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...

Abstract. The study of isotopic variation in snowmelt from seasonal snowpacks is useful for understanding snowmelt processes and is important for accurate hydrograph separation of spring runoff. However, the complex and variable nature of processes within a snowpack has precluded a quantitative link between the isotopic composition of the original snow and its melt. This work studies the isotopic composition of new snow and its modification by snow metamorphism and melting. To distinguish individual snowstorms, we applied solutions of rare earth elements to the snow surface between storms. The snowmelt was isotopically less variable than the snowpack, which in turn was less variable than the new snow, reflecting isotopic redistribution during metamorphism and melting.

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

A study was conducted to evaluate toenail arsenic concentrations as a biologic marker of drinking water arsenic exposure. Study subjects were controls in a US population-based case-control study of nonmelanoma skin cancer, randomly selected from drivers' license records (those < 65 years of age) and Medicare enrollment files (those > or = 65 years of age). Between 1994 and 1997, a total of 540 controls were interviewed and toenail samples of sufficient weight were collected from 506 (93.7%) of these. Beginning in 1995, a sample of tap water was taken from the participants' homes; a total of 217 (98.6%) water samples were obtained from the 220 subjects interviewed. Arsenic determinations were made from toenail samples using neutron activation analysis. Water samples were analyzed using hydride-generation magnet sector inductively coupled mass spectrometry. Among 208 subjects with both toenail and water measurements, the correlation (r) between water and nail arsenic was 0.65 (p < 0.001) among those with water arsenic concentrations of 1 microg/liter or higher and 0.08 (p = 0.31) among those with concentrations below 1 microg/liter (overall r = 0.46, p < 0.001). Our data suggest that toenail samples provide a useful biologic marker for quantifying low-level arsenic exposure.