An inflow and intrusion event in the Little Belt at the North Sea–Baltic Sea transition and a related sub-surface bloom of Pseudo-nitzschia pseudodelicatissima

Abstract A combined inflow and intrusion event was studied in the Little Belt region in the southwest Kattegat during autumn 1999. A persistent period of outflow from the Baltic Sea towards the North Sea was replaced by a strong southward flow in the opposite direction. This reverse of flow direction was related to a change in dominant wind direction and prevailed for about 14 days as demonstrated by current measurements. The study is based on CTD-casts at seven stations, ADCP current data, water sampling for nutrient analyses ((NH 3 +NH 4 ), (NO 2 +NO 3 ), PO 4 and SiO 2 ), and phytoplankton species composition. The reversal caused a considerable increase in bottom water salinity reaching up to 10 at several stations. The inflow water reached into a region of strong mixing where surface and bottom water became nearly fully mixed in the Little Belt constriction where high current speeds prevail. An intrusion of mixed water developed when current again turned into a flow out of the Little Belt and the intrusion was clearly recognised as a band of high fluorescence. The increase in fluorescence was related to a strong bloom of the potential toxic diatom Pseudo-nitzschia pseudodelicatissima . Cell concentrations in the intrusion reached up to 3249 cells ml −1 . Nutrient concentrations were consistently higher in the inflow bottom water as compared to surface concentrations. The bloom was supposedly fuelled by the mixing of nutrient rich bottom water into the photic zone. The occurrence and frequency of these intrusion events are discussed in relation to general hydrographical conditions in the region, as well as the intrusions contribution to the primary production. A conceptual model for the development of the intrusion is proposed.