Denitrification in sediments of the River Colne estuary, England

Abstract

The River Colne is a muddy macrotidal estuary on the east coast of England, with strong decreasing gradients of NO 3 -and NH 4 + down the estuary due to inputs from the river and upstream sewage treatment works. The rates of uncoupled (D w ) and coupled (D n ) denitrification in the sediments at 4 sites were measured monthly by 15 N isotope-pairing technique from October 1996 to March 1998. There was a significant spatial and temporal variation in the rates of D w and D n . The highest rates were observed at Site 1 and the lowest at Site 4, corresponding to the highest NO 3 -concentrations in the water column and highest organic contents in the sediments at Site 1 and the lowest at Site 4. Short-term kinetic experiments showed that rates of D w increased with increasing NO 3 -concentration in the water column, up to 500 M NO 3 -at Site 2 (the middle site along the estuary), and were not saturated up to 600 M NO 3 -at Site 1 or 180 M NO 3 -at Site 4. Field observations and kinetics experiments indicated a large sediment potential for D w at all sites. Nitrate availability limited the rates of D w and was the main factor regulating the rates of D w in this estuary. It was calculated that 32 to 44% of total oxidised nitrogen inputs or 20 to 25% of the total inorganic nitrogen inputs (total oxidised nitrogen + ammonium) into the estuary by the River Colne and the sewage treatment works were removed from the estuary by uncoupled denitrification (D w ) and total denitrification (D w + D n ) respectively, in the sediments before they were transported into the North Sea. Microphytobenthos, mainly diatoms, assimilated 4.1% of the ammonium inputs or 2.3% of total inorganic nitrogen inputs into the estuary and temporarily retained the assimilated nitrogen in the estuary. They also affected the rates of denitrification in a complex manner, by regulating the depth of oxygen penetration by photosynthetic oxygen production and respiratory oxygen uptake. Photosynthesis inhibited the rates of D w but stimulated D n. The overall effect of microphytobenthos on denitrification was to increase the percentages of D n in the total denitrification. The loads of nitrogen through the River Colne estuary to the North Sea was significantly attenuated by the removal of nitrate from the water column by D w and of nitrate produced from ammonium via nitrification in the sediment by D n .