MEPS 203:109-122 (2000)  -  doi:10.3354/meps203109

ABSTRACT: The River Colne is a muddy macrotidal estuary on the east coast of England, with strong decreasing gradients of NO₃^- and NH₄⁺ 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 ¹⁵N 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 NO₃^- 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 NO₃^- concentration in the water column, up to 500 µM NO₃^- at Site 2 (the middle site along the estuary), and were not saturated up to 600 µM NO₃^- at Site 1 or 180 µM NO₃^- 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.

KEY WORDS: Denitrification · Primary production · Microphytobenthos · Benthic diatoms · Estuaries · Nitrogen