The influence of both hydrologic regime and temperature on production and decomposition processes in low-gradient blackwater river-floodplain ecosystems will be investigated. The major goal of this research is to develop a model to predict the magnitude of green plant production, and the carbon (energy) dynamics associated with consumer and decomposer activities in these ecosystems as a function of abiotic conditions. Results from previous NSF-funded research will provide most of the data needed to build the model. New field and laboratory research will be conducted to supply data that is essential to the model-building effort, but is unavailable either in extant data sets developed by Meyer and Benke or in the literature. For the past six years Meyer and Benke have studied the organic carbon dynamics and trophic pathways in the Ogeechchee River and its floodplain in the lower coastal plain of Georgia. In this new research they will synthesize their previous work in a model that uses two abiotic "driving" variables, "hydrodynamic regime" and temperature to predict a series of biotic characteristics of the system including; system primary production and respiration, secondary production, and the dynamics of bacteria. If the model is carefully crafted it will be a useful tool for clearly outlining the possible affects on the biology of black water rivers of the southeastern United States of disturbances such as damming and dredging, and changes in thermal-regime like those associated with power-plant cooling. New field and laboratory research will address the following issues: availability of dissolved organic carbon to bacteria in a river's water column; the role of protozoa in controlling bacterial populations through consumption; the importance of protozoa as food sources for macroinvertebrates; and the importance secondary production in the floodplain. This new research is key to our understanding of the structure and function of blackwater rivers. The research team that will conduct these studies is strong. Drs. Meyers and Benke are experienced stream ecologists, and Dr. Weigert, a consultant on the modeling task, is a world-recognized ecological modeler. The institutional settings for the project will provide excellent scientific interaction and a synergistic environment of colleagues and related research. The Ecosystem Studies Program recommends award of this grant.