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
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 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 (<20% of the total phytoplankton carbon) in all
seasons, and was dominated by autotrophic/mixotrophic flagellates. Species with
a preference towards nutrient-enriched conditions, like the diatom Skeletonema
marinoi, dominated the microphytoplankton fraction. S. marinoi was the most
abundant in spring/winter (up to 2.86×106 cells L−1) above the halocline (making
a 96% contribution to the microphytoplankton). The potentially toxin-producing
diatom Pseudo-nitzschia pseudodelicatissima was recorded at abundances greater
than 105 cells L−1, together with Thalassionema frauenfeldii, as well as the
dinoflagellates Prorocentrum micans and the potentially harmful P. minimum.
The higher values of phytoplankton biomass and the dominance of phytoplankton
species or groups with preferences for nutrient-enriched conditions appear to be
consistent with the oligo-mesotrophic status of this specific ecosystem.
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