In the eastern part of the Black Sea harmful algal bloom problem is exists in the Paliastomi lagoon. Till 1934 Paliastomi was known as a fresh-water lake in the Black Sea coastal zone; than it was artificially connected to the Black Sea and is now a lagoon. Studying of this problem has begun since 1964. The identification of phytoplankton in previous years showed that blooms were caused by excessive growth of brackish-water toxic species Nodularia spumigena f.litorea, Anabaenopsis elenkinii and Protoperidinium globulus var. ovatum, and non- toxic Rhizosolenia fragilissima and Anabaenopsis arnoldii. These events repeated periodically in August-September and were associated with fish kills. Researches done by us in 1998-99 showed obvious changes of phytoplankton species in Paliastomi lagoon. The samples collected in 1998 (August- September) revealed the presence 76 species and samples collected in 1999 (September-August) revealed the presence 55 species, among them were: Cyanophyta, Bacillariophyta, Pyrrophyta, Xantophyta, Euglenophyta , Chlorophyta, in all 55 taxons. The species identified include Microcystis aeruginoza Kutz, Anabaena flos-aqua (Lyngb.), Oscilatoria sp., Cyclotella kuertzingiana, Coscinodiscus lacustris Grun. Most of phytoplankton species were fresh-water forms unlike from previous years. It has become clear that brackish-water and marine species has changed to fresh-water species. It may be caused by frequent lock of the channel in Paliastomi during the recent two years and by lessen of marine water from the Black Sea. Paliastomi is located in conservation of Kolkhi Lowland and is under the integrative management. The program of Integrative Management of the Black Sea Coastal Zone includes assessment of risk posed by harmful algal blooms in the Black Sea coastal environment and restoration the stock of fishes (during the last decade species of fishes reduced from 39 to 10) and other living resources in this area. It is considered that one of the reasons of eutrophication and harmful algal bloom events in Paliastomi is flowing of contaminated waters. Monitoring of harmful algal bloom is going for restoration the biodiversity in Paliastomi lagoon.