Phytoplankton has been quantitatively analysed in the Baltic Sea since the 1970s with increasing frequency of sampling over the last decade. Prior to this only few qualitative studies have been carried out, providing insufficient material to estimate conditions from before the intensification of eutrophication in the Baltic Sea to be considered as reference conditions.
In the Gulf of Bothnia, the outer coastal areas have not been substantially impacted by anthropogenic nutrients inputs. Thus the distribution of phytoplankton data from these sites may therefore represent reference conditions. However, adopting this approach for all Finnish coastal waters, summer chlorophyll a reference conditions vary between 1.2 to 3.4 μg l-1. Paleoecological studies from the Laajalahti bay (which is a shallow bay in the south coast of Finland) suggest a reference value for annual summer chlorophyll a of 10 μg l-1. However, this value may not be representative for other shallow bays in southern Finland. In the Kattegat our investigations suggest reference values of approximately 2.1 μg l-1 for the mesohaline coastal waters, although this is only a rough estimate that needs to be supported by other studies.
Using relationships between secchi depths and phytoplankton biomass is a feasible approach to establish reference conditions, given that there are sufficiently longtime series of historical secchi depth measurements and that other light attenuating substances would have had trends comparable to that of phytoplankton. In Finnish coastal waters this approach suggests reference conditions from 1.6 to 2.0 μg l-1, i.e. a narrower span compared to the values established from reference sites. Consistent relationships between chlorophyll and secchi depths are found basin-wide in the Baltic Sea, and this approach is an option for establishing reference conditions provided that the underlying assumptions are carefully examined and their implications evaluated.
Relationships between nutrient levels and phytoplankton biomass have been established for annual means, whereas it is difficult to link the spring bloom biomass to the winter nutrient level due to the strong spatial and temporal variability of the spring bloom intensity and the inadequate amount of data to verify such variability in most monitoring programs. It should be stressed that these relationships are associated with a degree of uncertainty, and that the predicted reference conditions from these relations have a similar degree of uncertainty.
The spring blooms in the Baltic Sea are mainly composed of diatoms. Several diatom species tend to increasingly dominate the total biomass with increasing nutrient levels, particularly with nitrogen. Species frequently observed in the southern Baltic Sea, the Gulf of Riga and Gulf of Finland, such as Skeletonema sp., Thalassiosira sp. and Chaetoceros sp. all showed increased dominance of the spring biomass with higher DIN levels (winter nutrient concentrations). Therefore, we propose that the reference conditions for these spring bloom indicator species could be estimated using reference conditions for nitrogen. Again, it should be acknowledged that such reference conditions are inherently uncertain due to the considerable variation in data.
The frequency of summer phytoplankton blooms was linked to the external nitrogen input to the coastal Kattegat. Using this relationship, with an adjustment for spatial variation, we suggest that in the reference conditions 3 to 5% of all summer observations would be blooms. This is, of course, a rough estimate and more detailed analyses on a regional level are needed to confirm these values.
Phytoplankton species abundance is a difficult indicator to assess from monitoring data, as the number of species recognised in a sample highly depends on the taxonomical skills of the person analysing the sample. Moreover, the taxonomy of phytoplankton is constantly developing and the awareness of new types of species is increasing. These
factors will impact the use and reliability of species abundance and diversity in the classification of coastal waters. At least, robust and unbiased indicators of the structural changes of phytoplankton communities need to be developed before phytoplankton species composition can be applied for classification of coastal waters of the Baltic Sea.