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 denitrifi cation rate using the isotope pairing technique; and (2) estimation of mass balances established on the basis of hydrodynamic numerical mode lling. Denitrification rates were found to range from 100 to 1600 mu m ol N m(-2) d(-1), and varied considerably from February to May. The se asonal and spatial variation in denitrification rate was positively re lated to the water column NO3- concentration. The average annual denit rification 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 bala nces. Annual denitrification and nitrogen retention in the estuary acc ounted for approximately 2% and 3%, respectively, of total nitrogen in put from the catchment area. These low values are explicable by the hi gh 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 estuari ne denitrification generally amounts to 40 to 50 % of the nitrogen inp ut. The findings indicate that in addition to nitrogen input from the land, consideration must also be given to the water residence when est uarine nitrogen retention is being estimated.