may be vulnerable to a variety of compounds, including the herbicide atrazine. Atrazine is used extensively throughout the Midwest and has been known to enter the Mississippi River through surface runoff and ground water discharge. The River transports the compound downstream to the delta, where it is discharged into Louisiana’s coastal estuaries. Due to the high amount of sugarcane production in the southeastern part of the state, atrazine also has the potential to enter these systems indirectly through agricultural runoff, and adversely affect native aquatic organisms. Because it is a photosynthesis inhibitor, phytoplankton communities may be especially susceptible to atrazine exposure. The phytoplankton stress response in these systems may be critical because phytoplankton form the base of the food web and are essential to the production of the entire ecosystem. The purpose of this study was to determine the extent of atrazine contamination in Louisiana’s estuaries, and its effect on local phytoplankton stress response. Field samples were taken under low and high flow and nutrient conditions from Breton Sound and Barataria Estuary. The results showed that atrazine was consistently present in these systems at low levels. Local phytoplankton from Barataria Estuary were also grown in microcosm and exposed to an atrazine dilution series under low and high nutrient conditions to determine the phytoplankton stress response. The treatment groups that received 5 ppb and 50 ppb atrazine treatments under high nutrient conditions exhibited an extended lag phase and entered into the exponential growth phase several days after the control groups. Overall, communities in nutrient enriched treatment groups exhibited higher growth response, oxygen production, and were healthier than non-enriched groups, indicating that atrazine exposure may induce a stress response in phytoplankton communities under low nutrient conditions.