S14 Mercury fate in marine ecosystems: From sources to consumers

Elevated levels of methylmercury (MeHg) in fish is a global human health and environmental concern and coastal systems, which are little studied, could be important conduits for the production and trophic transfer of MeHg. Water, sediment, and biota were therefore sampled to study MeHg production and bioaccumulation in both contaminated and pristine estuaries from Maine to New Jersey in 2008 and 2009. Sediment measurements (core and grab samples) included solid phase and pore water MeHg and total mercury (Hg), rates of methylation and demethylation using stable isotopes, organic matter content by loss on ignition (LOI), and pore water dissolved organic carbon (DOC) and sulfide. Water column parameters included dissolved and particulate MeHg and total Hg, total suspended solids (TSS), nutrients, and DOC. Sediment total Hg spans 3 orders of magnitude among estuarine sites (0.01 to 20 nmol g^-1), and correlates well with organic matter (LOI) in a range of 1 to 18%. Sediment MeHg ranges from 0.2 to 50 pmol g^-1 among locations, with elevated MeHg and total Hg in anthropogenically impacted areas (e.g., Mill Creek, NJ) as compared to rural/residential estuaries (e.g., Waquoit Bay, MA). Among intensively sampled locations, average sediment MeHg was highest (7.9 to 39 pmol g^-1; 0.4 to 0.6% of total Hg [%MeHg]) in cores (8 cm) from the contaminated Hackensack (NJ) site, and less at Portsmouth (NH, 3.4 to 7.7 pmol g^-1; 0.2 to 0.3% MeHg) and Wells (ME, 0.2 to 2.6 pmol g^-1; 2.0 to 2.8% MeHg). Elevated %MeHg at the Wells site was associated with comparatively low levels of sediment total Hg (0.01 to 0.1 nmol g^-1) and reduced organic matter (0.5 to 3.7%), and suggests a higher methylation rate as compared to the other sites. Increased rates of methylation at Wells are supported by lower pore water inorganic Hg (HgII) partitioning coefficients (K_D; 1.7 to 3.1 L kg^-1), indicating greater bioavailability of HgII to methylating bacteria. Preliminary observations suggest that organic matter is the main control over MeHg production among the geographically separated estuaries included in this study. We will discuss these relationships and the major factors controlling MeHg formation and fate in the presentation.

Mercury (Hg) speciation measurements were performed on board the R/V Knorr during the first leg of the U.S. GEOTRACES North Atlantic Ocean zonal section in Fall 2010. All four of the principal mercury species (monomethylmercury, dimethylmercury, elemental Hg, and total Hg) were determined in high vertical and horizontal resolution at eight stations off the coast of North Africa, an ocean region that experiences significant coastal upwelling, atmospheric dust deposition, and associated high levels of biological productivity. Preliminary results suggest that filtered total Hg has both scavenging and nutrient-type vertical distributions at different locations in the northeast Atlantic. Elemental Hg, in contrast, has strong nutrient-like profiles, with deep water concentrations as much as 50% of total Hg. This is in contrast to the western side of the basin where elemental Hg in deep water is much less abundant. Monomethylmercury (MMHg) distributions have pronounced mid-water maxima associated with the oxygen minimum zone. Increased levels in the oxygen minimum likely result from either in situ methylation or isopycnal transport from the margin. MMHg levels in the mixed layer and North Atlantic Deep Water on the eastern side of the ocean basin are similar to those we have recently measured in the western North Atlantic. High-quality Hg speciation results from the international GEOTRACES program will help constrain oceanic budgets and cycling models for toxic and bioaccumulative MMHg in the ocean.

Gulf of Mexico coastal residents consume more fish than inland populations. Consumption advisories for mercury are in place for a number of Gulf of Mexico fish species and efforts to curb anthropogenic mercury sources in the region have included regulations on waste incinerators and other products. To understand how mercury exposures in Gulf coast residents will be affected by pollution abatement efforts and changes in environmental quality in the Gulf of Mexico, it is first necessary to understand the fraction of exposure originating from consumption of locally caught fish species. Here we present a synthesis of data from the Gulf of Mexico to estimate mercury exposure sources among different demographic groups by combining fish mercury data with dietary information. Where possible, we delineate methyl mercury exposures in residents that could be attributed to fish and shellfish originating from the Gulf. We present results of probabilistic modeling of exposure levels among the population that are based on our data synthesis. Results shows that consumption of Gulf of Mexico fish represents a substantial portion of Gulf coast residents methyl mercury exposure and are exposed to higher intakes than the U.S population as a whole, primarily as a result of higher fish consumption rates. We also discuss key data limitations and uncertainties related to species-specific consumption data, origin of fish and variability, and meal sizes.

The Gulf of Maine, including the Bay of Fundy, borders New England in the U.S. and the Canadian provinces of New Brunswick and Nova Scotia. Here we review all available data on temporal and spatial patterns in mercury concentrations in the Gulf of Maine to better understand the exposure pathway for humans and wildlife in this region and anticipate the effects of future changes in loading and climate on Hg exposures. Despite significant reductions in Hg emissions in Eastern North America over the past several decades, response to loading reductions has been highly variable throughout the Gulf of Maine. Watershed contributions of Hg, that respond more slowly to changes in atmospheric deposition, dominate inputs of Hg in many Gulf of Maine embayments. In addition, the legacy of pollution from large point sources of Hg is still apparent in some regions such as Penobscot, Casco and Great bays. In general, the estuaries on the U.S. side of the Gulf of Maine appear to be much more impacted by historical Hg sources than the Bay of Fundy. Long-term biological concentration trends are best documented by seabird blood and egg data showing declines in concentrations since the 1970s in some systems but no clear trends in others. Results of our review suggest that the recovery times of Gulf of Maine estuaries are highly varied and depend largely on the physical and biological properties of specific systems and the extent of historical point sources. Biological exposures show clear differences between pelagic and benthic dominated food chains in estuarine ecosystems. The high degree of pelagic-benthic coupling in this systems means there may be a significant lag in many food webs due to slow declines in sediment concentrations. Human exposures in the region are reviewed based on dietary survey and exposure analyses from New England and Grand Manan Island. While the population in Grand Manan Island consumed predominantly low-mercury, pelagic fish and had low blood Hg concentrations, results from New England were more varied and included fish from many other systems. We conclude that additional monitoring programs for sediment, water and biota are needed to better diagnose temporal trends in mercury concentrations in Gulf of Maine fisheries.

In San Francisco Bay, mercury (Hg) contamination in the Bay and its watershed stems from a substantial legacy of historic Hg and gold mining, as well as inputs from atmospheric deposition, urban runoff, and other pathways. Elevated methylmercury (MeHg) concentrations in sport fish have led to consumption advisories for the Bay and for many upstream reservoirs. Detailed studies have documented significant risks to several bird species. Spatial and temporal trends observed in MeHg in the food web and recent work examining Hg stable isotopes indicate that Hg in sediment, derived largely from mining but also from other sources, is available for methylation and bioaccumulation, even though much of the Hg entered, and continues to enter, the aquatic environment as cinnabar or roasted cinnabar mine waste. Available long-term time series suggest that food web concentrations were higher in tidal marshes in the early 1900s, but have shown little or no decline in the Bay or its tidal marshes since the 1970s. Studies of the vertical and horizontal distribution of Hg in sediment indicate that past inputs have been thoroughly mixed throughout the system in this shallow and dynamic estuary.

This situation poses a challenge for managers aiming to reduce MeHg in the Bay food web. Contemporary Hg inputs or legacy Hg pools may be enough to fuel food web accumulation far into the future. Controllable inputs of Hg should certainly be minimized, but controlling in situ net MeHg production is also likely needed to achieve measurable and timely reductions in food web MeHg. Some facets of the Hg problem appear to be tractable. Highly managed habitats (i.e., reservoirs and salt ponds) offer the greatest opportunities for intervening in the MeHg cycle. It may also be possible to adjust the design of restored tidal wetlands to minimize MeHg bioaccumulation. Discrete areas of elevated uptake on the Bay margin may also present opportunities for intervention. Other systemic changes are also underway that could conceivably either significantly enhance or hinder efforts to reduce food web MeHg. Declining sediment supply to the Bay has increased water clarity and may enhance photodemethylation, but also may increase bed erosion and remobilization of legacy Hg sediment deposits. Climate change and sea level rise may also drive changes in food web MeHg in ways that are difficult to forecast.

An interesting feature of mercury biogeochemistry in the Mediterranean is that several fish species from the Mediterranean show higher concentrations of Hg than same fish species in the Atlantic ocean, although the concentrations of total-Hg in the open waters of both oceans are similar. Elevated Hg levels have been noted in environmental matrices from the Mediterranean regions adjacent to known mercury anomalies, yet, the data do not clearly indicate that these contaminated sites have contributed to open waters or to lower trophic level species. Recent studies indicated that the main source of MeHg in organisms in the coastal areas is related to methylation in sediments, while net mercury methylation in the open ocean occurs in the water column and is linked to organic matter regeneration promoted by the presence of small-sized nano- and picophytoplankton, that dominate under oligotrophic conditions. Relatively large portion of mercury in waters is present as dissolved gaseous Hg, originating from photochemical, biologically mediated mechanisms and/or diffusion from deeper layer either due to biological and/or to tectonic activity which is typical of the Mediterranean region. Recent studies on distribution and cycling of Hg and a mass balance were implemented. It was shown that the total-Hg exchanges at the straits are not unbalanced, while mercury entering the western Mediterranean is mainly in inorganic Hg forms and is exported to the Atlantic partially as methylated species. It has been shown that exchange with the atmosphere is the most important source/sink of Hg for the water compartment. Measurements have shown that the evasion of Hg varied between the different seasons with the highest evasion during the autumn and an estimate of yearly evasion from the Mediterranean Sea surface was 77 tons. Based on policy target scenarios and integrated models developed, it is expected that Hg concentration in the Mediterranean will decrease by 2020 for about 3 to 12% as compared to 2005. Based on mercury measured in sea food, it was indicated that a consumption of certain fish species may pose a health risk. A review of health related studies in the Mediterranean, particularly the effects of prenatal Hg exposure through fish consumption on neurodevelopment, will be presented and discussed in the light of inconsistencies found in similar studies around the world. Any fish advisories should carefully be balanced against the benefit of the fish consumption in the region.

The public faces fish consumption advice from a variety of sources. Diverse perspectives have influenced messaging related to fish consumption, including toxicological, nutritional, ecological, and economic viewpoints. For example, federal and state agencies, concerned about exposure to toxicants in fish such as methylmercury and polychlorinated biphenyls, have issued advisories targeting the general public, sport fishers, pregnant women, and young children. These advisories recommend that consumption of fish of certain types, or from certain sources, be limited or avoided. However, national organizations of physicians and nutritionists encourage oily fish consumption as a way to increase dietary intake of the long-chain omega-3 fatty acids that may prevent cardiovascular disease and improve neurological development. Meanwhile, environmental groups have recommended that consumers avoid certain fish based on concerns about species depletion, farming methods, site of origin, or type of harvesting. Furthermore, many guidelines do not provide detailed information on the range of fish available, and conversely consumers may not have access to the facts they need to make informed choices, for example how or where a fish was caught. Frequency of consumption and species type consumed are also influenced by taste, cultural tradition, recreational habits, budget constraints, and the availability of alternative foods.

Thus, the consumer who wants to know "which fish should I eat?" is likely to find no advice, or at times directly contradictory advice. For example, farm-raised salmon is high in omega-3 fatty acids and very low in mercury, but environmental groups consider it a “Fish to Avoid” because aquaculture may adversely impact ecosystem integrity and wild fish stocks. While some experts worry that a complicated or overly detailed message may confuse the public, it is likely that much of the public is already confused.

No review exists in the scientific literature that lays out viewpoints on fish consumption from different perspectives together in the same paper. In this analysis, we will summarize the issue of fish consumption advice from nutritional, toxicant, ecological, and economic points of view. We will identify areas of overlap and disagreement among these viewpoints. We will also review experience with previous advisories, and discuss the key elements of a good public advisory. The opinions are those of the authors and do not necessarily reflect the policies of the US EPA or any other institution.

Methylmercury (MeHg) is a known neurotoxicant, and emerging data indicate that it may have other adverse health consequences. Importantly, MeHg passes not only through the blood brain barrier but also the placenta of pregnant women, resulting in higher MeHg in fetal than maternal circulation. As with other metals, the developing fetus is especially vulnerable to MeHg exposure. This was exemplified by the occurrence of Minamata disease in Japan when pregnant women consumed highly MeHg contaminated seafood causing extreme fetal abnormalities and neurotoxicity (i.e., microcephaly, blindness, severe mental and physical development retardation).

More subtle neurologic effects have been observed in populations with moderate levels of MeHg exposure from regular consumption of fish and/or marine mammals, including associations of MeHg biomarkers at birth with decrements in memory, attention, language, and visuo-motor skills in childhood. A growing body of literature has explored the effects of lower levels of MeHg exposure and the impact of MeHg on a variety of health outcomes both in adults and children. Findings include adverse effects on the cardiovascular system, birth weight, immune function, and other pathways. Methylmercury is more readily absorbed than inorganic forms, and absorbed primarily in the intestines, reabsorbed through the enterohepatic system, and fecally excreted. Methylmercury excretion rates vary widely among individuals and involve glutathione conjugation by selenium dependent glutathione-S-transferases (GSTs). Scant data exist on the relations between GST polymorphisms and blood levels of MeHg, and their potential modifying effects on health outcomes. Additionally, the potential confounding or modifying effects of other factors (e.g., other nutrient/contaminants) have been explored only to a limited extent.

This paper provides a synthesis of the current state of knowledge on the human health effects of low-level MeHg exposure. It concentrates on recent studies that inform risk at common exposure levels, focusing on the epidemiologic literature of MeHg concentrations measured in biologic tissue. We examine the following questions: 1) What are the key health effects of lower, prevalent levels of MeHg exposure in the general population; 2) What is the recent evidence; 3) What are potential confounders or modifiers of human health risks (synergistic or antagonistic); 4) What important gaps exist in our knowledge; and 5) How can recent evidence help guide policies regarding the reduction of MeHg exposure worldwide. The opinions are those of the authors and do not necessarily reflect the policies of the U.S. EPA.