Recently, mercury contamination has made major headlines in local, national, and international news. Most articles deal with mercury poisoning from human consumption of fish. Fish bioaccumulate mercury as methylmercury from the foods they consume; therefore, larger and longer-lived fish are highest in methylmercury. Food webs in lake systems are relatively well understood and methylmercury can be traced from the primary producers up to top predators, usually game fish or fish-eating birds. The lack of migration in closed lake systems makes determining the food web easier; however movement of biota in and out of riverine systems makes food webs more difficult to map. Diets of stream biota can change based on water temperature, local currents, light penetration, water velocity, substrate, and local nutrient availability, among many other factors.
Previous studies have shown that MeHg biomagnifies as it moves up aquatic food chains to top predators (U.S. Environmental Protection Agency, 2001). Most of these studies have been conducted in lakes, reservoirs, and wetlands and have focused on top predator or game fish, the water column, and bed sediment (Krabbenhoft, 1996; Krabbenhoft et al., 1998). Few detailed studies of mercury bioaccumulation have been conducted in streams and rivers, despite their importance for recreational and subsistence fishing.
Based on data from lakes, mercury accumulation in phytoplankton or suspended algae, the base of the aquatic food web in lakes and reservoirs, is the single largest step in bioaccumulation, with bio-concentration factors (bioaccumulation factors hereon) typically in the 4 to 6 range between water and phytoplankton (Watras and Bloom, 1992). Phytoplankton blooms in lakes resulted in reduced bioaccumulation in algal-rich eutrophic lake systems due to decreases in the concentration of mercury per algal cell (Pickhardt et al., 2002). Periphyton or attached benthic algae are important in the transfer of mercury in the wetland environment of the Everglades (Cleckner et al., 1998; Cleckner et al., 1999).
In rivers, periphyton are primary producers and the base of the food web. Although periphyton are an important aspect in bioaccumulation and trophic transfer of MeHg to organisms higher on the food chain, there has been little research to date on bioaccumulation of MeHg in natural periphyton communities in rivers. The emphasis of this study is on the natural periphytic communities.
The United States Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program began a study in 2003 to examine mercury in precipitation, stream water, riverbed sediment, sediment porewater, predator fish, forage fish, benthic macroinvertebrates, and periphyton (the focus of this paper), methylation potential in river sediment, and dissolved organic carbon in water (Brigham et al., 2003). This periphyton study was a collaborative study between USGS and United States Environmental Protection Agency (USEPA) to address an important aspect of bioaccumulation, and complement the other aspects of the study. This paper focuses on mercury levels in periphyton and the physicochemical environment from which periphyton are sampled.
This study uses information from three active mercury study areas of the NAWQA/Toxics study: Western Lake Michigan Drainages (WMIC), Willamette Basin (WILL), and Georgia-Florida Coastal Plain (GAFL). Two to three rivers in each of the three basins were chosen for the NAWQA/Toxics study to represent one urban site and one or two reference/non-cultivated (low and high percent wetland) sites. Periphyton was collected twice in one year (spring and fall) at each of the eight streams.
Periphyton samples were collected using current NAWQA Protocols (Moulton et al., 2002) and ultra-clean techniques . Two composite periphyton samples, (1) periphyton on rocks or woody snags and (2) periphyton on sediment, were collected from each of the eight NAWQA sites during each sampling event. The periphyton sampling was closely coordinated with sampling of invertebrates, forage fish, water, and sediment for mercury at each site both in time and space.
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