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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

Bosak S., Šilović T., Ljubešić Z., Kušpilić G., Pestorić B., Krivokapić S., Viličić D.

Oceanologia

2012

No. 54 (2)

255-286

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.

Distribution of phytoplankton along the thermohaline gradient in the north-eastern Adriatic channel; winter aspect

Viličić D., Kuzmić M., Bosak S., Šilović T., Hrustić E., Burić Z.

Oceanologia

2009

No. 51 (4)

495-513

The distribution of phytoplankton and its relation to the hydrographic features in the north-eastern Adriatic was investigated in February 2008. The area of interest included a thermohaline gradient in the channel situated between the coast and the islands lying parallel to the coast. The gradient is controlled by the influx of oligotrophic karstic riverine water at the south-eastern end, submarine springs in the middle part, and warmer offshore waters at the north-western end of the channel. The change of temperature and salinity in the estuarine transition zone was accompanied by abundant diatoms and dinoflagellates below the halocline, with dominant chain-forming diatoms (Chaetoceros, Bacteriastrum) in abundances reaching 5 ×105 cells dm-3. The impact of coastal submarine springs detected by infrared remote sensing resulted in the growth of cyanobacteria in the nitrogen-depleted surface waters. The greater contribution of picoplankton, as well as of nanoplanktonic coccolithophorids and cryptophytes, in the outer channel system indicated their preference for oligotrophic conditions. Flow cytometric counts of nanophytoplankton were 10-30 times greater than inverted microscope counts. Cyanobacteria were about five times more abundant than picoeukaryotes. The study demonstrates how different techniques (remote sensing and in situ investigations) can be useful in understanding the biological and hydrographic set-up in the specific oligotrophic eastern Adriatic coastal environment.