Modelling the dynamics of harmful blooms of Chattonella sp. in the Skagerrak and the Kattegat

The presentation shows observations, satellite images and model results describing the growth and spreading of Chattonella sp. flagellates in the Skagerrak and the Kattegat. Chattonella sp. is a harmful alga that may cause fish kills due to damage of the gills. Calm weather, stable water column stratification, and low turbulence may facilitate the onset of a Chattonella bloom. Results from the three-dimensional hydrodynamical model HIROMB (High Resolution Operational Model for the Baltic Sea) are used as forcing of a transport model that computes vertical and horizontal transports of chemical and biological compounds. A modified version of the Swedish Coastal and Ocean Biogeochemical model (SCOBI) is used to describe the temporal evolution of the phytoplankton spring blooms in the year 2001 when Chattonella was abundant and 2002 when only small amounts of Chattonella were observed. A comparison with satellite images and cell counts indicates that the model captures the main transport patterns of phytoplankton in the surface layers of the offshore areas. The Chattonella bloom of the model starts in the quite shallow parts of the western Kattegat and in the stratified coastal areas of the northern Skagerrak. The coastal waters near the river Göta Älv of Sweden also indicate a tendency of an increased occurrence of Chattonella. Chattonella is observed in the model during both years but the occurrence of Chattonella is more significant in the year 2001 than in 2002. Keywords: Algal blooms; Chattonella sp.; Diatoms; HAB; Marine ecology; Modelling; North Sea; Skagerrak; Kattega