Establishment of baseline concentrations and elucidation of environmental processes controlling the bioavailability and bioaccumulation of mercury and other toxic metals in the Lake Maurepas Basin.

Mark W. Hester, Assaf A. Abdelghani, Kyle R. Piller, and Jonathan M. Willis

Project Overview

Many inhabitants of the basin and worldwide consumers of the seafood from our region are concerned about the prevalence of mercury, a well known and highly toxic contaminant, in the fish and other seafood they eat regularly.

The mercury we come in contact with comes both from water runoff and falls directly from the air in the form of atmospheric deposition. In oxygen-deprived (anoxic) systems like the Lake Pontchartrain Basin, we have an extra problem; the mercury deposited into anoxic waters quickly undergoes a chemical process called methylation, in which elemental mercury becomes the much more toxic methylated mercury.

Anoxic freshwater systems are so good at turning regular mercury into methylated mercury that they are known as global sources of the toxin. Once present in a system, it tends to be consumed by small animals, and once it is consumed it tends to stick inside the bodies of animals indefinitely. As larger fish eat smaller fish and invertebrates, the toxin builds to high levels, a process called bioaccumulation. Humans at the end of the food chain tend to eat the largest fish with the largest levels of methylated mercury, putting the most vulnerable parts of the population, such as infants and pregnant women, at risk.

This project seeks to quantify the concentration and bioaccumulation of regular and methylated mercury in the Lake Pontchartrain Basin wetlands. We hope to answer questions such as:

1) What are the current concentrations of methyl and total mercury in the Lake Maurepas wetland soils and interstitial water, and how do they vary seasonally? Although the concentration of either mercury or methyl mercury in the Lake Maurepas wetlands is currently unknown, methyl mercury concentration is anticipated to vary seasonally with the greatest concentrations in the summer due to microbial activity.

2) Do the dominant herbaceous plant species of the Lake Maurepas wetlands bioaccumulate mercury, and if so, in what portion of the plant does it tend to be located? It is anticipated that all of the plant species tested will bioaccumulate mercury to some extent and will tend to store mercury in root material.

3) How do soil conditions affect mercury cycling, and what happens if the conditions change (such as would happen in a freshwater diversion)? Anoxic soil conditions with moderate levels of available sulfate and carbon are expected to lead to maximum methyl mercury production, with increased levels of nitrate tending to result in reduced production.

4) How is the bioaccumulation of mercury by crawfish affected by mercury concentration, plant species, and soil conditions? Invertebrate mercury bioaccumulation is expected to be positively correlated with mercury concentrations in both ambient water and plant tissues and to a lesser extent soil mercury concentrations.