Benithic-pelagic coupling of nutrient metabolism along an estuarine eutrophication gradient |
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Authors: | Kamp-Nielsen Lars |
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Affiliation: | (1) Freshwater-Biological Laboratory, University of Copenhagen, Helsingørsgade 51, 3400 Hillerod, Denmark |
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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. |
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Keywords: | Sediment-water exchange nutrients estuaries eutrophication |
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