Abstract
Low productivity in aquatic ecosystems is associated with reduced individual growth of fish and increased concentrations of methylmercury (MeHg) in fish and their prey. However, many stream-dwelling fish species can use terrestrially-derived food resources, potentially subsidizing growth at low-productivity sites, and, because terrestrial resources have lower MeHg concentrations than aquatic resources, preventing an increase in diet-borne MeHg accumulation. We used a large-scale field study to evaluate relationships among terrestrial subsidy use, growth, and MeHg concentrations in two stream-dwelling fish species across an in-stream productivity gradient. We sampled young-of-the-year brook trout (Salvelinus fontinalis) and Atlantic salmon (Salmo salar), potential competitors with similar foraging habits, from 20 study sites in streams in New Hampshire and Massachusetts that encompassed a wide range of aquatic prey biomass. Stable isotope analysis showed that brook trout used more terrestrial resources than Atlantic salmon. Over their first growing season, Atlantic salmon tended to grow larger than brook trout at sites with high aquatic prey biomass, but brook grew two-fold larger than Atlantic salmon at sites with low aquatic prey biomass. The MeHg concentrations of brook trout and Atlantic salmon were similar at sites with high aquatic prey biomass and the MeHg concentrations of both species increased at sites with low prey biomass and high MeHg in aquatic prey. However, brook trout had three-fold lower MeHg concentrations than Atlantic salmon at low-productivity, high-MeHg sites. These results suggest that differential use of terrestrial resource subsidies reversed the growth asymmetry between potential competitors across a productivity gradient and, for one species, moderated the effect of low in-stream productivity on MeHg accumulation.
Highlights
Stream food webs are tightly linked to terrestrial food webs in nearby riparian zones through the exchange of resource subsidies [1]
Mean brook trout d13C values were closer to predictions for a terrestrial diet and mean Atlantic salmon d13C values were closer to predictions for an aquatic diet
Brook trout d15N did not correspond as closely with predicted d15N for a fully aquatic diet as Atlantic salmon (Figure 1), and the regression relationship between brook trout d15N and aquatic prey d15N (brook trout d15N = 6.14+0.22, r2 = 0.29, RMSE = 0.69, F1,18 = 7.5, P = 0.01) was weaker than that for salmon (Atlantic salmon d15N = 5.5+0.61, r2 = 0.83, RMSE = 0.57, F1,18 = 90.1, P,0.0001). Both the intercepts (t = 5.9, df = 36, P,0.001) and slopes (t = 3.9, df = 36, P,0.001) of the d15N regressions were significantly different between brook trout and salmon
Summary
Stream food webs are tightly linked to terrestrial food webs in nearby riparian zones through the exchange of resource subsidies [1]. Many stream-dwelling salmon and trout species (Family Salmonidae, or salmonids) eat terrestrial insects that fall into streams, while terrestrial spiders and birds in riparian areas eat adult aquatic insects that emerge to breed [2]. Such subsidies are often energetically important for consumers, but they may be a key pathway for the transfer of toxic contaminants [3]. Methylmercury (MeHg) and other contaminants that accumulate predominantly in aquatic food webs are transferred into terrestrial food webs when terrestrial predators eat aquatic prey [4]. Resource subsidies go both directions, and very few studies have addressed the effect of the reciprocal subsidy of terrestrial prey into aquatic environments on accumulation of MeHg and other contaminants in fish [6]
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