Mercury in Caddo Lake
Caddo Lake has two features that cause it to be highly contaminated with mercury:
1) It is located “deep in the piney woods” of East Texas and therefore in an area with high mercury deposition from the atmosphere. As conifers, pine trees increase mercury deposition under their forest canopy.
2) The western end of Caddo Lake is a wetland that is choked with vegetation. Wetlands are conducive to the creation of methylmercury, the toxic form of mercury that enters aquatic food chains.
Mercury contamination of fish
To explore the effect of habitat on mercury in fish, largemouth bass were collected in the open water and forested wetland habitat. Largemouth bass were selected for the study because they are a popular gamefish and are top predators in lakes with high levels of mercury (go to mercury biomagnification for more information about how mercury increases up the food chain).
Mercury in largemouth bass increased with length of the fish and mercury concentrations in large largemouth bass were 10-times higher than in small bass. Large largemouth bass in the forested wetland have concentrations of mercury that are much higher than in the open water.
Mercury-contaminated spiders and songbirds
Mercury doesn’t just stay in the lake, it can be transported out of the lake when insects emerge and transport the mercury to terrestrial consumers. Therefore mercury contamination is not just confined to the aquatic food chain, but also involves the surrounding terrestrial food chain including spiders and birds.
Long-jawed orb weaver spiders are predators that can become contaminated with high concentrations of methylmercury by feeding on small-bodied emergent aquatic insects. The spiders spin webs along the shoreline of lakes to capture emergent insects. Several investigators have proposed using the long-jawed orb weaver spider as an indicator species for monitoring sites polluted with biomagnifying contaminants.
Long-jawed orb weavers were collected in and along the shorelines of Caddo Lake. Methylmercury concentrations in spiders were significantly different in river, wetland, and open-water habitats of Caddo Lake. This study supports the hypothesis that shoreline spiders such as the long-jawed orb weavers may have potential as an indicator species of mercury contamination of aquatic systems.
Mercury concentrations in spiders were high enough to pose a risk to nestling songbirds that feed on spiders and had the highest risk in the river and wetland habitats. This mercury threat to songbirds may not be unique to Caddo Lake and may be found in other mercury-contaminated lakes with forested wetlands that are commonly found throughout the southeastern United States.
Sources of Information:
- Chumchal, M.M., R.W. Drenner, B. Fry, K.D. Hambright, and L.W. Newland. 2008. Habitat-specific differences in mercury concentrations in a top predator from a shallow lake. Transactions of the American Fisheries Society 137:195–208.
- Gann, G., C. Powell, M.M. Chumchal, R. Drenner. 2015. Mercury-contaminated terrestrial spiders pose a potential risk to songbirds at Caddo Lake, Texas/Louisiana, USA. Environmental Toxicology and Chemistry, 34:303-306.
For more information about our mercury research, go to our Aquatic Ecology Lab website.