Exchange of Water and Nutrients Between the Skagerrak and the Kattegat

Simultaneous CTD and current measurements have been made repeatedly at several stations in a cross-section between the Danish island Læsø and the Swedish west coast. The cross-section is situated in the northern Kattegat which forms the outer part of the Baltic estuary. Parallel nutrient and oxygen measurements were made in order to obtain a suitable set of background data for ecological modelling of the Kattegat. Here, this dataset is used to calculate fluxes of water, nutrients and oxygen, as functions of salinity. A division is made between the biologically passive winter period, November to February (when the inorganic nutrients components are nearly conservative) and a biologically active ʹsummerʹ period from March to October. A striking feature is the much stronger inflow of high salinity deep water during winter, approximately 64 000 m3 s-1, compared to only 28 000 m3 s-1 during summer. The inflow is concentrated to salinities higher than 33 PSU. The surface water outflow occurs in the interval 23 to 27 PSU. While the inflow of oceanic deep water to the Kattegat occurs on the deeper eastern side of Læsø, the outflow of surface water from the Baltic seems to be dominated by the flux west of Læsø.