The aim of the present studies was to determine the role of phytoplankton in detection of environmental changes in the micro- and macronutrients, as well as to estimate the use of phytoplankton in the assessment of environmental quality of the Eastern Baltic.

Phytoplankton as bioindicator of nutrient limitation.

Field studies of phytoplankton structure and experiments carried out with natural phytoplankton assemblages and test strains showed that nutrient limitation in the Gulf of Riga varies seasonally and spatially. In spring, in regions subjected to anthropogenic impact the main limiting factor is phosphorus. In spring – summer period, nitrogen is limiting the algal growth potential (AGP) in the south-west and central regions, but phosphorus and nitrogen are equally limiting in south-eastern nearshore waters. In summer, when river inputs are lowest, nitrogen limitation occurs in the entire Gulf.

The influence of nutrients availability.

Experimental studies of the effect of nutrient enrichment on the growth rate and community structure of summer phytoplankton of the Gulf of Riga showed that the most diatoms species preferred relatively high concentrations of inorganic nitrogen and from all inorganic nitrogen compounds ammonium was used the first. The dominance of some cyanobacterial and chlorophytes species, when all reserves of inorganic nitrogen were almost exhausted, but significant amounts of phosphorus and silicon still remained available, could be explained by the ability to sustain growth from uptake of dissolved organic nitrogen. Even if the uptake of nitrogenous organic substances was not as efficient as the uptake of inorganic nitrogen sources, it still could be believed that dissolved organic matter sustains growth in some phytoplankton species.
Experimental results were confirmed by field studies, showing the dominance of diatoms in areas with high nitrogen load (regions of river outflows and nearshore areas); an intensive development of nitrogen-fixing cyanobacteria in regions removed from the effect of river outflows and an increase of non-nitrogen-fixing cyanobacteria in nearshore waters with high concentrations of organic mater.

Potentially toxic phytoplankton as bioindicator of DIN/DIP ratio and of the presence of dissolved organic matter (DOM).

A long-term study of the phytoplankton communities structure showed an increase of abundance of potentially toxic species in the Gulf of Riga since the end of 1980ies, connected with a decreased use of fertilisers in the Latvian agriculture. Studies of the role of environmental factors revealed that a decrease of DIN/DIP ratio and an increase of concentration of dissolved organic nitrogen could be considered as the most important factors stimulating development of harmful species. The development of potentially toxic species at a low ratio of inorganic nitrogen to phosphorus can be explained by ability of nitrogen-fixing cyanobacteria to utilise molecular nitrogen, and by the mixotrophic nutrition of some dinoflagellates, green algae and non- nitrogen-fixing cyanobacteria.

The use of phytoplankton in determination of toxic factors.

Results showed a great difference in the toxic tolerance and adaptation capability of the phytoplankton species to the impact of heavy metals. The high sensitivity of some algal species to heavy metals offer a potential for their use in the control of environmental quality of Baltic water, as well as for the industrial control of waste water toxicity at different stages of its treatment, for identification of deviations in technological process or treatment devices, and for determining the efficiency of waste water treatment.

Conclusions.

Studies of the use of phytoplankton for assessment of environmental quality of the Eastern part of the Baltic Sea revealed about possibility to use algae for identification of changes in environmental micro- and macrocomponents; detection of limiting nutrients and determination of the presence of toxic factors. The practical importance of the work lies in the potential use of phytoplankton as a sensitive bioindicator in assessment of the water quality of the Baltic Sea.

e-mail: maija@hydro.edu.lv