Storage, redistribution and net export of dissolved and sediment-bound nutrients, Vejle Fjord, Denmark

Abstract

Sixty three percent of the nitrogen (total transported 2041 × 10³ kg y⁻¹) and 17% of the phosphorus (total 159 × 10³ kg y⁻¹) supplied from terrestrial sources to Vejle Fjord during the period September 1988 to October 1989 is exported to the Kattegat. The sediment nutrient concentrations in the estuary are mainly governed by hydrography and resuspension. The general wind-induced circulation consists of outgoing currents along the southern side and ingoing currents along the northern side of the estuary. The sediments in shallow water on the southern side had higher concentrations of nutrients.

Resuspension resulted in large differences between gross sedimentation and net sedimentation, especially in shallow water. Gross sedimentation of total-N in shallow water was 819 × 10³ kg y⁻¹ compared to a net sedimentation of 19 × 10₃ kg y⁻¹. The shallow water areas in the estuary (10% of the area), had a net sedimentation of total-N which was less than 1% of the supply.

Wave-induced resuspension only occurs in exposed parts of the deep water area, when wind velocities exceed 10 m s⁻¹ The concentration of nutrients in the sediments was found to increase with distance from the river (the source) and with increasing depth, as a result of resuspension near the river mouth in the inner part of the estuary. In sheltered parts of the estuary there was no wave-induced resuspension and the net sedimentation equals gross sedimentation. The rate of sedimentation in deep water areas was 12.2 g m⁻² y⁻¹ for total-N and 2.1 g m⁻² y⁻¹ for total-P.