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 rivers? The 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: Florida?s 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.
The springs of north Florida feed unique river systems that are imperiled by activities affecting the aquifer from which they flow. Increasing nutrients, declining flow, and changes in water sources all represent important potential impacts on these ecosystems that have high value, both culturally and economically. This project focused on understanding more about how water and pollutants move through the aquifer, and what happens to them when the reach the surface and become rivers. In particular, we sought to understand what happens to nutrients (specifically nitrogen and phosphorus, but also trace nutrients like metals) in the river as plants respond to day-night cycles in light. Our work has provided new insights into how and when river ecosystems process nutrients, how these nutrient cycles interact, and how plants affect them. These insights are valuable for understanding the impacts of human activities on these ecosystems, and for gaining a deeper understanding of the two-way interaction between organisms and their environment. Among the many important findings of this work was that phosphorus and nitrogen uptake by plants occurs at different times of day (nitrogen during the day, phosphorus at night). We observed similar variation in the timing of plant uptake across a suite of metals. This shows that nutrients with strong day-night signals in concentration are affected by many overlapping processes. We used new sensors, all of which provide measurements at a temporal resolution that has, to date, not been widely possible, to start to pull those overlapping processes apart, and understand how and when they work. The project also yielded important insights into how pollutants are reduced in the aquifer, and how variation in climate and water use affect the time that water spends in the aquifer before emerging from the springs. These are integral questions to managing the aquifer, which is the primary source of drinking water for the region, and for understanding how long it will take for remedial actions at the land surface to show as improvements in the quality of the water. This work allowed us to better understand the links between aquifer threats and ecosystem responses, and identified flow and not nutrients as the principal threat. One important outcome was a new theoretical basis for understanding when nutrients are likely to exert strong ecosystem level effects on flowing waters, and by extension what levels are unlikely to cause local ecological change (recognizing that nutrients that may not cause an effect in one place may have important effects downstream). Finally, this project allowed us to better understand the carbon cycle in these spring fed karst rivers. There has been a lingering question about how a river in this karst setting maintains a open channel, and our work points to an important role of plants in maintaining the channel, which is a valuable illustration of the important and often underappreciated role than organisms play in shaping the land surface.
