Comparing nearshore benthic and pelagic prey as mercury sources to lake fish: the importance of prey quality and mercury content

Abstract

Mercury (Hg) bioaccumulation in fish poses well-known health risks to wildlife and humans through fish consumption. Yet fish Hg concentrations are highly variable, and key factors driving this variability remain unclear. One little studied source of variation is the influence of habitat-specific feeding on Hg accumulation in lake fish. However, this is likely important because most lake fish feed in multiple habitats during their lives, and the Hg and caloric content of prey from different habitats can differ. This study used a three-pronged approach to investigate the extent to which habitat-specific prey determine differences in Hg bioaccumulation in fish. This study first compared Hg concentrations in common nearshore benthic invertebrates and pelagic zooplankton across five lakes and over the summer season in one lake, and found that pelagic zooplankton generally had higher Hg concentrations than most benthic taxa across lakes, and over a season in one lake. Second, using a bioenergetics model, the effects of prey caloric content from habitat-specific diets on fish growth and Hg accumulation were calculated. This model predicted that the consumption of benthic prey results in lower fish Hg concentrations due to higher prey caloric content and growth dilution (high weight gain relative to Hg from food), in addition to lower prey Hg levels. Third, using data from the literature, links between fish Hg content and the degree of benthivory, were examined, and showed that benthivory was associated with reduced Hg concentrations in lake fish. Taken together, these findings support the hypothesis that higher Hg content and lower caloric content make pelagic zooplankton prey greater sources of Hg for fish than nearshore benthic prey in lakes. Hence, habitat-specific foraging is likely to be a strong driver of variation in Hg levels within and between fish species.

Keywords: Aquatic food webs; Bioenergetics; Biomagnification; Contaminants; Growth dilution; Trophic transfer.

Fig. 1

Mean Hg concentrations (±SE) in zooplankton (2001) and benthic invertebrates (2003) in three lakes (n = 3 per taxon per lake; n = 2 for snails in Gregg Lake, and amphipods in Horseshoe and Post Pond). Groups with the same letter indicate nonsignificant differences. Taxa include amphipods (AMPH), chironomids (CHIR), dragonflies (libellulidae, DRA-L and gomphidae, DRA-G), snails (planorbidae, SNA-P and viviparidae, SNA-V), damselflies (DAMS), crayfish (CRA), unionids (UNIO) and zooplankton (ZOOP).

Fig. 2

Seasonal Hg concentrations (ng g⁻¹ dry weight) in zooplankton, coenagrionid odonates (damselflies) and amphipods from Post Pond, 2004. Means ± SE are shown for each month, n = 3 for each taxon per month. Filled squares show the proportion of Daphnia comprising the zooplankton assemblage at each time point (based on μg dry weight L⁻¹).

Fig. 3

The predicted range of Hg concentrations for given fish sizes based on four habitat-specific diet scenarios.

Fig. 4

The effect of invertebrate prey caloric content on fish growth rate and fish Hg concentration.

Fig. 5

PCA biplot on mean values across fish species. Hg content is negatively related to benthivory, and positively related to fish length and trophic position.