PHYTOPLANKTON COMMUNITY IN RELATION TO SALINITY FRONTS AT THE ENTRANCE TO THE GULF-OF-FINLAND, BALTIC SEA

Horizontal and temporal distributions of phytoplankton populations, in cluding the mosi abundant mixotrophic and heterotrophic flagellates, w ere studied during a cyanobacterial bloom at the entrance to the Gulf of Finland. The study area was characterized by quasi-persistent salin ity fronts. We examined the responses of different species to horizont al discontinuities in surface layer salinity, and compared their distr ibutions. Several species showed significant (p<0.05) differences (ana lysis of variance and t-test) in abundance in different water masses. In the early phase of the bloom, the front had a marked effect on the spatial distribution of the filamentous cyanobacteria Nodularia spumig ena Mertens and Aphanizomenon flos-aquae Ralfs ex Bornet et Flahault. Biomass decreased steeply at the low salinity side of the front. Durin g later bloom stages, spatial separations of cyanobacterial bloom spec ies disappeared. In the microplankton size group, the abundances of Di nophysis norvegica Claparede & Lachmann, Dinophysis acuminata Clapared e & Lachmann, Protoceratium reticulatum (Claparede sc Lachmann) Butchl i, Oocystis lacustris Chodat, Amphidinium crassum Lohmann and Ebria tr ipartita (Schumann) Lemmermann were also spatially separated along dif ferent water masses in the frontal zone. In the nanoplankton size grou p (< 20 mu m), flagellates (Chrysochromulina spp. and Cryptomonadales spp.) showed increasing abundances towards the low salinity water and tended to form distinct maxima, situated at or adjacent to the salinit y fronts. Frontal zones at the entrance to the Gulf of Finland may reg ulate the phytoplankton community composition.