Denitrification and nitrous oxide concentrations in the Humber estuary, UK, and adjacent coastal zones

Denitrification rates and nitrous oxide (N₂O) concentrations are presented for the Humber estuary and adjacent coastal zones covering the period from January 1995 to December 1996. Estuarine sediment denitrification rates, measured in April and August 1996, varied between 1.2 ± 1.1 (95% CI) to 10 ± 0.7 mmole N m^-2 d^-1. Highest rates were recorded at Brough (8.4 ± 0.5 mmole N m^-2 d^-1 in April and 10 ± 0.7 mmole N m^-2 d^-1 in August) situated in the inner Humber estuary concomitant with high nitrate concentrations in overlying water (~ 500 μmol l^-1), high sediment organic carbon and high abundance of macrofauna on both sampling occasions. Other sites (Skeffling-outer estuary, and Goxhill/Pyewipe-middle estuary) showed lower rates in April and August, but a significant increase in rates occurred in August related to increases in macrofauna abundance and temperature. The extreme inner site at Blacktoft showed consistently low denitrification rates as a consequence of a low sediment organic carbon content. Sediment denitrification removed an estimated 25% of the inorganic nitrogen load entering the Humber system from rivers annually. Nitrous oxide was supersaturated throughout the estuary with a distinct maximum observed at low salinities. Highest supersaturations of N₂O occurred in summer with a strong seasonal trend. Water column nitrification at the turbidity maximum was considered to be the main source of N₂O. The estuary represents a source of N₂O to the atmosphere and the North Sea throughout the year. Denitrification rates in adjacent coastal sediments were typically two orders of magnitude lower than the Humber estuary rates. Highest rates occurred at a site in the Humber plume (190 μmol m^-2 d^-1). A coastal transect revealed that highest rates occurred in the more coastal sites with a reduction in rates towards the open sea. An examination of N₂O in the surface waters of the North Sea revealed that supersaturation was restricted to coastal zones with a strong relationship to salinity. The source of supersaturated coastal N₂O appeared to be estuarine inputs.