Sensors in Boxes: New Tools for Understanding Riverine Nutrient Sources, Sinks, and Pathways

Abstract:

Rivers are the primary conduits for water and solute movement to the sea, and are increasingly recognized as venues for intensive biogeochemical reactivity. As water and solutes move downstream and exchange with adjacent sediments, important reactions occur that can reduce or enhance solute export. Potential ecosystem degradation due to elevated nitrogen and phosphorus availability makes retention of these solutes particularly important. Our knowledge of riverine nutrient processing has 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). While sensor-based methods have emerged for open-channel reach-scale dynamics, they hold limited experimental value, and can be ambiguous about retention or release pathways. To better constrain rates and pathways of nutrient retention, we deployed the same sensors in clear benthic chambers. This allowed us to examine nutrient dynamics at below-ambient concentrations, assess benthic controls on net nutrient fluxes, and enumerate the pathways, timing and kinetics of retention. We present results from the Rainbow River, a nutrient rich spring-fed river in Florida. Chambers revealed rapid net N retention dominated by denitrification, which clearly follows first-order kinetics, suggesting source limitation. A second N retention pathway is plant assimilation, revealed from diel concentration variation. This pathway covaried with biomass density and plant type (i.e., vascular plant vs. filamentous algae), and followed zero-order kinetics (independent of concentration). Phosphate dynamics were dominated by rapid P diffusion from the sediments. There was also evidence of diel phosphate variation, the magnitude of which was consistent with autotrophic uptake, but which retained a small fraction of the diffusive flux. Concordance between measurements of nutrient processing and ecosystem metabolism at chamber and whole-reach scales is quite promising. Our results suggests benthic boxes and in situ sensors can provide an important new tool for disaggregating river systems, and therefore better understand riverine dynamics.