Litter quality and stream food webs: a new paradigm for understanding interactions between microbes and invertebrates.

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Leaves that fall into streams and rivers in autumn can provide an important food resource for organisms that live in and around them. When leaves enter the water, bacteria and fungi rapidly colonize them and begin to decompose them. The leaves and microbial colonizers provide food for insect larvae that live in the water, which in turn, provide food for fish, and ultimately upon emergence, birds, bats and lizards. This study will test how leaves from different trees impact the food webs of stream and river ecosystems. The researchers will assess the fate of leaves from different tree species to determine whether the leaves are primarily converted into carbon dioxide by the microbial decomposers or whether the leaves are primarily a food resource for insect larvae. Scientists have assumed that leaves that decompose quickly are a better food source for insects and this research challenges that assumption by suggesting that leaves that decompose more slowly, because of their particular chemical composition, are a better food resource for insects. The research will also determine which microbes colonize specific leaf types and how leaf types along with their microbial colonizers affect the nutritional quality for insects that eat them. Understanding how different tree species affect the animals living in and near streams will guide managers to plant trees that are most beneficial. The researchers will also study how the temperature and water quality of streams affects how insects use leaves. This could help land managers understand if certain stream conditions make leaves a better resource for insects.  This research project will train undergraduate and graduate students and work with high school teachers and students. Results will be shared with the governmental and non-governmental organizations that are working to improve stream health.The researchers will use multiple state-of-the-art techniques to test how different leaf species transfer energy and nutrients to microbes and insects. First, investigators will grow twelve different plant species in the green house using 13C and 15N to label the leaves that will be feed to insects. After the leaf senescence, the researchers will collect the leaves and incubate them in streams in Arizona and New Hampshire, using the stable isotopic signatures to trace the transfer of leaf carbon and nitrogen to microbes and insects. Insects that eat the leaves in the stream will be brought back to the laboratory where they will be analyzed with an isotope ratio mass spectrometer to measure exactly how much of the leaf carbon and nitrogen ends up in their tissues. Second, the researchers will use stable isotope probing and DNA sequencing to study which microbes are a good food source for insects versus which microbes compete with insects by quickly using up the carbon and nitrogen stored in leaves. Third, scientists will construct stream mesocosms to control temperature and water quality to test how stream conditions affect the nutritional quality of leaves.