Long-term changes in phytoplankton in the Kattegat, the Belt Sea, the Sound and the western Baltic Sea

The development in biomass and dominant groups and species of phytoplankton was examined at six monitoring stations in the Kattegat. the Belt Sea, the Sound and the western Baltic Sea. Since the initiation of the monitoring in 1979 the biomass of phytoplankton has decreased significantly. The decrease correlated with Danish nitrogen loads to the Danish Straits but also with increases in surface water temperature. The decline in biomass was mainly due to less pronounced spring blooms in the Kattegat and the Belt Sea and reduced biomass during late summer-autumn on all stations. Diatoms have been the overall dominant phytoplankton group on all stations but the western Baltic Sea. However. their contribution to the total biomass has varied considerably throughout the monitoring period as during the years 1987-1989 phytoplankton was dominated by an unusually high dinoflagellate biomass. A qualitative comparison showed that the dominant species (diatoms and dinoflagellates) found in plankton net tows taken during spring and autumn around year 1900 were still among the dominant species during the monitoring period 1979-2006. However. several of the dominant species found during the latter period were not present in the samples taken around year 1900 despite that they should have been captured by the methods used. (C) 2008 Elsevier B.V. All rights reserved.