Changes in benthic biota and nitrogen sources concurrent with declining fish diversity in the Conasauga River
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The Conasauga River in Georgia and Tennessee is a site of recent, rapid decline in biodiversity of fishes and mollusks associated with a downstream gradient of increasing agricultural land use. The purpose of this research was to quantify other changes in the Conasauga and to compare location of changes with that of declines. Elevations in δ15N stable isotopes in primary consumer tissues indicated hotspots of anthropogenic nitrogen input located upstream of fish declines. Biomass, abundance, richness and composition of benthic macroinvertebrates shifted downstream and several individual taxa exhibited threshold declines upstream of fish declines and in conjunction with loss of an aquatic macrophyte. Algal accrual was more influenced by time period than location and was greatest during a period with long duration between rain events. If algal accrual is a mechanism by which eutrophication alters biotic communities in the Conasauga, then increased prevalence of drought might have dire consequences.