100-years-changes in the phytoplankton community of Kiel Bight (Baltic Sea)

Literature data from 1905/06, 1912/13 and 1949/50 were compared with recent data (2001–2003) from Kiel Bight in order to investigate changes in phytoplankton composition and biomass, which may serve as indicators of environmental changes. In terms of biomass, diatomophyceae and dinophyceae are by far the most important groups. Their ratio is still close to unity. The share of diatomophyceae increased strongly in years with exceptionally high summer blooms (2001) or exceptionally early spring blooms (2003). The summer and autumn blooms of Chaetoceros and Skeletonema, detected in the early 20th century, are replaced by other diatoms (Cerataulina pelagica, Dactyliosolen fragilissimus, Proboscia alata, Pseudo-nitzschia spp.). Chaetoceros and Skeletonema are still important components of the spring blooms. Now as before, the autumn blooms are dominated by Ceratium spp., sometimes also by diatoms. Newly appearing bloom-forming species are mostly potentially toxic (Dictyocha speculum, Prorocentrum minimum, Pseudo-nitzschia spp.). The total phytoplankton biomass has roughly doubled in the course of the last century. The reference condition for phytoplankton biomass in Kiel Bight in the sense of the Water Framework Directive was defined at 55 mg C m− 3 (± 10%, annual mean). The mean annual biomass of diatomophyceae and dinophyceae was 25 mg C m− 3 (± 40%) for each, indicating that the sum of their carbon biomass amounted to 90% (± 10%) of the total phytoplankton biomass on an annual average. Diatomophyceae represented at least 80% of carbon biomass in the spring bloom peak at the beginning of the 20th century.