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**This study was funded by the Norwegian Cooperation Programme on Research and Higher Education with countries in the Western Balkans: ‘Marine Science and Coastal Management in the Adriatic, Western Balkans. An education and research network (2006–2009)’ and by the Ministry of Science, Education and Sport of the Republic of Croatia (Project Nos. 119-1191189-1228, 098-0982705-2729 and 001-0013077-0845).

AbstractThe species composition and size-structure of the phytoplankton community in the Boka Kotorska Bay (SE Adriatic Sea) were analysed with respect to abundance and carbon biomass, together with the physico-chemical parameters, with the aim of evaluating the predefined oligo-mesotrophic status of this transitional water ecosystem. Three stations located in the inner part of the Bay were sampled with seasonal frequency in 2008/2009. Picophytoplankton cells were counted using flow cytometry; nanophytoplankton and microphytoplankton were identified and counted by light microscopy. The relative importance of the picoplankton in the Bay, in terms of both abundance and biomass, during all the investigated seasons emphasized their significance in the phytoplankton community. Picocyanobacteria (Synechococcus) constituted a significant part of the summer assemblages with regard to both abundance (up to 3.38×108 cells L−1) and carbon biomass (up to 73% of total phytoplankton carbon). The contribution of the nanophytoplankton was found to be generally low (