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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: 9.81). 06/2007; 104(18):7477-82. DOI: 10.1073/pnas.0611261104
Source: PubMed

ABSTRACT 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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    • "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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    • "As an example, Riget et al. (2000) found that Hg concentrations were 10–153 higher in lake-resident Arctic char Salvelinus alpinus than in co-occurring anadromous individuals. In addition, given all else equal, animals that grow slowly and inefficiently have been shown to accumulate more Hg per unit biomass than animals that grow quickly and efficiently , a phenomenon known as growth dilution (Simoneau et al. 2005; Karimi et al. 2007). Given the differences in growth rates and feeding ecology of ammocoetes (slow, inefficient growth and poor quality food resources) versus parasitic-phase (rapid, efficient growth on a high quality food source) lamprey , growth dilution may play an important role in determining relative Hg concentrations and the magnitude and direction of Hg transport in lamprey and anadromous fish in general. "
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    • "The increase of individual abundance is associated with a lower degree of trace metal partitioning (Chen and Folt, 2005; Luengen and Flegal, 2009). This dilution phenomenon has been reported for rapidly growing algal blooms, which resulted in lower Hg concentrations in both algae and higher trophic level plankton (Sunda and Huntsman, 1998; Chen and Folt, 2005; Pickhardt et al., 2005; Karimi et al., 2007; Pickhardt and Fisher, 2007). Organism size can also influence Hg uptake efficiency, with smaller organisms accumulating trace metals more rapidly than larger organisms , due to the greater surface area to volume ratio (S:V) of the former (Fisher et al., 1983). "
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    ABSTRACT: In lake foodwebs, pelagic basal organisms such as bacteria and phytoplankton incorporate mercury (Hg2+) from the dissolved phase and pass the adsorbed and internalized Hg to higher trophic levels. This experimental investigation addresses the incorporation of dissolved Hg2+ by four plankton fractions (picoplankton: 0.2–2.7 μm; pico + nanoplankton: 0.2–20 μm; microplankton: 20–50 μm; andmesoplankton: 50–200 μm)obtained fromfour Andean Patagonian lakes, using the radioisotope 197Hg2+. Species composition and abundancewere determined in each plankton fraction. In addition, morphometric parameters such as surface and biovolume were calculated using standard geometric models. The incorporation of Hg2+ in each plankton fraction was analyzed through three concentration factors: BCF (bioconcentration factor) as a function of cell or individual abundance, SCF (Surface concentration factor) and VCF (volume concentration factor) as functions of individual exposed surface and biovolume, respectively. Overall, this investigation showed that through adsorption and internalization, pico + nanoplankton play a central role leading the incorporation of Hg2+ in pelagic foodwebs of Andean lakes. Larger planktonic organisms included in the micro- and mesoplankton fractions incorporate Hg2+ by surface adsorption, although at a lesser extent. Mixotrophic bacterivorous organisms dominate the different plankton fractions of the lakes connecting trophic levels through microbial loops (e.g., bacteria–nanoflagellates–crustaceans; bacteria–ciliates–crustaceans; endosymbiotic algae–ciliates). These bacterivorous organisms, which incorporate Hg from the dissolved phase and through their prey, appear to explain the high incorporation of Hg2+ observed in all the plankton fractions.
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