Stoichiometric controls of mercury dilution by growth

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 (Ϸ95%) 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.contaminants ͉ food quality ͉ heavy metals ͉ nutrient stoichiometry ͉ plankton M ethylmercury (MeHg) poses a serious human and wildlife health risk primarily through fish consumption, so understanding the key factors driving MeHg accumulation in fish has become a global priority (1). Rapid somatic growth rates are hypothesized to reduce mass-specific MeHg concentration (burden) in fish and other aquatic organisms (2, 3) by the process of somatic growth dilution (SGD). SGD occurs when rapid growth results in a disproportionate increase in the net rate of biomass gain relative to MeHg gain. The relative quality of food consumed can strongly influence growth rates in aquatic organisms. Moreover, food quality for aquatic consumers varies widely across lakes and seasons (4), thus potentially contributing to the large variation in Hg concentrations observed in lake fish in situ. Despite its potential importance, the influence of food quality on MeHg accumulation has not been well examined.At present, evidence for somatic growth dilution of MeHg, whether due food quality or other factors, is sparse and somewhat contradictory. Thus far, our understanding of SGD has been limited to inferences drawn from field correlations between somatic growth rates and Hg concentrations in fish. In most of these studies, the many possible factors driving SGD (e.g., temperature, food availability, food quality, activity level, and stress) are not controlled and are often confounded. Negative correlations between somatic growth rate and concentrations of total Hg (5-8) and other contaminants, such as Pb (9

The fate of mercury in Arctic terrestrial and aquatic ecosystems, a review

“…Individuals with faster growth rates will have lower MeHg concentrations -the so-called 'biodilution effect'. [163][164][165][166] Once MeHg has been incorporated into microbial communities at the base of the food chain, subsequent biomagnification factors for MeHg concentrations at successively higher trophic levels range from ,4 to 10 (see Fig. 7b).…”

Mercury and Arsenic Bioaccumulation and Eutrophication in Baiyangdian Lake, China

“…Moreover, fish that consume benthic prey appear to bioaccumulate lower Hg burdens than those predominantly consuming pelagic prey [Watras et al, 1998;Power et al, 2002;Essington and Houser, 2003;Gorski et al, 2003]. Metal bioaccumulation within a trophic level is also related to nutrient status, algal densities, and growth rates [Pickhardt et al, 2002;Karimi et al, 2007].…”

Seasonal Variations in the Use of Profundal Habitat among Freshwater Fishes in Lake Norsjø, Southern Norway, and Subsequent Effects on Fish Mercury Concentrations

“…This seasonal pattern is likely caused by ABD and SGD [11,[15][16][17][18][19][20][21], as dilution lowers Tot-Hg-concentration during the growth season (summer). A. charr may then be starving during winter, which leads to near significantly higher Tot-Hg-concentrations [22], and significantly higher Tot-Hg-concentrations in spring before the onset of the growth season.…”

“…In addition to trophic position, Hg-concentrations in fish are well documented to increase with increasing age [8,11] and length [8,[11][12][13][14]. Contrarily, increasing weight at the same length or age results in lower Hg-concentrations, either by somatic growth dilution (SGD) [11,[15][16][17][18][19][20][21], or by further concentrating Hg during starvation [22]. The combination of these two effects results in seasonal variations in Hg-concentrations in fish [23][24][25][26][27][28][29][30], however, some studies suggest that this is not the case in all populations [31][32][33].…”