Picoplankton community structure in the southeastern Adriatic Sea

The Adriatic Sea, located in the northernmost part of the Med Sea, represents an important study site for for rapid climatology impacts, due to its topographical, hydrological features and complex circulation patterns. Its southern part is a highly oligotrophic environment where picophytoplankton are the main primary producers and the carbon/energy is transported mainly via the microbial food web. The aim of this study was to investigate the spatial distribution of the picoplankton community along the transect from Dubrovnik to South Adriatic Pit. The samples were taken at eight stations in early March 2016 as a part of BIOTA (Bio-tracing Adriatic Water Masses) project. Bacteria, picocyanobacteria and heterotrophic nanoflagellates were determined and enumerated by flow cytometry. Synechococcus dominated the surface photosynthetic community with maximal abundances up to 2.7 x 104 cells/mL with the high numbers recorded at stations both closer and further from the nearshore area (stations P120, P800). Prochlorococcus abundances were below 2.0 x 104 cells/mL, except at station P600 where 2.5 x 104 cells/mL were recorded. Picoeukaryotes were more abundant at stations farther from the coast, with abundances up to 9.2 x 103 cells/mL. Heterotrophic bacteria were separated into HNA and LNA subpopulations with total numbers up to 1.8 x 105 and 8.8 x 105 cells/mL, respectively. HNA populations dominated at offshore stations reaching their maximum at depths below the euphotic zone. Heterotrophic flagellates were present in low numbers, between 1.3 x 102 and 1.2 x 103, and generally were more abundant at the surface of nearshore stations.