Aquatic insects provide an energy subsidy to riparian food webs. However, most empirical studies have considered the role of subsidies only in terms of magnitude (using biomass measurements) and quality (using physiologically important fatty acids), negating an aspect of subsidies that may affect their impact on recipient food webs: the potential of insects to transport contaminants (e.g., mercury) to terrestrial ecosystems. To this end, I used empirical data to estimate the magnitude of nutrients (using physiologically important fatty acids as a proxy) and contaminants (total mercury (Hg) and methylmercury (MeHg)) exported by insects from rivers and lacustrine systems in each continent. The results reveal that North American rivers may export more physiologically important fatty acids per unit area (93.0 ± 32.6 Kg Km-2 year-1) than other continents. Owing to the amount of variation in Hg and MeHg, there were no significant differences in MeHg and Hg among continents in lakes (Hg: 1.5 × 10-4 to 1.0 × 10-3 Kg Km-2 year-1; MeHg: 7.7 × 10-5 to 1.0 × 10-4 Kg Km-2 year-1) and rivers (Hg: 3.2 × 10-4 to 1.1 × 10-3 Kg Km-2 year-1; MeHg: 3.3 × 10-4 to 8.9 × 10-4 Kg Km-2 year-1), with rivers exporting significantly larger quantities of mercury across all continents than lakes. Globally, insect export of physiologically important fatty acids by insect was estimated to be ~43.9 × 106 Kg year-1 while MeHg was ~649.6 Kg year-1. The calculated estimates add to the growing body of literature, which suggests that emerging aquatic insects are important in supplying essential nutrients to terrestrial consumers; however, with the increase of pollutants in freshwater systems, emergent aquatic insect may also be sentinels of organic contaminants to terrestrial consumers.
Keywords: aquatic ecosystems; docosahexaenoic acid; eicosapentaenoic acid; food webs; subsidies.