Spatiotemporal Trends in Fish Mercury From a Mine‐dominated Ecosystem: Clear Lake, California

Abstract

Abstract. Clear Lake, California, USA, receives acid mine drainage and mercury (Hg) from the Sulphur Bank Mercury Mine, a U.S. Environmental Protection Agency (U.S. EPA) Superfund Site that was active intermittently from 1873 to 1957 and partially remediated in 1992. Mercury concentrations were analyzed primarily in four species of Clear Lake fishes: inland silversides (Menidia beryllina, planktivore), common carp (Cyprinus carpio, benthic scavenger/omnivore), channel catfish (Ictalurus punctatus, benthic omnivorous predator), and largemouth bass (Micropterus salmoides, piscivorous top predator). These data represent one of the largest fish Hg data sets for a single site, especially in California.Spatially, total Hg (TotHg) in silversides and bass declined with distance from the mine, indicating that the mine site represents a point source for Hg loading to Clear Lake. Temporally, fish Hg has not declined significantly over 12 years since mine site remediation. Mercury concentrations were variable throughout the study period, with no monotonic trends of increase or decrease, except those correlated with boom and bust cycles of an introduced fish, threadfin shad (Dorosoma petenense). However, stochastic events such as storms also influence juvenile largemouth bass Hg as evidenced during an acid mine drainage overflow event in 1995.Compared to other sites regionally and nationally, most fish in Clear Lake exhibit Hg concentrations similar to other Hg-contaminated sites, up to ;2.0 mg/kg wet mass (WM) TotHg in largemouth bass. However, even these elevated concentrations are less than would be anticipated from such high inorganic Hg loading to the lake. Mercury in some Clear Lake largemouth bass exceeded all human health fish consumption guidelines established over the past 25 years by the U.S. Food and Drug Administration (1.0 mg/kg WM), the National Academy of Sciences (0.5 mg/kg WM), and the U.S. EPA (0.3 mg/kg WM). Mercury in higher trophic level fishes exceeds ecotoxicological risk assessment estimates for concentrations that would be safe for wildlife, specifically the nonlisted Common Merganser and the recently delisted Bald Eagle.Fish populations of 11 out of 18 species surveyed exhibited a significant decrease in abundance with increasing proximity to the mine; this decrease is correlated with increasing water and sediment Hg. These trends may be related to Hg or other lake-wide gradients such as distribution of submerged aquatic vegetation.

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

“…The rapid elimination of IoHg relative to MeHg (three to five times faster) is also consistent with the rapid reduction of concentrations upon resumption of zooplankton foraging (Trudel andRasmussen 1997, Wang andWong 2003). Limited data for silversides from Clear Lake indicate that the percentage of TotHg as MeHg is highly variable (20-60%) and the proportion of MeHg is relatively low in comparison to larger fish from the lake (Suchanek et al 2008d ).…”

“…1), an abandoned Hg mine that was responsible for ;100 Mg of Hg being deposited into the sediments between 1872 and 1957 (Suchanek et al 2003. Although the relative proportion of MeHg to total (primarily inorganic) Hg (TotHg) is low compared to other contaminated sites (Suchanek et al 2008a) in the lake are still commonly found with muscle tissue concentrations of 1.0-1.5 lg/g (ppm) wet mass (WM), and a human consumption advisory based on Hg has been in effect since 1986 (Suchanek et al 2008d, e).…”

“…Muscle tissue was chosen for TotHg analysis because it has been identified as the primary repository of MeHg in fishes and serves as a strong proxy for whole-body concentrations (Goldstein et al 1996, Suchanek et al 2008d ). All sampled fish were weighed before and after drying to determine moisture content.…”

Mercury Residues and Productivity in Osprey and Grebes From a Mine‐dominated Ecosystem

“…The other chemical analyses of most feathers by X-ray fluorescence (XRF) has already been described by Cahill et al (1997), where total Hg is reported, although MeHg dominates feather residues (Thompson andFurness 1989a, b, Cahill et al 1997). Methylmercury also dominates total Hg (TotHg) residues in various Clear Lake fish species (Suchanek et al 2008c) selected as prey by these two important piscivorous birds. Mercury concentrations derived from both of these analytical methods are comparable (see Cahill et al 1997).…”

“…Osprey and grebe feather residues (left y-axis; means 6 SE) over time plotted over residue levels in juvenile largemouth bass (right y-axis) from the Oaks Arm of Clear Lake (bass residue data are taken from Suchanek et al [2008c]). Sample sizes are given in Results and discussion: Osprey residues .…”