Contribution of emerging aquatic insects to lateral transfers of Carbon and Nutrients: significance for the integrated landscape budget along a gradient of agricultural pressure

Biogeochemists usually consider the matter fluxes from land to streams and rivers and, no or negligible fluxes in the opposite direction. However, winged aquatic insects emerging from water bodies (e.g. lakes, ponds, perennial and ephemeral streams, wetlands) are recognized to provide nutritional subsidies to terrestrial consumers. Therefore, we hypothesized that nutrient (N, P) and carbon exports by emergent aquatic insects can be substantial to adjacent terrestrial ecosystems. Especially, we assumed these exports to increase with the density of the hydrographic network. Moreover, we advocate that the quantification of these lateral biotic fluxes of matter between terrestrial and aquatic ecosystems will improve element budgets at the landscape scale. We quantified these lateral fluxes to compare them to the input and output fluxes at the catchment scale for five sites along a gradient of agricultural intensification. The sites have different agricultural contexts and were selected within the French Long-Term Socio-Ecological Research network of Zones Ateliers (LTSER RZA, ZA Brest Iroise, Loire and Armorique). On each site, a headwater stream and its adjacent landscape were monitored for identification and quantification of emerging aquatic insects. Quantification of the corresponding biomass (abundance and diversity) relied on trapping insects using sticky traps at different distances from their aquatic sources during the emergence season. Input and output fluxes were determined for the smallest catchment nested within the sampling site for which agricultural and water quality data were available. Landscape features, agricultural pressures, and when available data on stream water quality were compared within nested catchments. Our results showed that in catchments with higher river nutrient exports due to agricultural activities, the stream network supplies relatively high production of emerging aquatic insects, which support a feedback flux of N and C to adjacent (0 to 100 m from source) terrestrial ecosystem.