Midges deliver aquatic resources and enrich terrestrial arthropod food webs

Background/Question/Methods

 

Aquatic insects are a major conduit of resources moving from freshwater to riparian habitats and can subsidize terrestrial food webs.  However little is known about the cumulative effects of several years of aquatic insect deposition.  Mývatn, a lake in northern Iceland, has extraordinary midge (Chironomidae) emergences that result in large inputs of biomass and nutrients to terrestrial arthropod communities.  We examined the effects of midges on riparian arthropod food webs over 3 years by blocking midge inputs and measuring arthropod densities and stable isotope (δ¹³C, δ¹⁵N) responses in paired 4-m² control and midge-exclusion plots.  Midge exclusion treatments effectively eliminated midge deposition while allowing ground level predators and detritivores to move between plots.

Results/Conclusions

Densities of terrestrial arthropods in our plots were unaffected by midge exclusion.  However, the δ¹³C values (an indicator of aquatic vs. terrestrial C sources) of the terrestrial arthropod community were significantly enriched in control (midge-access) compared to midge-exclusion plots.  This pattern was consistent across time and for a variety of predatory and detritivorous consumers and indicates a reliance on aquatic-derived resources (midges) in open control plots.  Reliance on aquatic resources varied based on trophic position and taxonomic group, presumably due to mobility and diet breadth.  The ability of arthropod populations in midge-exclusion plots to maintain densities equal to those with access to midge-derived resources suggests that other resources derived from terrestrial primary production (i.e., living and dead plant biomass) is sufficient to sustain the arthropod food web in the absence of midges for at least 3 years.  Accumulated plant biomass may be a result of long-term (1000 yrs.) midge deposition at this site and represents a key reservoir for storage of aquatic-derived nutrients in the terrestrial system.  Aquatic insect deposition delivers nutrients that can stimulate primary productivity and thus promote the accumulation of plant biomass, the development of arthropod communities and soil formation.