Abstract

In freshwater food webs, consumers often prefer high-quality autochthonous OM over allochthonous OM. However, scarcity may force reliance on the abundant but lower-quality allochthonous OM. We examined how the dietary contribution of autochthonous OM varied with its in-habitat quantity. We manipulated light incidence over natural ecosystems to create a gradient of autochthonous OM quantity. We tested whether autochthonous OM contributed proportionally its quantity to consumer diets or was preferentially foraged. Autochthonous OM contributed more to diets when scarce but stabilized at a limit despite further increases, indicating a minimum quantity to trigger an autochthonous-based energy pathway and a saturation point. Light incidence and periphyton phosphorus, proxies for OM quantity and quality, positively influenced dietary contributions, while its interaction with chlorophyll-a negatively affected it---likely by reducing OM quality or quantity. We concluded that OM quantity and quality shape energy pathways in freshwater food webs, with quantity as the primary driver.