Mercury accumulation in fish is a global public health concern, because fish are the primary source of toxic methylmercury to humans. Fish from all lakes do not pose the same level of risk to consumers. One of the most intriguing patterns is that potentially dangerous mercury concentrations can be found in fish from clear, oligotrophic lakes whereas fish from greener, eutrophic lakes often carry less mercury. In this study, we experimentally tested the hypothesis that increasing algal biomass reduces mercury accumulation at higher trophic levels through the dilution of mercury in consumed algal cells. Under bloom dilution, as algal biomass increases, the concentration of mercury per cell decreases, resulting in a lower dietary input to grazers and reduced bioaccumulation in algal-rich eutrophic systems. To test this hypothesis, we added enriched stable isotopes of Hg to experimental mesocosms and measured the uptake of toxic methylmercury (CH 3 200 Hg ؉ ) and inorganic 201 Hg 2؉ by biota at several algal concentrations. We reduced absolute spike detection limits by 50 -100 times compared with previous techniques, which allowed us to conduct experiments at the extremely low aqueous Hg concentrations that are typical of natural systems. We found that increasing algae reduced CH 3Hg ؉ concentrations in zooplankton 2-3-fold. Bloom dilution may provide a mechanistic explanation for lower CH 3Hg ؉ accumulation by zooplankton and fish in algal-rich relative to algal-poor systems. N utrient enrichment with subsequent eutrophication is one of the most important problems impacting lakes worldwide (1, 2). Increased nutrient concentrations produce algal blooms, which in turn alter concentrations of nutrients, gases, pH, and metal ions in the water (3). It is our hypothesis that by increasing algal abundance, nutrient enrichment also alters Hg inputs to lake food webs. Mercury concentrations in fish have been related to metal burdens in their zooplankton prey (4-8), but the connection between Hg accumulation by zooplankton and increasing algal density under nutrient enrichment has not been established. It is critical to discern this association because algae can concentrate Hg from the aqueous phase (e.g., by 100-10,000ϩ times) and thus provide the greatest inputs of Hg to the food chain (9, 10). Here we report how an induced algal bloom affects the accumulation of methyl and inorganic Hg in the cladoceran Daphnia after 2 and 3 weeks of grazing on algae labeled with stable isotopes of Hg. Daphnia is a common zooplankton herbivore and known to be a major food for planktivorous fish (11), therefore factors affecting Hg burdens in this ''keystone'' (12, 13) prey taxon may have important ramifications for predicting CH 3 Hg ϩ burdens in fish across lakes of varying trophic status.We experimentally tested the hypothesis that at equal initial concentrations of aqueous Hg, an increase in algae will result in a decrease in Hg uptake-by zooplankton grazers. Our rationale for this hypothesis was that the concentration of metal per cel...

Recent studies have emphasized the need for understanding the accumulation and fate of metal contaminants at different trophic levels and across a broad spectrum of lake types. To address both issues, metal concentrations (Hg, Zn, Cd, As, and Pb) were measured in the water, two size fractions of zooplankton, and fish from 20 lakes in contaminated to pristine watersheds in the northeastern United States. Our goals were to examine links between watershed characteristics and aqueous metal levels in lakes and relationships between aqueous concentrations, metal burdens in different plankton groups and in fish. Two pairs of metals, (1) Hg and Zn and (2) As and Pb, exhibited strong similarities both in the factors that predict their concentrations in water and in the patterns of accumulation in particular trophic levels. Aqueous concentrations of Hg and Zn were highest in cool water lakes, whereas As and Pb were highest in more eutrophic lakes in agricultural areas. Aqueous Cd concentrations were closely correlated with the land-use variables, percentage of agricultural land, and road densities. Similarly, Hg and Zn both biomagnified from small plankton (45-202 m) to macrozooplankton (Ͼ202 m) and from macrozooplankton to fish. In contrast, bioaccumulation of both As and Pb diminished with increasing trophic level. Although aqueous metal and zooplankton metal levels were not significant predictors of As and Pb levels in fish, metal levels in zooplankton were predictive of Hg and Zn in fish, suggesting that sources of bioaccumulation differ for different metals. Our findings demonstrate the importance of investigating upper and lower trophic levels separately, to fully understand metal transfer pathways in aquatic food webs.Metals transferred through aquatic food webs to fish, humans, and other piscivorous animals are of environmental and human health concern. High levels of Hg in fish from apparently pristine lakes have resulted in the adoption of conservative fish consumption advisories in many states (Håkanson et al. 1988;DiFranco et al. 1995;USEPA 1997;Yeardley et al. 1998). However, pathways of metal movement from land to water and then through aquatic food webs are not well understood. This makes it difficult to extrapolate findings from single systems to other lakes or to account for the significant variation in metal levels found in the fish from different lakes within the same geographic regions. Our broad goal is to elucidate factors across a variety of lake types that determine metal levels in water, in fish, and in the zooplankton, which are the primary dietary conduit of metal from water to pelagic feeding fish.Although knowledge of metal movement in freshwater systems has grown significantly in recent years, there are still large gaps in our understanding. For example, most past studies of metals focus on a few taxa, single metals, or transfer mechanisms in a small portion of the food web (Prahalad

Phytoplankton concentrate mercury from their aqueous surroundings and represent the primary entry point for Hg in aquatic food webs. We used 203Hg to compare the uptake of inorganic mercury, Hg(II), and methylmercury, MeHg, in four phytoplankton species (a diatom, a chlorophyte, a cryptophyte, and a cyanobacterium) in two waters containing different concentrations of dissolved organic carbon (DOC). At steady state, volume concentration factors (VCFs) for Hg(II) in the four species were similar and ranged from 0.5 to 5 x 10(4) for both water types, whereas VCFs for MeHg exceeded those for Hg(II) and ranged from 1.3 to 14.6 x 10(5). The VCFs for MeHg in the three eukaryotic cells in the high DOC water were 2-2.6 times greater than those in the low DOC water, but the VCFs for the prokaryote were similar in both waters. Higher cell surface area to volume ratios correlated with increased MeHg concentrations but not with Hg(II). In both water types, VCFs of Hg(II) were similar for living and heat-killed cells, but the VCFs of MeHg were 1.5-5.0 times greater in living cells, suggesting an active uptake component for MeHg. Hg(II) and MeHg were entirely bound to cell surfaces of the dead cells, whereas 59-64% of the MeHg and 9-16% of the Hg(II) in living cells entered the cytoplasm.

Rapid growth could significantly reduce methylmercury (MeHg) concentrations in aquatic organisms by causing a greater than proportional gain in biomass relative to MeHg (somatic growth dilution). We hypothesized that rapid growth from the consumption of high-quality algae, defined by algal nutrient stoichiometry, reduces MeHg concentrations in zooplankton, a major source of MeHg for lake fish. Using a MeHg radiotracer, we measured changes in MeHg concentrations, growth and ingestion rates in juvenile Daphnia pulex fed either high (C:P ‫؍‬ 139) or low-quality (C:P ‫؍‬ 1317) algae (Ankistrodesmus falcatus) for 5 d. We estimated Daphnia steady-state MeHg concentrations, using a biokinetic model parameterized with experimental rates. Daphnia MeHg assimilation efficiencies (Ϸ95%) and release rates (0.04 d ؊1 ) were unaffected by algal nutrient quality. However, Daphnia growth rate was 3.5 times greater when fed high-quality algae, resulting in pronounced somatic growth dilution. Steady-state MeHg concentrations in Daphnia that consumed high-quality algae were one-third those of Daphnia that consumed low-quality algae due to higher growth and slightly lower ingestion rates. Our findings show that rapid growth from high-quality food consumption can significantly reduce the accumulation and trophic transfer of MeHg in freshwater food webs.contaminants ͉ food quality ͉ heavy metals ͉ nutrient stoichiometry ͉ plankton M ethylmercury (MeHg) poses a serious human and wildlife health risk primarily through fish consumption, so understanding the key factors driving MeHg accumulation in fish has become a global priority (1). Rapid somatic growth rates are hypothesized to reduce mass-specific MeHg concentration (burden) in fish and other aquatic organisms (2, 3) by the process of somatic growth dilution (SGD). SGD occurs when rapid growth results in a disproportionate increase in the net rate of biomass gain relative to MeHg gain. The relative quality of food consumed can strongly influence growth rates in aquatic organisms. Moreover, food quality for aquatic consumers varies widely across lakes and seasons (4), thus potentially contributing to the large variation in Hg concentrations observed in lake fish in situ. Despite its potential importance, the influence of food quality on MeHg accumulation has not been well examined.At present, evidence for somatic growth dilution of MeHg, whether due food quality or other factors, is sparse and somewhat contradictory. Thus far, our understanding of SGD has been limited to inferences drawn from field correlations between somatic growth rates and Hg concentrations in fish. In most of these studies, the many possible factors driving SGD (e.g., temperature, food availability, food quality, activity level, and stress) are not controlled and are often confounded. Negative correlations between somatic growth rate and concentrations of total Hg (5-8) and other contaminants, such as Pb (9

High Hg concentrations in freshwater fish are a concern for human health, yet we lack a clear understanding of the mechanisms that produce high Hg concentrations in fish. Controlled studies in natural surface waters that quantify the uptake and retention of Hg in fish tissues following exposures from the aqueous phase and from invertebrate prey diets are rare. Using 203Hg, we contrasted the accumulation of inorganic Hg (HgI) and methylmercury (MeHg) from the dissolved phase and from invertebrate food in mosquitofish (Gambusia affinis) feeding on Daphnia pulex (representing a pelagic food chain) and in redear sunfish (Lepomis microlophus) feeding on amphipods (Hyallela sp., representing a benthic/macrophyte-based chain). Experiments were conducted with environmentally realistic Hg concentrations in two freshwaters from the San Francisco Bay Delta (CA, USA) with significantly different dissolved organic carbon (DOC) concentrations. Mercury uptake rates following aqueous exposures were consistently higher for fish in the water with lower DOC, whereas efflux rates were similar for both water types. Approximately 50% of the ingested Hg, associated with invertebrate prey was lost from mosquitofish, and 90% or more from sunfish, within 48 h. Assimilation efficiencies for ingested MeHg for both fish were 86 to 94%, substantially higher than those for HgI regardless of water type. Biokinetic modeling using the parameters determined in these experiments accurately predicted Hg burdens for fish in the San Francisco Bay Delta system. Despite considerable accumulation of HgI from both aqueous and dietary exposure routes, the high assimilation efficiencies and slow loss of MeHg from dietary sources are the principal determinants of predicted Hg burdens in both fish species.

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Hg and As are widespread contaminants globally and particularly in Asia. We conducted a field study in Baiyangdian Lake, the largest lake in the North China Plain, to investigate bioaccumulation and trophic transfer of potentially toxic metals (total mercury and arsenic) in sites differing in proximity from the major point sources of nutrients and metals. Hg concentrations in fish and As concentrations in water are above critical threshold levels (US Environmental Protection Agency based) considered to pose some risk to humans and wildlife. Hg concentrations in biota are within the range of concentrations in lakes in the Northeast US despite the high levels of Hg emission and deposition in China whereas As concentrations are much higher. Dissolved concentrations of both Hg and As decrease with increasing chlorophyll concentrations suggesting that there is significant uptake of metal from water by algae. These results provide evidence for algal blooms controlling dissolved metal concentrations and potentially mitigating the trophic transfer of Hg to fish. This study also underscores the need for further investigation into this contaminated ecosystem and others like it in China that are an important source of fish and drinking water for consumption by local human populations.

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