Mercury (Hg) is a global pollutant that affects human and ecosystem health. We synthesize understanding of sources, atmosphere-land-ocean Hg dynamics and health effects, and consider the implications of Hg-control policies. Primary anthropogenic Hg emissions greatly exceed natural geogenic sources, resulting in increases in Hg reservoirs and subsequent secondary Hg emissions that facilitate its global distribution. The ultimate fate of emitted Hg is primarily recalcitrant soil pools and deep ocean waters and sediments. Transfers of Hg emissions to largely unavailable reservoirs occur over the time scale of centuries, and are primarily mediated through atmospheric exchanges of wet/dry deposition and evasion from vegetation, soil organic matter and ocean surfaces. A key link between inorganic Hg inputs and exposure of humans and wildlife is the net production of methylmercury, which occurs mainly in reducing zones in freshwater, terrestrial, and coastal environments, and the subsurface ocean. Elevated human exposure to methylmercury primarily results from consumption of estuarine and marine fish. Developing fetuses are most at risk from this neurotoxin but health effects of highly exposed populations and wildlife are also a concern. Integration of Hg science with national and international policy efforts is needed to target efforts and evaluate efficacy.

http://pubs.acs.org/doi/pdf/10.1021/es305071v

Mercury as a Global Pollutant: Sources, Pathways, and Effects

Mercury biogeochemical cycling in the ocean and policy implications.

We document a systematic decline in 202 Hg, 1 199 Hg and 1 201 Hg values with the depth at which fish feed. We show that these mercury isotope trends can be explained only if monomethylmercury is produced below the surface mixed layer, including in the underlying oxygen minimum zone, that is, between 50 and more than 400 m depth. Specifically, we estimate that about 20‐40% of the monomethylmercury detected below the surface mixed layer originates from the surface and enters deeper waters either attached to sinking particles, or in zooplankton and micronekton that migrate to depth. We suggest that the remaining monomethylmercury found at depth is produced below the surface mixed layer by methylating microbes that live on sinking particles. We suggest that microbial production of monomethylmercury below the surface mixed later contributes significantly to anthropogenic mercury uptake into marine food webs.

/pdf/methylmercury-production-below-the-mixed-layer-in-the-north-53hmrf3ffd.pdf

Methylmercury production below the mixed layer in the North Pacific Ocean

Tracing the origin of nutrients is a fundamental goal of food web research but methodological issues associated with current research techniques such as using stable isotope ratios of bulk tissue can lead to confounding results. We investigated whether naturally occurring delta C-13 patterns among amino acids (delta C-13(AA)) could distinguish between multiple aquatic and terrestrial primary production sources. We found that delta C-13(AA) patterns in contrast to bulk delta C-13 values distinguished between carbon derived from algae, seagrass, terrestrial plants, bacteria and fungi. Furthermore, we showed for two aquatic producers that their delta C-13(AA) patterns were largely unaffected by different environmental conditions despite substantial shifts in bulk delta C-13 values. The potential of assessing the major carbon sources at the base of the food web was demonstrated for freshwater, pelagic, and estuarine consumers; consumer delta C-13 patterns of essential amino acids largely matched those of the dominant primary producers in each system. Since amino acids make up about half of organismal carbon, source diagnostic isotope fingerprints can be used as a new complementary approach to overcome some of the limitations of variable source bulk isotope values commonly encountered in estuarine areas and other complex environments with mixed aquatic and terrestrial inputs.

/pdf/tracing-carbon-sources-through-aquatic-and-terrestrial-food-4nepktas8v.pdf

Tracing Carbon Sources through Aquatic and Terrestrial Food Webs Using Amino Acid Stable Isotope Fingerprinting

There have been numerous reports of plastic debris accumulation in surface waters of the central North Pacific Subtropical Gyre. Further, incidences have been reported of plastic ingestion by different marine organisms, including seabirds and small planktivorous fishes. Diet studies (2007 to 2012) of predatory pelagic fishes from this general region showed repeat observa- tions of anthropogenic marine debris ingestion in 7 species (n = 595 individuals examined). Inci- dence rates ranged from <1% in Gempylus serpens to 58% in Lampris sp. (small-eye). Of all indi- viduals 19% contained some form of marine debris, the majority of which was some form of plastic or fishing-related line. Surprisingly, species with the highest incidences of debris ingestion are thought to be primarily mesopelagic and unlikely to come into contact with surface waters con- taining known debris fields. Ingested debris pieces were found to be positively buoyant in seawa- ter mimicking different depths. These observations are the first of their kind in scope and number, and suggest that more attention should be given to marine debris in subsurface waters as well as to poorly understood organismal and food web implications.

/pdf/plastic-for-dinner-observations-of-frequent-debris-ingestion-4c639j3kca.pdf

Plastic for dinner? Observations of frequent debris ingestion by pelagic predatory fishes from the central North Pacific

Mercury distribution in the oceans is controlled by complex biogeochemical cycles, resulting in retention of trace amounts of this metal in plants and animals. Inter- and intra-specific variations in mercury levels of predatory pelagic fish have been previously linked to size, age, trophic position, physical and chemical environmental parameters, and location of capture; however, considerable variation remains unexplained. In this paper, we focus on differences in ecology, depth of occurrence, and total mercury levels in 9 species of commercially important pelagic fish (Thunnus obesus, T. albacares, Katsuwonus pelamis, Xiphias gladius, Lampris guttatus, Coryphaena hippurus, Taractichthys steindachneri, Tetrapturus audax, and Lepidocybium flavobrunneum) and in numerous representatives (fishes, squids, and crustaceans) of their lower trophic level prey sampled from the central North Pacific Ocean. Results indicate that total mercury levels of predatory pelagic fishes and their prey increase with median depth of occurrence in the water column and mimic concentrations of dissolved organic mercury in seawater. Stomach content analysis results from this study and others indicate a greater occurrence of higher-mercury containing deeper-water prey organisms in the diets of the deeper-ranging predators, X. gladius, T. obesus, and L. guttatus. While present in trace amounts, dissolved organic mercury increases with depth in the water column suggesting that the mesopelagic habitat is a major entry point for mercury into marine food webs. These data suggest that a major determinant of mercury levels in oceanic predators is their depth of forage.

/pdf/the-influence-of-depth-on-mercury-levels-in-pelagic-fishes-26ycjpcout.pdf

J. John Kaneko

Papers

The influence of depth on mercury levels in pelagic fishes and their prey