Benithic-pelagic coupling of nutrient metabolism along an estuarine eutrophication gradient

Abstract

The shallow, brackish (11–18% 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−1. 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 µmol N m⁻² h⁻² achieved at loading rates of about 25–125 g N m⁻² yr⁻¹.