Trophic structure and function of stream food webs along a gradient of metal contamination

Abstract

Bioenergetics and food webs have long been central themes in ecology and have greatly expanded our understanding of nature. Within a food web framework, estimates of consumption and energy flow through populations facilitate linking population-level contaminant effects to ecosystem-level effects. For this purpose, we used estimates of population production, diet composition, and bioenergetic principles of aquatic insects to quantify food web linkages in five Rocky Mountain headwater streams that varied in metal contamination. Our overall goals were to 1) link the effects of contaminants on individuals and populations to effects on species assemblages, functional guilds, and ecosystem process; 2) determine the influence of food web and assemblage structure on ecosystem functions; and 3) determine the role of species in ecosystem processes. Quantitative sampling of benthic macroinvertebrates, chemistry, and habitat occurred monthly from May-November. We explicitly quantified uncertainty in our analyses by bootstrapping samples of population density, biomass, diet composition, and energetic efficiencies. Relative to reference streams, population production of metal-sensitive mayflies was lower in light and moderately polluted streams, and absent in the stream with highest Zn levels. Although periphyton food quality and quantity were lower in polluted streams than in reference streams, individual growth of herbivorous mayflies was similar among streams. We therefore concluded that among-stream variation in population production resulted from differences in population abundances caused by metal sensitivity. Low production and consumption of heptageniids had a large impact on ecosystem functions because these taxa dominate energetic fluxes from primary producers to the remaining food web. Slight reductions in heptageniid production were associated with lower total energy flow in a stream with metal levels considered safe for aquatic life. A similar pattern was observed for leaf-shredding insects and leaf litter decomposition. Collectively, our results indicate that some (e.g., energetic fluxes from herbivory and leaf litter decomposition) ecosystem processes in Rocky Mountain streams are controlled by a small number of species, many of which are sensitive to anthropogenic stressors. Consequently, even relatively small reductions in population abundances and species composition may have far-reaching effects on ecosystem functions.