True autochthony and allochthony in aquatic-terrestrial resource fluxes along a landuse gradient

Freshwater and terrestrial ecosystems are connected via reciprocal cross-boundary resource fluxes, where terrestrially derived (allochthonous) organic matter is a critical energy source to freshwater food webs. Therefore, some proportion of aquatic-to-terrestrial resource fluxes, which consist primarily of emergent aquatic insects, are of allochthonous origin (i.e., recycled terrestrial matter). Landuse activities modify basal resources and consumer community composition in aquatic systems and, thereby, aquatic resource fluxes to terrestrial systems. The origin of aquatic terrestrial resource fluxes and alterations to them caused by land use should be considered to understand these fluxes better. Resource fluxes at the aquatic terrestrial interface were measured at 10 streams along a forest-to-agriculture gradient. Autochthony in emergent aquatic insects ranged from 10 to 97%. Hence, aquatic fluxes to terrestrial systems drive the flux of matter of aquatic origin (true aquatic flux) and the recycling of terrestrial matter to the terrestrial environment. Land use indirectly affected autochthony of emergent aquatic insects via changes in water chemistry, high-quality resource availability, and in-stream consumer composition. Chironomidae diet shifts along the landuse gradient strengthened the aquatic flux to the land. For every 10% increase in agricultural land cover, aquatically derived matter deposited on land via emergent Chironomidae increased 0.005 g dry mass m(-2) d(-1). Plecoptera strengthened the aquatic flux and the recycling of terrestrial matter via changes in abundance across the landuse gradient. Aquatically derived matter deposited on land via emergent Plecoptera increased 0.002 g dry mass m(-2) d(-1) for every 10% increase in coniferous forest cover. Qualitative changes in resource fluxes across the aquatic terrestrial interface may be driven indirectly by the influences of land use on diets and composition of emergent aquatic insects.