| Abstract [eng] |
Curonian Lagoon is a shallow transitional water basin located in the south-eastern part of the Baltic Sea. The southern and central parts of the lagoon contain freshwater due to discharge from the Nemunas River, while the salinity in the northern part varies from 0 to 8 PSU, depending on winds activity affecting brackish water inflow from the Baltic Sea. The investigation was carried out in the fresh-brackish water mixing zone (Influence zone of Baltic Sea), in the central part and Nemunas River influence zone in July-August 2004 - 2006. Changes in physico-chemical parameters, chlorophyll a concentration, phytoplankton and toxic algae cell density were monitored. Totally 223 species and varieties mainly belonging to Chlorophyceae (43 %) and Cyanophyceae (32 %) were found. 26 algae species from 3 algae classes (Cyanophyceae, Chlorophyceae and Dinophyceae) were identified as potential toxic species in the northern part of Curonian Lagoon during 2004 and 2006 summer time. Dominated toxic species Ahpanizomenon flos-aquae, Microcystis aeruginosa, M. viridis, M. wesenbergii, Woronichinia compacta. Phytoplankton biomass in Curonian Lagoon surface ranged from 12,27 to 50,22 mg/l. The peak of phytoplankton (33,11 mg/l) and potential toxic algae (28,67 mg/l) biomass in 2004 summer time was observed near by Klaipeda Strait, were Aphanizomenon flos-aquae contain 36 % from total biomass. In 2005 summer time the highest phytoplankton (50,22 mg/l) and toxic algae (21.46 mg//l) biomass were in the influence zone of Nemunas River. Marine toxic cyanobacteria species Nodularia spumigena were found in all investigating Curonian Lagoon areas. All toxic algae species and Microcystis cyanobacteria recruitment in water column per day was investigated in the littoral sites located by Juodkrante. The highest toxic algae biomass (3,84 mg/l and 6,25 mg/l) at dark period (2:00) was near bottom. Baltic Sea brackish water (salinity was till 4,4 PSU) influence vertical distribution of toxic algae during daytime. Their largest biomass (7,83 mg/l). Twice highest Microcystis biomass (4.91 mg/l) during dark period was in the surface layer. Phytoplankton growth is controlled by the supply of limiting nutrients, usually nitrogen or phosphorus. The Curonian Lagoon may be N- or P-limited depending on the volume of inputs from marine sources. The experiment of enclosure nutrient enrichment shows, that nutrient limitation plays an important role in succession of phytoplankton. Experiment using different nutrient (N and P) manipulations were performed in 70 liter mesocosms of 4 days. Results revealed that phytoplankton and also toxic algae and heterocystous forming cyanobacteria development in the lagoon is strongly affected by nutrient concentration in the water. On the third day of experiment in the mesocosm with phosphorus (biomass – 5,04 mg/l) and with nitrogen (biomass – 4,58 mg/l) biomass of toxic algae species three times greater then in control. The same situation was estimated with heterocystous forming cyanobacteria. Using the algae species composition state, phytoplankton and toxic algae species abundance and biomass data, was indicated hypertrophic-eutrophic status of Curonian Lagoon water in 2004-2005 summer. |
