Intellectual Merit: Hydrologic, geomorphic and biogeochemical dynamics of river systems are coupled over time scales ranging from hours to millennia. Catchment inputs and flowpath properties influence geomorphic and biogeochemical processes through effects on water and solute delivery dynamics. Simultaneously, metabolic processes of river biota can exert reciprocal control on hydrology, morphology, and biogeochemistry. These coupled biotic-abiotic interactions lead to complex, non-linear responses that require high spatiotemporal density of data to understand. In spring-fed karst rivers, strong biotic-abiotic interactions are expected due to high plant production and low flow velocity, and are hypothesized to control both N and DO dynamics, with important ecosystem consequences, and also river water pH with implications for mineral dissolution and thus karst channel development. The magnitude of biotic controls on solute delivery, and links to stream channel dissolution, are poorly understood. Consequently, this proposal addresses 3 questions that link biotic and abiotic processing of spring-fed karst channels: 1) How do sub-surface flow paths vary with climate and flow regime, and how do they control the magnitude of spring water delivery, and the solute chemistry of the water 2) What are the hydrologic and geomorphic controls on riverine N processing 3) What are the biotic and hydrologic controls on channel dissolution in low-relief karst riversThe study site is the Ichetucknee River in north Florida where multiple gauged springs merge to form an 8-km long river, which is also gauged 5 km downstream of the springs. The work plan includes: 1) continuous measurements of water chemistry, utilizing state-of-the-art sensors at strategically selected spring vent and river sites, 2) synoptic water sampling for analytes not continuously measured or at non-sensor sites, and 3) analysis of archival data at Ichetucknee and other regionally important springs to test hypotheses more generally. Continuously recording sensors, that measure DO concentrations, T, stage, specific conductivity, and pH, will be deployed at two springs, representing two hydrochemical groupings of the Ichetucknee complex. These, along with continuous NO3 and turbidity sensors, will also be deployed at three locations downstream. The continuously measured data will be used to assess delivery variability and in-stream processing of N (e.g. assimilatory uptake versus dissimilatory loss) at diel, episodic, seasonal, and inter-annual time scales. Monthly synoptic sampling will link the biological processes to carbonate saturation state and dissolution. Analyses of archival data from springs across the region, will allow statistical analyses (auto/cross-correlation) of discharge vs. chemistry relationships spanning a larger population of springs.Broader Impacts: Floridas springs are significant regional cultural, economic, and ecological resources; observed ecological declines have made springs the subject of current regulatory consideration, with a focus on N pollution. As regulatory options are debated, poor mechanistic understanding of how springs process N limits the ability to make recommendations for how to manage them. Multiple local and state regulatory agencies and stakeholder groups (e.g., Ichetucknee Springs Basin Working Group, Florida Department of Environmental Protection, Florida Springs Task Force) allow translation of scientific knowledge into policy and public education. Results from this proposed work will be presented regularly to stakeholders at public and agency meetings. A significant portion of the research will be conducted at a minority serving institution (Florida International University); under-represented groups will be recruited to work as students and post-doctoral candidates. Co-locating this work with a WATERS test-bed location will expand the expertise gained about sensor networks and data infrastructure during the preliminary studies. This work will provide additional technological advances toward building sensing platforms, allowing critical information about river systems to be collected across process and management relevant time-scales.