Decoupled reciprocal subsidies of biomass and fatty acids in fluxes of invertebrates between a temperate river and the adjacent land

Abstract

Streams and riparian areas are tightly coupled through reciprocal trophic subsidies, and there is evidence that these subsidies affect consumers in connected ecosystems. Most studies of subsidies consider only their quantity and not their quality. We determined the bidirectional exchange of organisms between the Kowie River and its riparian zone in South Africa using floating pyramidal traps (to measure insect emergence) and pan traps (to capture infalling invertebrates). The exchanges of biomass were variable spatially (three sites) and temporally (four seasons), with emergence declining about two orders of magnitude between summer (169–1402 mg m⁻² day⁻¹) and winter (3–28 mg m⁻² day⁻¹) across all sites, while invertebrate infall declined by a much smaller range from summer (413–679 mg m⁻² day⁻¹) to winter (11–220 mg m⁻² day⁻¹). Conversely, the absolute flux of physiologically important highly unsaturated fatty acids contained in the emergent and infalling arthropods peaked at comparable values in summer (emergence = 0.3–18 mg m⁻² day⁻¹ and infall = 0.3–3 mg m⁻² day⁻¹) and declined less in winter (emergence = 0.01–0.51 mg m⁻² day⁻¹ and infall = 0.01–0.03 mg m⁻² day⁻¹). During some seasons, there was no net flux of essential fatty acids, but there was generally a net flow of highly unsaturated fatty acids from river to land, even when land-to-river inputs dominated by biomass. Thus, quantitative net fluxes of biomass were decoupled from net fluxes of qualitatively key nutrients, establishing the importance of considering both the quality and the quantity of trophic subsidies.