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Unusual occurrence of planktonic diatom species, Chaetoceros bacteriastroides and Chaetoceros pseudosymmetricus, was noticed in three different marine ecosystems of Adriatic Sea: the Krka Estuary and Telaščica Bay in the Central Adriatic, and in southern Adriatic offshore. From 2010 to 2015, these two Chaetoceros species were recorded in heterogeneous environmental conditions and in a very low abundances. Both species are regarded as very rare in world oceans, and consequently knowledge of their distribution and ecology is rather poor. Primarily described from tropical waters and showing Indo-Pacific distribution, C. bacteriastroides and C. pseudosymmetricus findings in Adriatic represent the northernmost records in world's oceans and seas. For C. pseudosymmetricus this is also the first occurrence in European seas. Areal expansion and introduction of new phytoplankton species in the Adriatic Sea might be related to different circulation regimes in the Ionian Sea and the concurrent rise in sea temperature in the Mediterranean in the last decade. Recent investigations have shown that entering currents, of either Atlantic/Western Mediterranean or Eastern Mediterranean origin, modify the composition of the plankton community in the Adriatic by bringing different newcomers.

The species composition and size-structure of the phytoplanktoncommunity in the Boka Kotorska Bay (SE Adriatic Sea) were analysed withrespect to abundance and carbon biomass, together with the physico-chemicalparameters, with the aim of evaluating the predefined oligo-mesotrophicstatus of this transitional water ecosystem. Three stations locatedin the inner part of the Bay were sampled with seasonal frequencyin 2008/2009. Picophytoplankton cells were counted using flowcytometry; nanophytoplankton and microphytoplankton were identifiedand counted by light microscopy. The relative importance of thepicoplankton in the Bay, in terms of both abundance and biomass,during all the investigated seasons emphasized their significancein the phytoplankton community. Picocyanobacteria (Synechococcus)constituted a significant part of the summer assemblages withregard to both abundance (up to 3.38 × 108 cellsL-1)and carbon biomass (up to 73% of total phytoplankton carbon).The contribution of the nanophytoplankton was found to be generallylow (Skeletonema marinoi, dominated the microphytoplanktonfraction. S. marinoi was the most abundant in spring/winter(up to 2.86 imes 106 cells L-1) above the halocline(making a 96% contribution to the microphytoplankton). The potentiallytoxin-producing diatom Pseudo-nitzschia pseudodelicatissimawas recorded at abundances greater than 105 cells L-1, togetherwith Thalassionema frauenfeldii, as well as the dinoflagellatesProrocentrummicans and the potentially harmful P. minimum. The higher valuesof phytoplankton biomass and the dominance of phytoplankton speciesor groups with preferences for nutrient-enriched conditions appear tobe consistent with the oligo-mesotrophic status of this specific ecosystem.