BENTHIC-PELAGIC COUPLING OF NUTRIENT METABOLISM ALONG AN ESTUARINE EUTROPHICATION GRADIENT

The shallow, brackish (11-18 parts per thousand. salinity) Roskilde Fjord represents a eutrophication gradient with annual averages of chlorophyll, ranging from 3 to 25 mg chl a m-3. Nutrient loadings in 1985 were 11.3-62.4 g N m-2 yr-1 and 0.4-7.3 g P m-2 yr-1. A simple one-layer advection-diffusion model was used to calculate mass balances for 7 boxes in the fjord. Net loss rates varied from -32.2 to 17.9 g P m-2 yr-1 and from -3.3 to 66.8 g N m-2, corresponding to 74% of the external P-loading and 88% of the external N-loading to the entire estuary. Gross sedimentation rates measured by sediment traps were between 7 and 52 g P m-2 yr-1 and 50 and 426 g N m-2 yr-1, respectively. Exchangeable sediment phosphorus varied in annual average between 2.0 and 4.8 g P m-2 and exchangeable sediment nitrogen varied from 1.9 to 33.1 g N m-2. Amplitudes in the exchangeable pools followed sedimentation peaks with delays corresponding to settling rates of 0.3 m d-1. Short term nutrient exchange experiments performed in the laboratory with simultaneous measurements of sediment oxygen uptake showed a release pattern following the oxygen uptake, the changes in the exchangeable pools and the sedimentation peaks. The close benthic-pelagic coupling also exists for the denitrification with maxima during spring of 5 to 20 mmol N m-2 d-1. Denitrification during the nitrogen-limited summer period suggests dependence on nitrification. Comparisons with denitrification from other shallow estuaries indicate a maximum for denitrification in estuaries of about 250-mu-mol N m-2 h-1 achieved at loading rates of about 25-125 g N m-2 yr-1.