Development and dynamics of a coastal sub-surface phytoplankton bloom in the Southwest Kattegat, Baltic Sea

The study was based on CTD-casts performed on 5 consecutive days at 7 positions along a shallow- to deep-water transect during the spring bloom in the southwest Kattegat. The development, dynamics and fate of the sub-surface chlorophyll maximum (SCM) are described. The SCM developed at a constant rate of 1.4 mg chl a m−3 d−1, which is equal to a carbon fixation rate of 49.0 mgC m−3 d−1. The SCM developed at 6 m depth at low irradiance levels (10–43.0 μE m−2 s−1). Chl a concentrations reached up to 23 mg chl a m−3 in the centre of the SCM. Growth rates in the SCM were estimated at 0.82 d−1. The water column was only weakly stratified, with nutrients in excess in the whole of the water column. The SCM was relocated vertically to depths below the photic zone by wind-induced advection between the bay and the outer Kattegat. There were indications of a horizontal relocation of the SCM by an internal standing wave. Application of high spatial and temporal resolution made it possible to estimate key SCM parameters, such as growth rates [mg chl a m−3 d−1] and carbon fixation rate [mgC m−3 d−1], on the basis of chl a variations in time and space.