Collaborative Research: Headwater stream networks in a warming world: predicting heterotrophic ecosystem function using theory, multi-scale temperature manipulations and modeling

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Earth's river systems receive, transport, store, and break down a substantial proportion of the dead plant material produced in terrestrial ecosystems. Many of the steps in the breakdown of this organic matter are greatly affected by water temperature, which is increasing along with air temperature globally. While the effects of temperature on some of these steps are understood relatively well at small scales, it remains an open question how they interact as a complex, integrated ecosystem-level process to drive the effects of rising temperatures at the large spatial and temporal scales over which Earth's river systems operate. Consequently, research is needed that tests the effects of temperature on organic matter breakdown in whole ecosystems, with the explicit goal of scaling up these results to understand how entire river reaches and networks process organic matter. This project will use data from temperature manipulations at multiple spatial and temporal scales to: 1) inform ecological theory that uses basic principles to understand how the effects of temperature scale from individual organisms to entire ecosystems; and 2) build a model that simulates the effects of temperature on organic matter processing in an entire forest stream network. This research is important because rising temperatures may alter the global biogeochemical role of streams and rivers in currently unpredictable ways, making the results useful in the future management of river health. An education program will be an integral part of this project and will include the development of a citizen science project, as well as training of undergraduates, graduate students and research technicians. Fieldwork for this project will take place at the Coweeta Hydrologic Laboratory, North Carolina, U.S.A., a site with a decades-long history of research on organic matter processing in stream ecosystems. The first objective is to understand how warming affects the organisms (fungi and invertebrates) that are critically important in organic matter breakdown in streams. This component of the project will combine detailed laboratory-based studies of fungi, experiments in heated streamside channels and the warming of an entire reach of a small forest stream to understand how higher temperatures change community structure of fungal and animal decomposers. The second objective is to use the various temperature manipulations to understand how temperature changes pathways and rates of organic matter processing in streams and rivers. The fates of organic matter will be studied using laboratory investigations, streamside channels, and field experiments combined with organic matter budgets and food web descriptions in the experimentally warmed stream and a matching reference stream. The results of these first two objectives will be combined with extensive existing datasets available at the Coweeta site to inform the final objective: construction of an ecological model that predicts the effects of warming on organic matter processing in an entire stream network. This model will also allow incorporation of other important factors that could potentially change in the future, such as precipitation and the relative quality of organic matter that enters stream ecosystems. Together, this research will provide much needed information about how warming influences the important biogeochemical role of streams and rivers.