Relative contribution of rice and fish consumption to bioaccessibility-corrected health risks for urban residents in eastern China.

There are global concerns about dietary exposure to metal(loid)s in foods. However, little is known about the relative contribution of rice versus fish to multiple metal(loid) exposure for the general population, especially in Asia where rice and fish are major food sources. We compared relative contributions of rice and fish consumption to multi-metal(loid) exposure on the city-scale (Nanjing) and province-scale in China. The effects of ingestion rate, metal(loid) level, and bioaccessibility were examined to calculate modeled risk from Cu, Zn, total As (TAs), inorganic As (iAs), Se, Cd, Pb, and methylmercury (MeHg). Metal(loid) levels in rice and fish samples collected from Nanjing City were generally low, except iAs. Metal(loid) bioaccessibilities in fish were higher than those in rice, except Se. Calculated carcinogenic risks induced by iAs intake (indicated by increased lifetime cancer risk, ILCR) were above the acceptable level (1 0 -4) in Nanjing City (median: 3 × 10-4 for female and 4 × 10-4 for male) and nine provinces (1.4 × 10-4 to 5.9 × 10-4) in China. Rice consumption accounted for 85.0% to 99.8% of carcinogenic risk. The non-carcinogenic hazard quotients (HQ) for single metals and hazard index (HI) for multi-metal exposure were < 1 in all cases, indicating of their slight non-carcinogen health effects associated. In Guangdong and Jiangsu provinces, results showed that rice and fish intake contributed similarly to the HI (i.e., 42.6% vs 57.4% in Guangdong and 54.6% vs 45.4% in Jiangsu). Sensitivity analysis indicated that carcinogenic risk was most sensitive to rice ingestion rate and rice iAs levels, while non-carcinogenic hazard (i.e., HQ and HI) was most sensitive to ingestion rate of fish and rice, and Cu concentration in rice. Our results suggest that rice is more important than fish for human dietary metal(loid) exposure risk in China, and carcinogenic risk from iAs exposure in rice requires particular attention.

Antimicrobial Drug-Resistant Gram-Negative Saprophytic Bacteria Isolated from Ambient, Near-Shore Sediments of an Urbanized Estuary: Absence of ß-Lactamase Drug-Resistance Genes.

We assessed the prevalence of antimicrobial resistance and screened for clinically relevant ß-lactamase resistance determinants in Gram-negative bacteria from a large urbanized estuary. In contrast to the broad literature documenting potentially hazardous resistance determinants near wastewater treatment discharge points and other local sources of aquatic pollution, we employed a probabilistic survey design to examine ambient, near-shore sediments. We plated environmental samples from 40 intertidal and shallow subtidal areas around San Francisco Bay (California, USA) on drug-supplemented MacConkey agar, and we tested isolates for antimicrobial resistance and presence of clinically relevant ß-lactamase resistance determinants. Of the 74 isolates identified, the most frequently recovered taxa were Vibrio spp. (40%), Shewanella spp. (36%), Pseudomonas spp. (11%), and Aeromonas spp. (4%). Of the 55 isolates tested for antimicrobial resistance, the Vibrio spp. showed the most notable resistance profiles. Most (96%) were resistant to ampicillin, and two isolates showed multidrug-resistant phenotypes: V. alginolyticus (cefotaxime, ampicillin, gentamicin, cefoxitin) and V. fluvialis (cefotaxime, ampicillin, cefoxitin). Targeted testing for class 1 integrons and presence of ß-lactam-resistance gene variants TEM, SHV, OXA, CTX-M, and Klebsiella pneumonia carbapenemase (KPC) did not reveal any isolates harboring these resistance determinants. Thus, while drug-resistant, Gram-negative bacteria were recovered from ambient sediments, neither clinically relevant strains nor mobile ß-lactam resistance determinants were found. This suggests that Gram-negative bacteria in this well-managed, urbanized estuary are unlikely to constitute a major human exposure hazard at this time.

High retention of silver sulfide nanoparticles in natural soils.

Silver, either in ionic or nanoparticulate form, is widely used in consumer products. However, silver sulfide (Ag2S) are more likely to be the form that Ag enters the environment. The retention of silver sulfide nanoparticles (Ag2S-NPs) in natural soils is critical for bioavailability and toxicity but remains unclear. Here, we examined the retention of Ag2S-NPs in 11 natural soils with varying properties using batch assays. More than 99% of Ag2S-NPs were retained in soil solids, irrespective of soil properties. Such high retention of Ag2S-NPs, at least partially, explained the distinct differences in phytoavailability performed in soil vs. liquid media in the literature. Nanoparticles containing Ag and S were identified in representative soil solids by high resolution transmission electron microscopy equipped with an energy dispersive X-ray spectrometer. Iron-rich acidic soil had a high dissolution of Ag2S-NPs ranging from 47.1% to 61.7% in porewater. In contrast to Ag2S-NPs, silver nanoparticles (AgNPs) and Ag+ in these soils were less retained (as described by Freundlich model) and the retention was closely associated with soil properties. These findings highlight the unique behaviors of Ag2S-NPs in natural soils.

The association between community-associated Staphylococcus aureus colonization and disease: a meta-analysis.

BACKGROUND: Colonization with Staphylococcus aureus is a well-defined risk factor for disease in hospitals, which can range from minor skin infections to severe, systemic diseases. However, the generalizability of this finding has not been thoroughly investigated outside of the hospital environment. We aimed to assess the role of S. aureus colonization as a risk factor for disease in the community. METHODS: We performed a meta-analysis of observational studies and searched PubMed for articles published between December 1979 and May 23, 2016. We included cohort, cross-sectional, and case-control studies that reported quantitative estimates of both S. aureus colonization and disease statuses of all study subjects. We excluded studies on recently hospitalized subjects, long-term care facilities, surgery patients, dialysis patients, hospital staff, S. aureus outbreaks, and livestock-associated infections. Our meta-analysis was performed using random-effects analysis to obtain pooled odds ratios (ORs) to compare the odds of S. aureus disease with respect to S. aureus colonization status. RESULTS: We identified 3477 citations, of which 12 articles on 6998 subjects met the eligibility criteria. Overall, subjects colonized with S. aureus were more likely to progress to disease than those who were non-colonized: (OR 1.87, 95% CI 1.21-2.88, n = 7 studies). We observed a larger effect with methicillin-resistant S. aureus colonization (7.06, 4.60-10.84, n = 7 studies). However, the methicillin-sensitive S. aureus colonization was not associated with greater odds of disease (1.20, 0.69-2.06, n = 4 studies). Heterogeneity was present across studies in all of the subgroups: S. aureus (I2 = 95.0%, χ2 = 120.3, p < 0.001), MRSA (I2 = 92.8%, χ2 = 82.8, p = p < 0.001), and MSSA (I2 = 86.3%, χ2 = 21.8, p < 0.001). CONCLUSIONS: While the majority of papers individually support the assumption that colonization is a risk factor for S. aureus disease in the general population, there is marked heterogeneity between studies and further investigation is needed to identify the major sources of this variance. There is a shortage of literature addressing this topic in the community setting and a need for further research on colonization as a focus for disease prevention.

Bioaccessibility-corrected risk assessment of urban dietary methylmercury exposure via fish and rice consumption in China.

The role of seafood consumption for dietary methylmercury (MeHg) exposure is well established. Recent studies also reveal that rice consumption can be an important pathway for dietary MeHg exposure in some Hg-contaminated areas. However, little is known about the relative importance of rice versus finfish in MeHg exposure for urban residents in uncontaminated areas. Especially, the lack of data on MeHg bioaccessibility in rice hinders accurately assessing MeHg exposure via rice consumption, and its importance compared to fish. By correcting commonly used risk models with quantified MeHg bioaccessibility, we provide the first bioaccessibility-corrected comparison on MeHg risk in rice and fish for consumers in non-contaminated urban areas of China, on both city- and province-scales. Market-available fish and rice samples were cooked and quantified for MeHg bioaccessibility. Methylmercury bioaccessibility in rice (40.5±9.4%) was significantly (p<0.05) lower than in fish (61.4±14.2%). This difference does not result from selenium content but may result from differences in protein or fiber content. Bioaccessibility-corrected hazard quotients (HQs) were calculated to evaluate consumption hazard of MeHg for consumers in Nanjing city, and Monte Carlo Simulations were employed to evaluate uncertainty and variability. Results indicate that MeHg HQs were 0.14 (P50) and 0.54 (P90). Rice consumption comprised 27.2% of the overall dietary exposure to MeHg in Nanjing, while fish comprised 72.8%. Employing our bioaccessibility data combined with literature parameters, calculated relative contribution to MeHg exposure from rice (versus fish) was high in western provinces of China, including Sichuan (95.6%) and Guizhou (81.5%), and low to moderate in eastern and southern provinces (Guangdong: 6.6%, Jiangsu: 17.7%, Shanghai: 15.1%, Guangxi: 20.6%, Jiangxi: 22.8% and Hunan: 25.9%). This bioaccessibility-corrected comparison of rice versus fish indicates that rice consumption can substantively contribute to dietary MeHg exposure risk for urban populations in Asia, and should be regularly included in dietary MeHg exposure assessment.

Modeling the Emergence of Antibiotic Resistance in the Environment: an Analytical Solution for the Minimum Selection Concentration.

Environmental antibiotic risk management requires an understanding of how subinhibitory antibiotic concentrations contribute to the spread of resistance. We develop a simple model of competition between sensitive and resistant bacterial strains to predict the minimum selection concentration (MSC), the lowest level of antibiotic at which resistant bacteria are selected. We present an analytical solution for the MSC based on the routinely measured MIC, the selection coefficient (sc) that expresses fitness differences between strains, the intrinsic net growth rate, and the shape of the bacterial growth dose-response curve with antibiotic or metal exposure (the Hill coefficient [κ]). We calibrated the model by optimizing the Hill coefficient to fit previously reported experimental growth rate difference data. The model fit varied among nine compound-taxon combinations examined but predicted the experimentally observed MSC/MIC ratio well (R2 ≥ 0.95). The shape of the antibiotic response curve varied among compounds (0.7 ≤ κ ≤ 10.5), with the steepest curve being found for the aminoglycosides streptomycin and kanamycin. The model was sensitive to this antibiotic response curve shape and to the sc, indicating the importance of fitness differences between strains for determining the MSC. The MSC can be >1 order of magnitude lower than the MIC, typically by the factor scκ This study provides an initial quantitative depiction and a framework for a research agenda to examine the growing evidence of selection for resistant bacterial communities at low environmental antibiotic concentrations.

A multivariate analysis of CalEnviroScreen: comparing environmental and socioeconomic stressors versus chronic disease.

BACKGROUND: The health-risk assessment paradigm is shifting from single stressor evaluation towards cumulative assessments of multiple stressors. Recent efforts to develop broad-scale public health hazard datasets provide an opportunity to develop and evaluate multiple exposure hazards in combination. METHODS: We performed a multivariate study of the spatial relationship between 12 indicators of environmental hazard, 5 indicators of socioeconomic hardship, and 3 health outcomes. Indicators were obtained from CalEnviroScreen (version 3.0), a publicly available environmental justice screening tool developed by the State of California Environmental Protection Agency. The indicators were compared to the total rate of hospitalization for 14 ICD-9 disease categories (a measure of disease burden) at the zip code tabulation area population level. We performed principal component analysis to visualize and reduce the CalEnviroScreen data and spatial autoregression to evaluate associations with disease burden. RESULTS: CalEnviroScreen was strongly associated with the first principal component (PC) from a principal component analysis (PCA) of all 20 variables (Spearman ρ = 0.95). In a PCA of the 12 environmental variables, two PC axes explained 43% of variance, with the first axis indicating industrial activity and air pollution, and the second associated with ground-level ozone, drinking water contamination and PM2.5. Mass of pesticides used in agriculture was poorly or negatively correlated with all other environmental indicators, and with the CalEnviroScreen calculation method, suggesting a limited ability of the method to capture agricultural exposures. In a PCA of the 5 socioeconomic variables, the first PC explained 66% of variance, representing overall socioeconomic hardship. In simultaneous autoregressive models, the first environmental and socioeconomic PCs were both significantly associated with the disease burden measure, but more model variation was explained by the socioeconomic PCs. CONCLUSIONS: This study supports the use of CalEnviroScreen for its intended purpose of screening California regions for areas with high environmental exposure and population vulnerability. Study results further suggest a hypothesis that, compared to environmental pollutant exposure, socioeconomic status has greater impact on overall burden of disease.

Long-term variation in concentrations and mass loads in a semi-arid watershed influenced by historic mercury mining and urban pollutant sources.

Urban watersheds are significantly anthropogenically-altered landscapes. Most previous studies cover relatively short periods, without addressing concentrations, loads, and yields in relation to annual climate fluctuations, and datasets on Ag, Se, PBDEs, and PCDD/Fs are rare. Intensive storm-focused sampling and continuous turbidity monitoring were employed to quantify pollution at two locations in the Guadalupe River (California, USA). At a downstream location, we determined loads of suspended sediment (SS) for 14yrs., mercury (HgT), PCBs, and total organic carbon (TOC) (8yrs), total methylmercury (MeHgT) (6yrs), nutrients, and trace elements including Ag and Se (3yrs), DDTs, chlordanes, dieldrin, and PBDEs (2yrs), and PCDD/Fs (1yr). At an upstream location, we determined loads of SS for 4yrs. and HgT, MeHgT, PCBs and PCDD/Fs for 1yr. These data were compared to previous studies, climatically adjusted, and used to critically assess the use of small datasets for estimating annual average conditions. Concentrations and yields in the Guadalupe River appear to be atypical for total phosphorus, DDTs, dieldrin, HgT, MeHgT, Cr, Ni, and possibly Se due to local conditions. Other pollutants appear to be similar to other urban systems. On average, wet season flow varied by 6.5-fold and flow-weighted mean (FWM) concentrations varied 4.4-fold, with an average 7.1-fold difference between minimum and maximum annual loads. Loads for an average runoff year for each pollutant were usually less than the best estimate of long-term average. The arithmetic average of multiple years of load data or a FWM concentration combined with mean annual flow was also usually below the best estimate of long-term average load. Mean annual loads using sampled years were also less than the best estimate of long-term average by a mean of 2.2-fold. Climatic adjustment techniques are needed for computing estimates of long-term average annual loads.

Mechanistic understanding of low methylmercury bioaccessibility from crayfish (Procambarus clarkii) muscle tissue.

Recent research indicates that dietary exposure to mercury and other metals from crayfish consumption poses a human health concern, particularly in regions with high crayfish-consuming populations. To better understand consumption risk from methylmercury (MeHg), we quantified MeHg bioaccessibility in edible tail muscle of cooked red swamp crayfish (Procambarus clarkii, collected from seven cities in China), versus cooked fillet tissue of two finfish species: yellow croaker (Larimichthys polyactis) and snakehead (Channa argus). Results indicated that digestive solubilization rate (DSR) of MeHg in crayfish (7.8±3.9% for restaurant-crayfish and 9.8±0.8% for market-crayfish) was lower than the rate in yellow croaker (25.8±2.7%) and snakehead (26.2±4.7%) tissue, suggesting that relatively low MeHg bioaccessibility in crayfish may reduce dietary exposure to humans. Three possible mechanisms for the reduced MeHg DSR in crayfish tissue were examined: MeHg-Se interactions, MeHg subcellular fractionation, and Hg-amino acid binding. Selenium concentrations were comparable among the examined species, and no significant relationship was observed between tissue Se and MeHg DSR. Similarly, observed differences in subcellular fractionation of MeHg could not explain the species-specific MeHg DSR. Therefore, MeHg-Se interactions and MeHg subcellular fractionation do not explain the relatively low MeHg bioaccessibility in crayfish. Significantly higher cysteine and arginine content was found in crayfish than in the finfish. We suspect that the lower MeHg bioaccessibility of crayfish tail muscle may be attributed to the higher cysteine concentrations, and thus, stronger MeHg-protein binding in crayfish. These results support the interpretation that bioaccessibility differences will alter risk interpretations for MeHg, especially when comparing hazard across aquatic food types.

Decline in methylmercury in museum-preserved bivalves from San Francisco Bay, California.

There are ongoing efforts to manage mercury and nutrient pollution in San Francisco Bay (California, USA), but historical data on biological responses are limited. We used bivalves preserved in formalin or ethanol from museum collections to investigate long-term trends in methylmercury (MeHg) concentrations and carbon and nitrogen isotopic signatures. In the southern reach of the estuary, South Bay, MeHg in the Asian date mussel (Musculista senhousia) significantly declined over the study duration (1970 to 2012). Mean MeHg concentrations were highest (218ng/g dry weight, dw) in 1975 and declined 3.8-fold (to 57ng/g dw) by 2012. This decrease corresponded with closure of the New Almaden Mercury Mines and was consistent with previously observed declines in sediment core mercury concentrations. In contrast, across all sites, MeHg in the overbite clam (Potamocorbula amurensis) increased 1.3-fold from 64ng/g dw before 2000 to 81ng/g dw during the 2000s and was higher than in M. senhousia. Pearson correlation coefficients of the association between MeHg and δ13C or δ15N provided no evidence that food web alterations explained changing MeHg concentrations. However, isotopic composition shifted temporally. South Bay bivalve δ15N increased from 12‰ in the 1970s to 18‰ in 2012. This increase corresponded with increasing nitrogen loadings from wastewater treatment plants until the late 1980s and increasing phytoplankton biomass from the 1990s to 2012. Similarly, a 3‰ decline in δ13C from 2002 to 2012 may represent greater utilization of planktonic food sources. In a complimentary 90day laboratory study to validate use of these preserved specimens, preservation had only minor effects (<0.5‰) on δ13C and δ15N. MeHg increased following preservation but then stabilized. These are the first documented long-term trends in biota MeHg and stable isotopes in this heavily impacted estuary and support the utility of preserved specimens to infer contaminant and biogeochemical trends.

Are Chinese consumers at risk due to exposure to metals in crayfish? A bioaccessibility-adjusted probabilistic risk assessment.

Freshwater crayfish, the world's third largest crustacean species, has been reported to accumulate high levels of metals, while the current knowledge of potential risk associated with crayfish consumption lags behind that of finfish. We provide the first estimate of human health risk associated with crayfish (Procambarus clarkii) consumption in China, the world's largest producer and consumer of crayfish. We performed Monte Carlo Simulation on a standard risk model parameterized with local data on metal concentrations, bioaccessibility (φ), crayfish consumption rate, and consumer body mass. Bioaccessibility of metals in crayfish was found to be variable (68-95%) and metal-specific, suggesting a potential influence of metal bioaccessibility on effective metal intake. However, sensitivity analysis suggested risk of metals via crayfish consumption was predominantly explained by consumption rate (explaining >92% of total risk estimate variability), rather than metals concentration, bioaccessibility, or body mass. Mean metal concentrations (As, Cd, Cu, Ni, Pb, Se and Zn) in surveyed crayfish samples from 12 provinces in China conformed to national safety standards. However, risk calculation of φ-modified hazard quotient (HQ) and hazard index (HI) suggested that crayfish metals may pose a health risk for very high rate consumers, with a HI of over 24 for the highest rate consumers. Additionally, the φ-modified increased lifetime risk (ILTR) for carcinogenic effects due to the presence of As was above the acceptable level (10(-5)) for both the median (ILTR=2.5×10(-5)) and 90th percentile (ILTR=1.8×10(-4)), highlighting the relatively high risk of As in crayfish. Our results suggest a need to consider crayfish when assessing human dietary exposure to metals and associated health risks, especially for high crayfish-consuming populations, such as in China, USA and Sweden.

Human exposure to methylmercury from crayfish (Procambarus clarkii) in China.

Methylmercury (MeHg) accumulation in aquatic food raises global concerns about human exposure to MeHg. Crayfish is the world's third largest farmed crustacean species and a favorite aquatic food in many countries. However, human health hazard due to MeHg exposure via crayfish consumption is unclear, partly because appropriate survey data are lacking. We report on mercury concentrations and speciation in edible tail muscle of crayfish collected from restaurants in 23 Chinese cities. On average, MeHg constituted 99.1 % of mercury in tail muscle, and MeHg concentrations were comparable with those reported for fish in China. Variation in MeHg concentrations was not attributable to broad geographic region (i.e., provinces) or tail length. For different populations, potential health risk (characterized by hazard quotient or HQ) of MeHg exposure through crayfish consumption depended largely on crayfish consumption rates. In particular, a health hazard (HQ > 1) was found for high-rate consumers (i.e., 95 %ile or higher) in some cities in the middle and lower reaches of the Yangtze River (MLYR), during the peak consumption season. Our results suggest that more attention should be paid to dietary MeHg intake via crayfish consumption in China, particularly for communities with high consumption in MLYR.

Soil geochemistry and digestive solubilization control mercury bioaccumulation in the earthworm Pheretima guillemi.

Mercury presents a potential risk to soil organisms, yet our understanding of mercury bioaccumulation in soil dwelling organisms is limited. The influence of soil geochemistry and digestive processes on both methylmercury (MeHg) and total mercury (THg) bioavailability to earthworms (Pheretima guillemi) was evaluated in this study. Earthworms were exposed to six mercury-contaminated soils with geochemically contrasting properties for 36 days, and digestive fluid was concurrently collected to solubilize soil-associated mercury. Bioaccumulation factors were 7.5-31.0 and 0.2-0.6 for MeHg and THg, respectively, and MeHg accounted for 17-58% of THg in earthworm. THg and MeHg measured in soils and earthworms were negatively associated with soil total organic carbon (TOC). Earthworm THg and MeHg also increased with increasing soil pH. The proportion of MeHg and THg released into the digestive fluid (digestive solubilizable mercury, DSM) was 8.3-18.1% and 0.4-1.3%, respectively. The greater solubilization of MeHg by digestive fluid than CaCl2, together with a biokinetic model-based estimate of dietary MeHg uptake, indicated the importance of soil ingestion for MeHg bioaccumulation in earthworms.

A tiered assessment framework to evaluate human health risk of contaminated sediment.

For sediment contaminated with bioaccumulative pollutants (e.g., PCBs and organochorine pesticides), human consumption of seafood that contain bioaccumulated sediment-derived contaminants is a well-established exposure pathway. Historically, regulation and management of this bioaccumulation pathway has focused on site-specific risk assessment. The state of California (United States) is supporting the development of a consistent and quantitative sediment assessment framework to aid in interpreting a narrative objective to protect human health. The conceptual basis of this framework focuses on 2 key questions: 1) do observed pollutant concentrations in seafood from a given site pose unacceptable health risks to human consumers? and 2) is sediment contamination at a site a significant contributor to seafood contamination? The first question is evaluated by interpreting seafood tissue concentrations at the site, based on health risk calculations. The second question is evaluated by interpreting site-specific sediment chemistry data using a food web bioaccumulation model. The assessment framework includes 3 tiers (screening assessment, site assessment, and refined site assessment), which enables the assessment to match variations in data availability, site complexity, and study objectives. The second and third tiers use a stochastic simulation approach, incorporating information on variability and uncertainty of key parameters, such as seafood contaminant concentration and consumption rate by humans. The framework incorporates site-specific values for sensitive parameters and statewide values for difficult to obtain or less sensitive parameters. The proposed approach advances risk assessment policy by incorporating local data into a consistent region-wide problem formulation, applying best available science in a streamlined fashion.

Evidence for photochemical and microbial debromination of polybrominated diphenyl ether flame retardants in San Francisco Bay sediment.

Brominated diphenyl ethers (BDEs) are flame retardant compounds that have been classified as persistent organic pollutants under the Stockholm Convention and targeted for phase-out. Despite their classification as persistent, PBDEs undergo debromination in the environment, via both microbial and photochemical pathways. We examined concentrations of 24 PBDE congeners in 233 sediment samples from San Francisco Bay using Positive Matrix Factorization (PMF). PMF analysis revealed five factors, two of which contained high proportions of congeners with two or three bromines, indicating that they are related to debromination processes. One of the factors included PBDE 15 (4,4'-dibromo diphenyl ether, comprising 20% of the factor); the other included PBDE 7 (2,4-dibromo diphenyl ether; 12%) and PBDE 17 (2,2',4-tribromo diphenyl ether; 16%). The debromination processes that produce these congeners are probably photochemical debromination and anaerobic microbial debromination, although other processes could also be responsible. Together, these two debromination factors represent about 8% of the mass and 13% of the moles of PBDEs in the data matrix, suggesting that PBDEs undergo measurable degradation in the environment.

Predictors of mercury spatial patterns in San Francisco Bay forage fish.

Pollution reduction efforts should be targeted toward those sources that result in the highest bioaccumulation. For mercury (Hg) in estuaries and other complex water bodies, carefully designed biosentinel monitoring programs can help identify predictors of bioaccumulation and inform management priorities for source reduction. This study employed a probabilistic forage fish Hg survey with hypothesis testing in San Francisco Bay (California, USA). The goal was to determine what pollution sources, regions, and landscape features were associated with elevated Hg bioaccumulation. Across 99 sites, whole-body Hg concentrations in Mississippi silversides (Menidia audens) and topsmelt (Atherinops affinis) followed a broad spatial gradient, declining with distance from the Guadalupe River (Pearson's r = -0.69 and -0.42, respectively), which drains historic mining areas. Site landscape attributes and local Hg sources had subtle effects, which differed between fish species. Topsmelt Hg increased in embayment sites (i.e., enclosed sites including channels, creek mouths, marinas, and coves) and sites with historic Hg-contaminated sediment, suggesting an influence of legacy industrial and mining contamination. In 2008, Mississippi silverside Hg was reduced at sites draining wastewater-treatment plants. Fish Hg was not related to abundance of surrounding wetland cover but was elevated in some watersheds draining from historic Hg-mining operations. Results indicated both regional and site-specific influences for Hg bioaccumulation in San Francisco Bay, including legacy contamination and proximity to treated wastewater discharge.

Seasonal and annual trends in forage fish mercury concentrations, San Francisco Bay.

San Francisco Bay is contaminated by mercury (Hg) due to historic and ongoing sources, and has elevated Hg concentrations throughout the aquatic food web. We monitored Hg in forage fish to indicate seasonal and interannual variations and trends. Interannual variation and long-term trends were determined by monitoring Hg bioaccumulation during September-November, for topsmelt (Atherinops affinis) and Mississippi silverside (Menidia audens) at six sites, over six years (2005 to 2010). Seasonal variation was characterized for arrow goby (Clevelandia ios) at one site, topsmelt at six sites, and Mississippi silverside at nine sites. Arrow goby exhibited a consistent seasonal pattern from 2008 to 2010, with lowest concentrations observed in late spring, and highest concentrations in late summer or early fall. In contrast, topsmelt concentrations tended to peak in late winter or early spring and silverside seasonal fluctuations varied among sites. The seasonal patterns may relate to seasonal shifts in net MeHg production in the contrasting habitats of the species. Topsmelt exhibited an increase in Alviso Slough from 2005 to 2010, possibly related to recent hypoxia in that site. Otherwise, directional trends for Hg in forage fish were not observed. For topsmelt and silverside, the variability explained by year was relatively low compared to sampling station, suggesting that interannual variation is not a strong influence on Hg concentrations. Although fish Hg has shown long-term declines in some ecosystems around the world, San Francisco Bay forage fish did not decline over the six-year monitoring period examined.

Polychlorinated biphenyl spatial patterns in San Francisco Bay forage fish.

Industrialized waterways frequently contain nearshore hotspots of legacy polychlorinated biphenyl (PCB) contamination, with uncertain contribution to aquatic food web contamination. We evaluated the utility of estuarine forage fish as biosentinel indicators of local PCB contamination across multiple nearshore sites in San Francisco Bay. Topsmelt (Atherinops affinis) or Mississippi silverside (Menidia audens) contamination was compared between 12 targeted sites near historically polluted locations and 17 probabilistically chosen sites representative of ambient conditions. The average sum of 209 PCB congeners in fish from targeted stations (441±432 ng g(-1) wet weight, mean±SD) was significantly higher than probabilistic stations (138±94 ng g(-1)). Concentrations in both species were comparable to those of high lipid sport fish in the Bay, strongly correlated with spatial patterns in sediment contamination, and above selected literature thresholds for potential hazard to fish and wildlife. The highest concentrations were from targeted Central Bay locations, including Hunter's Point Naval Shipyard (1347 ng g(-1); topsmelt) and Stege Marsh (1337 ng g(-1); silverside). Targeted sites exhibited increased abundance of lower chlorinated congeners, suggesting local source contributions, including Aroclor 1248. These findings indicate that current spatial patterns in PCB bioaccumulation correlate with historical sediment contamination due to industrial activity. They also demonstrate the utility of naturally occurring forage fish as biosentinels of localized PCB exposure.

Nutrient supply and mercury dynamics in marine ecosystems: a conceptual model.

There is increasing interest and concern over the impacts of mercury (Hg) inputs to marine ecosystems. One of the challenges in assessing these effects is that the cycling and trophic transfer of Hg are strongly linked to other contaminants and disturbances. In addition to Hg, a major problem facing coastal waters is the impacts of elevated nutrient, particularly nitrogen (N), inputs. Increases in nutrient loading alter coastal ecosystems in ways that should change the transport, transformations and fate of Hg, including increases in fixation of organic carbon and deposition to sediments, decreases in the redox status of sediments and changes in fish habitat. In this paper we present a conceptual model which suggests that increases in loading of reactive N to marine ecosystems might alter Hg dynamics, decreasing bioavailabilty and trophic transfer. This conceptual model is most applicable to coastal waters, but may also be relevant to the pelagic ocean. We present information from case studies that both support and challenge this conceptual model, including marine observations across a nutrient gradient; results of a nutrient-trophic transfer Hg model for pelagic and coastal ecosystems; observations of Hg species, and nutrients from coastal sediments in the northeastern U.S.; and an analysis of fish Hg concentrations in estuaries under different nutrient loadings. These case studies suggest that changes in nutrient loading can impact Hg dynamics in coastal and open ocean ecosystems. Unfortunately none of the case studies is comprehensive; each only addresses a portion of the conceptual model and has limitations. Nevertheless, our conceptual model has important management implications. Many estuaries near developed areas are impaired due to elevated nutrient inputs. Widespread efforts are underway to control N loading and restore coastal ecosystem function. An unintended consequence of nutrient control measures could be to exacerbate problems associated with Hg contamination. Additional focused research and monitoring are needed to critically examine the link between nutrient supply and Hg contamination of marine waters.

Mercury isotopes link mercury in San Francisco Bay forage fish to surface sediments.

Identification of sources of biologically accessible Hg is necessary to fully evaluate Hg exposure in aquatic ecosystems. This study assesses the relationship between Hg in forage fish and Hg in surface sediments throughout San Francisco Bay (SF Bay) and evaluates processes influencing the incorporation of Hg into the aquatic food web. We measured the Hg stable isotope compositions of two nearshore fish species and compared them with previously reported analyses of colocated intertidal surface sediments. Fish δ(202)Hg values (mass-dependent fractionation) demonstrated a distinct spatial gradient within SF Bay that ranged from 0.60‰ in the south to -0.25‰ in the north. Fish δ(202)Hg values were consistently higher than sediment δ(202)Hg values by 0.73‰ (±0.16‰, 1SD). Fish and sediment δ(202)Hg values in SF Bay proper were well correlated (r(2) = 0.83), suggesting that sediment is a primary source of Hg to the nearshore aquatic food web. Fish Δ(199)Hg values (mass-independent fractionation) ranged from 0.46‰ to 1.55‰, did not correlate with sediment values, and yielded a Δ(199)Hg/Δ(201)Hg ratio of 1.26 (±0.01, 1SD; r(2) = 0.99). This mass-independent fractionation is consistent with photodegradation of MeHg to varying degrees at each site prior to incorporation into the food web.