Floods, Food Chains, and Ecosystem Processes in Rivers

Abstract

SUMMARY Disturbance regimens link species and ecosystems. For example, floods that scour channels in river drainage networks can also alter trophic networks that link river biota. The impacts of flood disturbance regimens on trophic structure vary among communities, depending on the attributes of constituent species. In the midwestern United States, where algivorous fish are the principal herbivores, floods may spatially rearrange predators and prey among pool habitats, but larger (reach) scale food chain patterns are not affected, or ifaltered, are rapidly restored. In California rivers, where fish faunas are rel-atively depauperate, invertebrates are the chief primary consumers. Here, hydrologic disturbance or its absence does affect food chainlength at the reach scale. Scouring floods allow weedy invertebrate species to dominate early suc-cessional primary consumer guilds. These species are resilient following phys-ical disturbance but subsequently vulnerable to predation; After prolonged low flow during drought, or in regulated channels with artificially stabilized hydrographs, lower trophic levels become dominated by armored or sessile taxa that are relatively invulnerable to predators. In these late successional communities, the biomass of primary producers is chronically suppressed, and energy flow to higher trophic levels appears to attenuate. In addition, other ecosystem functions may be changed, including nitrogen furation and river-watershed exchange mediated by floating algal mats