Phytoplankton size structure and species composition as an indicator of trophic status in transitional ecosystems: the case study of a Mediterranean fjord-like karstic bay

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 (<20% of the total phytoplankton carbon) in all seasons,and was dominated by autotrophic/mixotrophic flagellates. Specieswith a preference towards nutrient-enriched conditions, likethe diatom 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.