TIDAL CONTROLS ON DENITRIFICATION IN COASTAL STREAMBEDS

In tidal freshwater zones (TFZs), tidal pumping enhances exchange of oxygen-rich river water across the sediment-water interface and controls nitrogen cycling in riverbed sediments. A one-dimensional, coupled fluid flow and solute transport model was developed to quantify the influence of tidal pumping on redox zonation and nitrate removal in the TFZ of White Clay Creek (Delaware, USA). As tide rises, oxygen rich river water infiltrates into the bed and limits denitrification, which causes rates to decrease by ~9% relative to falling tide. Surface water nitrate concentration minimally affects denitrification rate, suggesting that most of the nitrate being removed is groundwater-sourced. Under tidal pumping conditions a deep, oscillating zone of aerobic respiration forms, which diminishes denitrification rates relative to non-tidal conditions. Removal rates of groundwater-borne nitrate should decrease along TFZs as tidal amplitude increases, given a uniform substrate. Denitrification hot spots should occur in less permeable, organic-rich sediment under low tidal ranges, typically near the upstream limits of TFZs.