Mine‐derived Mercury: Effects on Lower Trophic Species in Clear Lake, California

Abstract

Abstract. Considerable ecological research on mercury (Hg) has focused on higher trophic level species (e.g., fishes and birds), but less on lower trophic species. Clear Lake, site of the abandoned Sulphur Bank Mercury Mine, provides a unique opportunity to study a system influenced by mine-derived Hg. An exponentially decreasing gradient of total Hg (TotHg) away from the mine allowed us to evaluate Hg bioaccumulation in planktonic and benthic invertebrates and evaluate population-and community-level parameters that might be influenced by Hg. Studies from 1992-1998 demonstrated that TotHg in lower trophic species typically decreased exponentially away from the mine, similar to trends observed in water and sediments. However, a significant amount of invertebrate TotHg (;60% for sediment-dwelling chironomid insect larvae) likely derives from Hg-laden particles in their guts. Spatially, wholebody methylmercury (MeHg) did not typically exhibit a significant decrease with increasing distance from the mine. Temporally, TotHg concentrations in plankton and chironomids did not exhibit any short-term (seasonal or annual) or long-term (multiyear) trends. Methylmercury, however, was elevated during late summer/fall in both plankton and chironomids, but it exhibited no long-term increase or decrease during this study. Although data from a 50-yr monitoring program for benthic chaoborid and chironomid larvae documented significant population fluctuations, they did not demonstrate population-level trends with respect to Hg concentrations. Littoral invertebrates also exhibited no detectable population-or communitylevel trends associated with the steep Hg gradient. Although sediment TotHg concentrations (1-1200 mg/kg dry mass) exceed sediment quality guidelines by up to 7000 times, it is notable that no population-or community-level effects were detected for benthic and planktonic taxa. In comparison with other sites worldwide, Clear Lake's lower trophic species typically have significantly higher TotHg concentrations, but comparable or lower MeHg concentrations, which may be responsible for the discrepancy between highly elevated TotHg concentrations and the general lack of observed population-or community-level effects. These data suggest that MeHg, as well as TotHg, should be used when establishing sediment quality guidelines. In addition, site-specific criteria should be established using the observed relationship between MeHg and observed ecological responses.

Changes in Fish Diets and Food Web Mercury Bioaccumulation Induced by an Invasive Planktivorous Fish

“…Although our results clearly show a strong relationship between threadfin shad abundance, benthic-pelagic energy reliance, and Hg bioaccumulation, it is notable that benthic chironomids actually had lower MeHg concentrations (0.044 6 0.011 ppm DM) than zooplankton (0.059 6 0.008 ppm DM) analyzed from the same times and locations (Suchanek et al 2008c). It is intriguing that silverside and bass Hg concentrations increased in light of their dietary MeHg concentrations slightly decreasing.…”

“…Moreover, TotHg concentrations in benthic prey from the Oaks Arm are one to two orders of magnitude greater than those in zooplankton (Suchanek et al 2008c), indicating much higher IoHg burdens. The assimilation efficiency of MeHg from prey to predator ranges from 56% to 95%, whereas that of IoHg is ;10-27% (Wang and Wong 2003).…”

“…It is intriguing that silverside and bass Hg concentrations increased in light of their dietary MeHg concentrations slightly decreasing. This discrepancy is likely the result of some combination of increased Hg exposure from sediment consumption and increased inorganic Hg (IoHg) exposure and assimilation associated with chironomid prey (Suchanek et al 2008c). Other potential causal mechanisms such as reduced growth rate as well as cascading changes in zooplankton community structure and primary productivity do not appear to have occurred to an extent large enough to have driven these changes (C. A. Eagles-Smith, unpublished data).…”

“…Caloric content of chironomids and zooplankton are likely not different enough (Wissing and Hasler 1971) to result in sufficiently drastic changes in fish growth rates such that Hg concentrations would fluctuate to the extent detected here. In addition, zoobenthos biomass is sufficiently high that it is unlikely there was a significant change in energy expended during foraging (Suchanek et al 2008c).…”

Biomonitoring of lake sediments using benthic macroinvertebrates

“…Similar results were obtained for lakes. For example, benthic communities were analyzed in Clear Lake, California, USA [62], impaired by Hg. A decreasing gradient of total Hg in sediments from the mine hotspot pollution point allowed evaluation of the impact on population-level and community-level parameters.…”