ESTUARINE NITROGEN-RETENTION INDEPENDENTLY ESTIMATED BY THE DENITRIFICATION RATE AND MASS-BALANCE METHODS - A STUDY OF NORSMINDE FJORD, DENMARK

Nitrogen retention was studied in a small, shallow estuary (Norsminde Fjord, Denmark) by 2 independent methods: (1) measurement of denitrification rate using the isotope pairing technique; and (2) estimation of mass balances established on the basis of hydrodynamic numerical modelling. Denitrification rates were found to range from 100 to 1600 mu mol N m(-2) d(-1), and varied considerably from February to May. The seasonal and spatial variation in denitrification rate was positively related to the water column NO3- concentration. The average annual denitrification rate was 29 kg N ha(-1) yr(-1), or approximately 6 t N yr(-1) for the entire estuary. This compared well with the annual nitrogen retention of approximately 8 t N yr(-1) calculated from the mass balances. Annual denitrification and nitrogen retention in the estuary accounted for approximately 2% and 3%, respectively, of total nitrogen input from the catchment area. These low values are explicable by the high water exchange rate in the estuary, freshwater retention time being 1.5 to 13 d, with minimum values during the winter. The present study therefore lends no support to the widely held assumption that estuarine denitrification generally amounts to 40 to 50 % of the nitrogen input. The findings indicate that in addition to nitrogen input from the land, consideration must also be given to the water residence when estuarine nitrogen retention is being estimated.