Bioaccumulation syndrome: identifying factors that make some stream food webs prone to elevated mercury bioaccumulation

Abstract

Mercury is a ubiquitous contaminant in aquatic ecosystems, posing a significant health risk to humans and wildlife that eat fish. Mercury accumulates in aquatic food webs as methylmercury (MeHg), a particularly toxic and persistent organic mercury compound. While mercury in the environment originates largely from anthropogenic activities, MeHg accumulation in freshwater aquatic food webs is not a simple function of local or regional mercury pollution inputs. Studies show that even sites with similar mercury inputs can produce fish with mercury concentrations ranging over an order of magnitude. While much of the foundational work to identify the drivers of variation in mercury accumulation has focused on freshwater lakes, mercury contamination in stream ecosystems is emerging as an important research area. Here, we review recent research on mercury accumulation in stream-dwelling organisms. Taking a hierarchical approach, we identify a suite of characteristics of individual consumers, food webs, streams, watersheds, and regions that are consistently associated with elevated MeHg concentrations in stream fish. We delineate a conceptual, mechanistic basis for explaining the ecological processes that underlie this vulnerability to MeHg. Key factors, including suppressed individual growth of consumers, low rates of primary and secondary production, hydrologic connection to methylation sites (e.g., wetlands), heavily forested catchments, and acidification are frequently associated with increased MeHg concentrations in fish across both streams and lakes. Hence, we propose that these interacting factors define a syndrome of characteristics that drive high MeHg production and bioaccumulation rates across these freshwater aquatic ecosystems. Finally, based on an understanding of the ecological drivers of MeHg accumulation, we identify situations when anthropogenic effects and management practices could significantly exacerbate or ameliorate MeHg accumulation in stream fish.

Figure 1

Conceptual map of factors that affect methylmercury (MeHg) in individual fish. Environmental and ecological factors are in boxes, components of the mercury cycle are in ovals. Arrows between boxes indicate causal relationships. Arrows within boxes indicate the direction of change.

Figure 2

Relationship between mean mercury (Hg) concentrations in salmon and mercury concentrations in their prey (left panel) and relationship between the residuals from the salmon-prey regression, an index of trophic transfer, and salmon growth as mean final mass (right panel). Each point is a study site and different symbols indicate different years (■: 2005; +: 2006; ×:2007; □: 2008). Solid lines are simple linear fits to pooled data. The dashed line is the 1:1 relationship between salmon and prey Hg concentrations. Based on a subset of samples analyzed for Hg speciation, 96% of Hg in salmon and 85% of mercury in prey is methylmercury.

Figure 3

Expanded conceptual map showing factors at the food web level, with relationships at this level highlighted with heavy arrows. Symbols as in Figure 1.

Figure 4

Relationship between prey biomass and canopy cover (left panel) and prey mercury (Hg) concentration and prey biomass (right panel). Symbols as in Figure 2.

Figure 5

Expanded conceptual map showing factors at the stream level, with relationships at this level highlighted with heavy arrows. Symbols as in Figure 1.

Figure 6

Relationship between salmon salmon Hg concentration and pH (left panel) and prey Hg concentration and pH (right panel). Symbols as in Figure 2.

Figure 7

Expanded conceptual map showing factors at the watershed level, with relationships at this level highlighted with heavy arrows. Symbols as in Figure 1.

Figure 8

Relationships between salmon mercury (Hg) concentration and the percent forest cover (left panel) and wetland cover (right panel) in the watershed. Symbols as in Figure 2. The bracket indicates the range in small-fish Hg concentrations observed by Chasar et al. .

Figure 9

Expanded conceptual map showing factors at the region level, with relationships at this level highlighted with heavy arrows. Symbols as in Figure 1.