Denitrification rates across a temperate North Pacific estuary, Yaquina Bay, Oregon

The extent and temporal variability of denitrification activity was measured in Yaquina Bay, Oregon, over a year using sediment cores collected approximately monthly from August 2003 through August 31, 2004. Denitrification rates in sediments from a marine-dominated intertidal sand flat near the mouth of the estuary averaged 0.181 ±0.114 mmol m^-2 d^-1 whereas sediments in the estuary (5 stations) and river averaged 0.626 ±0.141 mmol m^-2 d^-1. Sediment cores from all estuarine sites indicated denitrification activity throughout the year and were within the values reported for other temperate estuaries. Denitrification rates decreased with depth from 0.4 mmol m^-2 d^-1 in the upper 2 to 5 cm of sediment to 0.006 mmol m^-2 d^-1 at 28 cm sediment depth, indicating denitrification occurred primarily in the upper 5 cm. There was no relationship between denitrification rate and nitrate concentrations in the overlying water column (r² = 0.16). Denitrification rates were lowest in areas with low sediment carbon content, particularly in the sandy intertidal areas at the mouth of the estuary (r² = 0.78). The results suggest that denitrification rates in this estuary were influenced primarily by the availability of organic carbon. The amount of nitrogen removed by denitrification was estimated to be 8.7 percent of the annual Yaquina River load for August 2003 through August 2004. The relatively low percent lost via denitrification may be due to high river discharge when the nitrogen load was greatest during winter storm events and dissolved nitrogen was exported directly from the estuary into the Pacific Ocean. Stable isotopes were used to investigate the carbon source. The carbon isotope data increased from -27 δ¹³C in the freshwater river to -21.5 δ¹³C at the seawater site, reflecting a typical change from terrestrial plant vegetation to phytoplankton carbon sources. Similar values for δ¹³C between suspended and benthic sediments indicated resuspension and mixing occurred during tidal inflow.