Stoichiometric controls of mercury dilution by growth. Proc Natl Acad Sci USA

Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 06/2007; 104(18):7477-82. DOI: 10.1073/pnas.0611261104
Source: PubMed


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 (approximately 95%) 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.

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    • "Total mercury content as a function of mass for spiders collected within and upstream of the Buffalo River AOC Bull Environ Contam Toxicol due to the highly convoluted intestinal configuration and reliance on a liquefied diet (Hopkin 1989). Thus, tissue growth might lead to a biodilution effect in larger spiders (Karimi et al. 2007). This study suggests more information is needed on spider/Hg dynamics in natural settings. "
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    ABSTRACT: Riparian communities can receive environmental contaminants from adjacent aquatic 'donor' habitats. We investigated mercury biotransport from aquatic to terrestrial habitats via aquatic insect emergence and uptake by riparian spiders at sites within and upstream of the Buffalo River Area of Concern (AOC), a site with known sediment Hg contamination. Mercury concentration in emerging midges was roughly 10× less than contaminated sediment levels with the AOC, but biomagnification factors from midges to spiders ranged from 2.0 to 2.65 between sites. There was a significantly negative body mass:total mercury relationship in spiders (p < 0.001), indicating that mercury depuration is rapid or tissue dilution occurs in these riparian predators. Spiders contained significantly more mercury than their midge prey and spiders upstream of the AOC had higher mercury concentrations than spiders from within the AOC. Collectively, these data indicate that riparian spiders can be good mercury sentinels in urban environments, and that riparian communities upstream from the AOC may be at greater risk to mercury than has been previously considered.
    Bulletin of Environmental Contamination and Toxicology 09/2015; DOI:10.1007/s00128-015-1658-6 · 1.26 Impact Factor
    • "Higher concentrations for the open ocean, where plankton biomass are lower, are consistent with the idea that concentrations decrease as biomass increases, and growth rates increase (i.e. growth dilution; Karimi et al., 2007; Hammerschnidt et al., 2013; Driscoll et al., 2012). It is evident that phytoplanktons concentrate more Hg and MeHg from the water in the oligotrophic open ocean, suggesting that the MeHg may be more bioavailable in offshore waters. "
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    ABSTRACT: Open ocean samples of phytoplankton and zooplankton were collected from the central Pacific on board the R/V Kilo Moana in October of 2011. The cruise traveled from Hawaii to Samoa, progressing through a High Nutrient Low Chlorophyll (HNLC) zone, and an equatorial upwelling region. Phytoplankton samples were size fractioned into 0.2–5 μm, 5–20 μm, and N20 μm samples. Methylmercury concentrations were 2.91 ± 2.58 pmol g−1 (wet weight) for the overall b200 μm size fractions, and highest around the HNLC region. Phytoplankton bioconcentration factors (logBCFs) averaged to 5.69±0.98 and were higher than the values found for coastal regions. Both %MeHg ([MeHg]/[Hg]) and logBCF values indicated that the lowest size fraction had the largest fraction of the HgT as MeHg, signifying enhanced accumulation of MeHg into smaller organisms. Zooplankton vertical net tows were completed from depths of 200 m up to ~10 m. Zooplankton samples were analyzed for carbon, nitrogen and sulfur in addition to Hg and MeHg at size fractionations of 0.2–0.5 mm, 0.5–1.0 mm, 1.0–2.0 mm and occasionally >2.0 mm. Zooplankton abundance and MeHg concentrations both peaked at Stations 3 and 5 (upwelling region). The %MeHg in the organisms was highest in the >2.0 mm size class, displaying MeHg bioaccumulation for increasing zooplankton sizes. Separate day and night net tows were collected at Stations 3 and 5 in order to investigate differences due to diurnal migration of zooplankton. There were higher concentrations of MeHg for all sizes in night collections of zooplankton at Station 5, but no discrepancy for Station 3. These results represent some of the few measurements of Hg and MeHg at the base of the open ocean food chain, and are significant as they represent the instigation of bioconcentration into the base of marine food webs.
    Marine Chemistry 07/2015; DOI:10.1016/j.marchem.2015.07.005 · 2.74 Impact Factor
    • "Higher zooplankton densities can also decrease Hg transfer to fish through zooplankton density dilution (Chen and Folt 2005). Other possible mechanisms that can lower fish Hg concentrations in eutrophic systems include shifts in algal cell size (Pickhardt and Fisher 2007) and growth dilution in zooplankton and fish (Simoneau et al. 2005; Karimi et al. 2007). However, lower fish Hg concentrations with eutrophication are not always observed as evidenced by higher Hg concentrations in top predators from temperate eutrophic reservoirs (Stone et al. 2011). "
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    ABSTRACT: Eutrophication can have opposite effects on mercury (Hg) bioavailability in aquatic systems, by increasing methylmercury (MeHg) production but reducing Hg biomagnification. Globally, the effect of eutrophication on Hg dynamics remains largely untested at lower latitudes such as eastern China, a productive subtropical ecoregion with Hg emission and deposition rates that are among the highest worldwide. Here, we quantify Hg (both MeHg and total Hg, THg) concentrations, Hg bioaccumulation, and Hg biomagnification rates in reservoir food webs across a gradient of eutrophication indicated by chlorophyll a (Chl a), zooplankton density, and total phosphorus (TP). We also assess the effect of hydrogeomorphic (HGM) features on Hg concentrations in water and biota. Water THg concentrations were strongly correlated with TP and were greater in reservoirs at higher elevations with short water retention times (WRT). Zooplankton and top predator THg concentrations were negatively correlated with Chl a, suggesting algal biodilution; evidence for zooplankton density dilution was also found when subtropical reservoirs were compared at a global scale with temperate lakes. Mercury bioaccumulation and biomagnification factors, respectively, did not correlate with increasing Chl a or zooplankton density suggesting no effect of plankton density on Hg trophic transfer. In subtropical reservoirs, eutrophication is associated with lower Hg concentrations in biota but does not explain Hg biomagnification; HGM features (i.e., elevation, WRT) and land use (i.e., % crop) appear to also influence Hg concentrations and bioaccumulation in reservoir food webs.
    03/2015; 60(2). DOI:10.1002/lno.10036
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