Abstract
Horizontal and temporal distributions of phytoplankton populations, including the most abundant mixotrophic and heterotrophic flagellates, were studied during a cyanobacterial bloom at the entrance to the Gulf of Finland. The study area was characterized by quasi-persistent salinity fronts. We examined the responses of different species to horizontal discontinuities in surface layer salinity, and compared their distributions. Several species showed significant (p<0.05) differences (analysis 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 spumigena Mertens and Aphanizomenon flos-aquae Ralfs ex Bernet et Flahault. Biomass decreased steeply at the low salinity side of the front. During later bloom stages, spatial separations of cyanobacterial bloom species disappeared. In the microplankton size group, the abundances of Dinophysis norvegica Claparéde & Lachmann, Dinophysis acuminata Claparéde & Lachmann, Protoceratium reticulatum (Claparéde & Lachmann) Bütchli, Oocystis lacustris Chodat, Amphidinium crassum Lohmann and Ebria tripartita (Schumann) Lemmermann were also spatially separated along different water masses in the frontal zone. In the nanoplankton size group < 20 µm), 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 salinity fronts. Frontal zones at the entrance to the Gulf of Finland may regulate the phytoplankton community composition.