Collaborative research: Defining ecosystem heterotrophic response to nutrient concentrations and ratios

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Streams and rivers are important sites of global carbon processing. Carbon in the form of dead leaves and wood (detritus) from the land enters streams and is colonized by bacteria and fungi (heterotrophic microorganisms). This complex of microorganisms and detrital carbon fuels food webs that support fish, amphibians and other consumers. Detrital carbon is consumed by organisms but is also converted to carbon dioxide by bacteria and fungi. Problematic to this important detritus-fuelled pathway is that human-derived nutrient inputs (nitrogen and phosphorus) stimulate microbially-driven release of carbon dioxide and decomposition of detritus, resulting in less available carbon for important ecosystem functions. In this study, laboratory incubations, streamside channels and whole-stream nutrient additions will be used to determine the concentrations and ratios of nitrogen and phosphorus that elicit such ecosystem changes. Knowledge of these threshold concentrations and ratios can guide management of aquatic ecosystems to maintain water quality for wildlife conservation and human utilization. Study findings can aid predictions of how excess nutrients affect the fate of detrital carbon in streams, which is relevant to global carbon budgets. Excess nutrient loading is the primary source of impairment to more than half of the freshwater and coastal ecosystems in the U.S. While hundreds of scientific studies have assessed autotrophic (e.g. algal) response to nutrient loading, few have tested nutrient effects on detritus, heterotrophic microorganisms and the larger aquatic organisms they support. The proposed research will advance our understanding of how aquatic ecosystems are affected by nutrients via detritus-fuelled pathways that potentially lead to carbon loss, a perspective that is currently lacking in management decisions related to nutrient enrichment. This project will provide critical research support to four early-career investigators, as well as experience and training for undergraduate and graduate students at three institutions in the southeastern U.S. The project will include public and K-12 educational components that will focus on how excess nutrients affect aquatic food webs and water quality.