Resuspension, ephemeral mud blankets and nitrogen cycling in Laholmsbukten, south east Kattegat

Abstract

Primary production in coastal waters is generally nitrogen-limited due to efficient nitrogen sink pathways, which therefore limit further eutrophication. In this context, the significance of ephemeral mud blankets at shallow depth has been characterised using a simple computational box model. Bed materials from several synoptic grab-sample surveys in Laholmsbukten, a shallow embayment of the Kattegat Sea, were analysed. Sediment trap and current meter data provided an assessment of fine material transport; the frequency of loose mud resuspension was estimated to vary between three and ten events per month in the spring, depending on the wind conditions.

Mud blankets appear to be deposited following major spring and fall phytoplankton blooms, they are only a few centimetres thick, they are composed mainly of pelletized organic material and detritus and they have a high water content (80% wet weight) and nitrogen content (0.5% dry weight). In the course of export from the bay, the muds undergo continuous resuspension, redeposition and biological breakdown. In one September day, the amount of particulate nitrogen redeposited in sediment traps was of equivalent magnitude to the entire mud blanket deposit (5 g N/m⁻² or a 4 mm deposit as a bay-wide mean). The presence of mud blankets is believe to control the nitrogen budget and water quality in the bay. A simple model proved compatible with observed mud blanket presence and rates of denitrification. Calm conditions are conducive to the loss of bound nitrogen and free oxygen within the bay as a result of prolonged denitrification and pelagic and benthic nitrogen regeneration. If fine particulates are rapidly exported from shallow bottoms, as in windy periods, the water quality of the entire Kattegat Sea is likely to be impaired.