Effect of stream degradation on PUFA export via emergent insects and transfer to riparian ground-dwelling spiders

Background/Question/Methods

During the last century, freshwater systems have undergone drastic alterations, potentially affecting cross-boundary resource transfers between aquatic and terrestrial ecosystems. One important connection likely affected is the export of omega-3 polyunsaturated fatty acids (PUFAs), especially eicosapentaenoic acid (EPA), by emergent aquatic insects. Because of their high omega-3 PUFA content, aquatic subsidies have been shown to be crucial for riparian specialists such as tree swallows. Besides total biomass, the contribution of different insect groups to export should be considered because of taxon-specific differences in PUFA content and in functional traits. One functional traits that should be investigated in this context is the mode of emergence. Stoneflies, in contrast to other aquatic insects, emerge by crawling to land instead of flying away directly from the water surface, making them more accessible to ground-dwelling predators. To fulfil EPA requirements riparian ground-dwelling predators, such as wolf spiders, might therefore need stoneflies. Stream degradation can cause a mismatch of available and required nutrients. In this study, we estimated emergent insect biomass and EPA export along two streams with different levels of habitat degradation. We used floating emergence traps to estimate biomass export and analyzed the fatty acid content of emergent insects and riparian ground-dwelling spiders.

Results/Conclusions

The EPA content in aquatic insects did not differ with the degree of habitat degradation and the total biomass export was also unaffected. However, habitat degradation substantially altered the contribution of crawling emergence to the total export in spring, with no biomass export by stoneflies at the most degraded, but up to 10 mg m-2 day-1 biomass export by stoneflies at more natural sites. The EPA content in ground-dwelling spiders was correlated with emergent stonefly biomass, making up on average only 18 % of total fatty acids at sites with no stonefly emergence, but 24 % of total fatty acids at the sites with highest stonefly emergence. Because immune function in ground-dwelling spiders has been connected to EPA levels, reduced crawling emergence might impact spider fitness and thus a loss of stoneflies and this important pathway leading to ground-dwelling predators might influence floodplain assemblages. Functional traits, like emergence mode as well as the nutritional quality of emergent aquatic insects, should be considered when assessing the overall effect of stream degradation on adjacent terrestrial ecosystems.