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Cohen, Matthew J ¹

¹ University of Florida

Rivers are the primary conveyance for water and solutes to the sea, and are increasingly recognized as venues for intensive biogeochemical reactivity. As water and solutes move downstream and exchange with adjacent sediments and slow velocity flowpaths, important reactions occur that can reduce or enhance solute export. Potential ecosystem degradation, both locally and downstream, due to elevated solute availability (e.g., nitrogen and phosphorus) makes understanding retention and generation dynamics a central challenge for river management. Our knowledge of riverine nutrient processing has recently been enhanced by new field sensors capable of high temporal resolution measurements that align with time scales of flow variation and ecosystem processes (e.g., event, diel time scales). These sensors can be deployed for both high temporal resolution measurements of concentration variation at fixed points (i.e., Eulerian sampling), as well high spatial resolution measurements along a river, enabling a Lagrangian view of ecosystem processes. Sensor measurements also allow nutrient use dynamics to be evaluated at the same time-scales as stream metabolism, and thereby inform the stoichiometry of system-scale primary production and respiration. In this talk, I will describe several applications of sensor measurements to river ecosystem dynamics. First, I will describe the utility of high resolution (sub-daily) for resolving both the rate and pathways of nutrient retention in rivers, even those where residence times are rapidly changing (i.e., not at base flow). Second, I will be describe how the high resolution sensing capability enables Lagrangian sampling from which inferences about geomorphic and biological controls on metabolism and nutrient retention can be inferred, enabling a new approach to riverscape ecology. Finally, I will describe a new experimental application where sensors are placed in clear benthic chambers from which we extract nutrient dynamics at below-ambient concentrations, assess benthic controls on net nutrient fluxes, explore nutrient limitation using full-factorial nutrient enrichment experiments, and enumerate the pathways, timing and kinetics of retention. Among the key findings of the most recent work is strong evidence of iron (Fe) limitation of primary production, and zero-order (i.e., concentration independent) kinetics of nitrogen and phosphorus use in streams. Strong concordance between measurements of nutrient processing and ecosystem metabolism at both whole-reach (using both Eulerian and Lagrangian reference frames) and highly localized benthic habitat scales is extremely promising for the future utility of sensors in the hydrologic sciences.