High potential subsidy of dry-season aquatic fauna to consumers in riparian zones of wet–dry tropical rivers

Abstract

Aquatic fauna provide an important subsidy to terrestrial consumers. In the wet-dry tropics, important subsidy from rivers to riparian-zone consumers is expected in the dry season, but this may vary depending on riparian-zone condition. We investigated potential subsidy of aquatic fauna to consumers in riparian zones in two highly seasonal rivers in Australia's wet-dry tropics. Stable carbon and nitrogen isotopes of invertebrate predators in riparian zones were closely aligned with aquatic invertebrates and emergent adult insects. Further, a considerable proportion (40-50%) of the observed vertebrate fauna in riparian zones were consumers of aquatic fauna, which included fish, crustaceans, invertebrates and flying adult insects with aquatic larval stages. For hydrologically-disconnected waterbodies, estimates of potential insect emergence and the proportion of vertebrate species (in riparian zones) that consume these insects both increased as indicators of riparian plant regeneration and condition improved. Our findings suggest that aquatic fauna provide important subsidies to terrestrial-zone consumers (both invertebrates and vertebrates) during the dry season, and that these transfers can be moderated by riparian zone condition. The wide home and foraging ranges of some consumers also suggest that the importance of these subsidies may extend far beyond the waterbody of origin. Human activities and climate-driven alteration of flow regimes and riparian zones that reduce the availability of dry-season waterbodies or degrade their riparian zones are likely to have negative impacts on aquatic-terrestrial linkages in these systems.