SEPARATING THE HYDROLOGICAL AND BIOLOGICAL CONTROLS ON DENITRIFICATION IN THE STREAMBED OF A LOW-RELIEF COASTAL CATCHMENT

Humans have dramatically altered the terrestrial nitrogen cycle. There is a large amount of uncertainty associated with the fate of anthropogenic nitrogen dispersed in the environment. Part of the uncertainty stems from the large degree of variability in the factors controlling and denitrification. The Eastern Shore of Virginia provides an ideal laboratory for studying some aspects of the nitrogen cycle due to large inputs of agriculturally-derived nitrogen to small coastal catchments. The Cobb Mill Creek watershed is a low-relief coastal catchment on the Eastern Shore of Virginia in the Virginia Coast Reserve Long-Term Ecological Research Site. Flow to Cobb Mill Creek is supplied by vertical upwelling of groundwater through the streambed and horizontal flow through the riparian zone. Pore water profiles of nitrate in the streambed indicate that sharp reductions in nitrate concentrations are coincident with a zone of high denitrification potential and high denitrifier abundance, suggesting that reductions are due to denitrification. We propose a simple first-order kinetic rate law for describing denitrification in the streambed of Cobb Mill creek with respect to nitrate. We show that the fraction of nitrate removed in the streambed is reasonably described as a function of groundwater travel time through the nitrate reduction zone and a characteristic biological rate coefficient. There is, however, a large degree of dispersion in the data due to the variability of both the controlling variables. We use a multiple linear regression technique to separate the effects of groundwater travel time and rate coefficients on nitrate removal in the streambed. Standard partial regression coefficients indicate that both groundwater travel time and reaction rate coefficients are important for explaining the observed variation in nitrate removal in the streambed. The multiple linear regression allows one to control for the variability in reaction rate coefficients to more clearly show the strong dependence of nitrate removal on groundwater travel time through the nitrate reduction zone.