Mercury trophic transfer in a eutrophic lake: the importance of habitat-specific foraging.

Mercury (Hg) trophic transfer and bioaccumulation in fish from a mine-impacted, eutrophic lake were examined in relation to foraging habitat, trophic position, and size. Diet analysis indicated that there were clear ontogenetic shifts in foraging habitats and trophic position. Pelagic diet decreased and benthic diet increased with increasing fish length in bluegill, black crappie, inland silverside, and largemouth bass, whereas there was no shift for prickly sculpin or threadfin shad. Stable carbon isotope values (delta13C) were inversely related to the proportion of pelagic prey items in the diet, but there was no clear relationship with benthic foraging. There were distinct differences between pelagic and benthic prey basal delta13C values, with a range of approximately -28 per thousand in pelagic zooplankton to approximately -20 per thousand in benthic caddisflies. Profundal prey such as chironomid larvae had intermediate delta13C values of approximately -24 per thousand, reflecting the influence of pelagic detrital subsidies and suppressing the propagation of the benthic carbon isotope signal up the food chain. Fish total mercury (TotHg) concentrations varied with habitat-specific foraging, trophic position, and size; however, the relationships differed among species and ages. When controlling for the effects of species, length, and trophic position, TotHg and delta13C were positively correlated, indicating that Hg trophic transfer is linked to benthic foraging. When examined on a species-specific basis, TotHg was positively correlated with delta13C only for bluegill, largemouth bass, and threadfin shad. However, diet-based multiple regression analyses suggested that TotHg also increased with benthic foraging for inland silverside and black crappie. In both species, benthic prey items were dominated by chironomid larvae, explaining the discrepancy with delta13C. These results illustrate the importance of foraging habitat to Hg bioaccumulation and indicate that pelagic carbon can strongly subsidize the basal energy sources of benthic organisms.

Changes in fish diets and food web mercury bioaccumulation induced by an invasive planktivorous fish.

The invasion, boom, collapse, and reestablishment of a population of the planktivorous threadfin shad in Clear Lake, California, USA, were documented over a 20-year period, as were the effects of changing shad populations on diet and mercury (Hg) bioaccumulation in nearshore fishes. Threadfin shad competitively displaced other planktivorous fish in the lake, such as inland silversides, young-of-year (YOY) largemouth bass, and YOY bluegill, by reducing zooplankton abundance. As a result, all three species shifted from a diet that was dominated by zooplankton to one that was almost entirely zoobenthos. Stable carbon isotopes corroborated this pattern with each species becoming enriched in delta13C, which is elevated in benthic vs. pelagic organisms. Concomitant with these changes, Hg concentrations increased by approximately 50% in all three species. In contrast, obligate benthivores such as prickly sculpin showed no relationship between diet or delta13C and the presence of threadfin shad, suggesting that effects of the shad were not strongly linked to the benthic fish community. There were also no changes in Hg concentrations of prickly sculpin. The temporary extirpation of threadfin shad from the lake resulted in zooplankton densities, foraging patterns, isotope ratios, and Hg concentrations in pelagic fishes returning to pre-shad values. These results indicate that even transient perturbations of the structure of freshwater food webs can result in significant alterations in the bioaccumulation of Hg and that food webs in lakes can be highly resilient.

Mine-derived mercury: effects on lower trophic species in Clear Lake, California.

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 (approximately 60% for sediment-dwelling chironomid insect larvae) likely derives from Hg-laden particles in their guts. Spatially, whole-body 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 community-level 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.

Spatiotemporal trends in fish mercury from a mine-dominated ecosystem: Clear Lake, California.

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 approximately 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.

The legacy of mercury cycling from mining sources in an aquatic ecosystem: from ore to organism.

Clear Lake is the site of an abandoned mercury (Hg) mine (active intermittently from 1873 to 1957), now a U.S. Environmental Protection Agency Superfund Site. Mining activities, including bulldozing waste rock and tailings into the lake, resulted in approximately 100 Mg of Hg entering the lake's ecosystem. This series of papers represents the culmination of approximately 15 years of Hg-related studies on this ecosystem, following Hg from the ore body to the highest trophic levels. A series of physical, chemical, biological, and limnological studies elucidate how ongoing Hg loading to the lake is influenced by acid mine drainage and how wind-driven currents and baroclinic circulation patterns redistribute Hg throughout the lake. Methylmercury (MeHg) production in this system is controlled by both sulfate-reducing bacteria as well as newly identified iron-reducing bacteria. Sediment cores (dated with dichlorodiphenyldichlorethane [DDD], 210pb, and 14C) to approximately 250 cm depth (representing up to approximately 3000 years before present) elucidate a record of total Hg (TotHg) loading to the lake from natural sources and mining and demonstrate how MeHg remains stable at depth within the sediment column for decades to millenia. Core data also identify other stresses that have influenced the Clear Lake Basin especially over the past 150 years. Although Clear Lake is one of the most Hg-contaminated lakes in the world, biota do not exhibit MeHg concentrations as high as would be predicted based on the gross level of Hg loading. We compare Clear Lake's TotHg and MeHg concentrations with other sites worldwide and suggest several hypotheses to explain why this discrepancy exists. Based on our data, together with state and federal water and sediment quality criteria, we predict potential resulting environmental and human health effects and provide data that can assist remediation efforts.

Time of day that Aedes sierrensis females enter oviposition sites prior to laying eggs.

An ovitrap with a time-segregated entrance was used to separate eggs laid by Aedes sierrensis females according to the time of day that females entered the ovitrap. During a 37-day period in Lake County, CA, females that entered the ovitrap between sunrise and sunset laid 82% of the total number of eggs collected. A daily peak in oviposition (eggs per hour) was produced by females that entered during the 2-h period ending at sunset. Overall, females that oviposited had entered the ovitrap throughout the diel cycle except for a 2-h period ending at sunrise. Those eggs laid by females that entered the ovitrap between 2 h after sunset and 2 h before sunrise provided the 1st evidence that Ae. sierrensis females are capable of locating oviposition sites during the night.

Natural variability in the seasonal occurrence and densities of adult populations of Ochlerotatus sierrensis.

This study examined the utility of Fay traps baited with carbon dioxide (Fay/CO2 traps) and ovitraps for monitoring adult Ochlerotatus sierrensis over time and space in the Coast Range of northern California. During a 3-year study in a dense oak woodland, Fay/CO2 traps collected adults from March to November at rates that were correlated with air temperature through peak activity periods ending in late June for males and late July for females. Variability in total annual rainfall did not explain the 3-fold difference in the numbers of females caught among years. Yearly collections were all male-biased, but sex ratios varied to the extent that early season densities of males did not reliably predict subsequent densities of females. Determinations of the seasonal activity periods of females by Fay/CO2 traps and ovitraps were similar except that adults were captured 2-4 wk before oviposition occurred. The availability of natural oviposition sites affected ovipositional totals into the ovitraps, precluding the use of egg counts as a direct measure of population size of females either among or within years. In a subsequent comparison of 3 sites, Fay/CO2 trap captures of females 1st peaked in mid-April at each location, but peak periods of host-seeking activity lasted 1, 7, and 18 wk in woodlands with open, moderate, and closed tree canopies, respectively. Significant differences in the densities of host-seeking females were not detected by the use of ovitraps because egg counts had similar magnitude and peaked between mid-May and mid-August at each location.