Bioaccumulation and biomagnification of mercury in African lakes: The importance of trophic status

Amanda E Poste,Derek C.G Muir,Stephanie J Guildford,Robert E Hecky

doi:10.1016/j.scitotenv.2014.10.094

Abstract

Despite the global prevalence of both mercury (Hg) contamination and anthropogenic eutrophication, relatively little is known about the behavior of Hg in eutrophic and hypereutrophic systems or the effects of lake trophic status on Hg uptake and trophodynamics. In the current study we explore Hg trophodynamics at 8 tropical East African study sites ranging from mesotrophic to hypereutrophic, in order to assess the influence of lake trophic status on Hg uptake and biomagnification. Comprehensive water, plankton and fish samples were collected for analysis of total mercury (THg) and stable carbon and nitrogen isotopic ratios. We found evidence that uptake of THg into phytoplankton tended to be lower in higher productivity systems. THg concentrations in fish were generally low, and THg trophic magnification factors (TMFs; representing the average increase in contaminant concentrations from one trophic level to the next) ranged from 1.9 to 5.6. Furthermore TMFs were significantly lower in hypereutrophic lakes than in meso- and eutrophic lakes, and were negatively related to chlorophyll a concentrations both across our study lakes, and across African lakes for which literature data were available. These observations suggest that THg concentrations were strongly influenced by trophic status, with year-round high phytoplankton and fish growth rates reducing the potential for high THg in fish in these productive tropical lakes.

Highlights

Several studies over the past decade have greatly increased our understanding of mercury (Hg) concentrations and trophodynamics in arctic, temperate and tropical systems (Campbell et al, 2003a; Swanson and Kidd, 2010; Kidd et al, 2012; Clayden et al, 2013; Lavoie et al, 2013)
We found that much of the variability in TMF across all sites could be explained by the negative relationship between TMF and chlorophyll a (r2 = 0.67, P = 1.88 × 10−5; Fig. 3b), and as such, in tropical African lakes, trophic status appears to be an important driver of total Hg (THg) uptake at the base of the food web, but of TMFs as well
Several studies have observed a negative relationship between lake primary productivity and fish Hg concentrations (Lange et al, 1993; Kidd et al, 1999; Rypel, 2010), the current study identifies a strong negative relationship between productivity and TMF, indicating that in these tropical African lakes, biomagnification of Hg may be moderated by lake trophic status

Summary

Introduction

Several studies over the past decade have greatly increased our understanding of mercury (Hg) concentrations and trophodynamics in arctic, temperate and tropical systems (Campbell et al, 2003a; Swanson and Kidd, 2010; Kidd et al, 2012; Clayden et al, 2013; Lavoie et al, 2013). It has been noted that in tropical African lakes, Hg concentrations in fish are ‘anomalously’ low relative to fish from temperate systems, despite total Hg (THg) concentrations in water. A.E. Poste et al / Science of the Total Environment 506–507 (2015) 126–136 where nutrient loading has been reduced. Eutrophic lakes often support highly productive fisheries, which are a source of daily subsistence upon which riparian populations can be heavily dependent, leading to natural concern about Hg concentrations in these systems

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Conclusion