MEASURING AND MODELING DENITRIFICATION IN A TIDAL RIVER

The tidal freshwater zone (TFZ) is the transition between fresh inland rivers and marine waters. The fate of nitrogen in TFZs is poorly understood, with tidal dynamics likely changing in-stream and near-stream rates of nitrogen cycling, and ultimately influencing nitrogen export to the coast. Here, we quantify respiration rates of denitrifying bacteria within the streambed sediment of tidal White Clay Creek and compare these rates to corresponding sediment characteristics, including grain size and organic matter content. Sediment cores and water samples were collected from four locations along a 2.5 kilometer reach. Samples from sediment and water column microcosm incubations were analyzed for O₂, N₂, and Ar content using MIMS (Membrane Inlet Mass Spectrometry). Temporal changes in the amounts of dissolved O₂ and N₂ were determined using O₂:Ar and N₂:Ar ratios. The streambed sediment in the TFZ is extremely heterogeneous, though sediment tends to be finer grained and more organic rich towards the mouth of the river. Generally, sediments of finer grain size and higher organic matter content showed elevated rates of denitrification. These results are used to develop numerical models of surface water-groundwater exchange, aerobic respiration, and denitrification along the TFZ. Together, field observations and models allow for upscaling of results to an entire TFZ to assess the influence of tidal dynamics and geology on potential nitrogen transformations within TFZs.