Blooms of Alexandrium spp. are a well-known phenomenon in Northern European waters. While A. tamarense/catenella, and A. pseudogonyaulax have been reported from marine waters, high densities of A. ostenfeldii are mainly observed at lower salinities in North Sea estuaries and the Baltic Sea, suggesting salinity as a driver of Alexandrium
species composition and toxin distribution. To investigate this
relationship, an oceanographic expedition through a natural salinity
gradient was conducted in June 2016 along the coasts of Denmark. Besides
hydrographic data, phytoplankton and sediment samples were collected
for analyses of Alexandrium spp. cell and cyst abundances, for toxin measurement and cell isolation. Plankton data revealed the predominance of A. pseudogonyaulax at all transect stations while A. ostenfeldii and A. catenella generally contributed a minor fraction to the Alexandrium community. High abundances of A. pseudogonyaulax
in the shallow enclosed Limfjord were accompanied by high amounts of
goniodomin A (GDA). This toxin was also detected at low abundances along
with A. pseudogonyaulax in the North Sea and the Kattegat.
Genetic and morphological characterization of established strains showed
high similarity of the Northern European population to distant
geographic populations. Despite low cell abundances of A. ostenfeldii,
different profiles of cycloimines were measured in the North Sea and in
the Limfjord. This field survey revealed that salinity alone does not
determine Alexandrium species and toxin distribution, but
emphasizes the importance of habitat conditions such as proximity to
seed banks, shelter, and high nutrient concentrations. The results show
that A. pseudogonyaulax has become a prominent member of the Alexandrium
spp. community over the past decade in the study area. Analyses of long
term monitoring data from the Limfjord confirmed a recent shift to A. pseudogonyaulax
dominance. Cyst and toxin records of the species in Kiel Bight suggest a
spreading potential into the brackish Baltic Sea, which might lead to
an expansion of blooms under future climate conditions.