Plankton communities in oligotrophic offshore environments in the eastern adriatic sea in relation to mesoscale hydrodynamic fluctuations

Oligotrophic marine systems respond rapidly to episodic nutrient enrichment with an increase in productivity at all trophic levels. In order to link the prevailing hydrodynamic processes with plankton indicators, we investigated the biomass, composition and production of plankton communities as well as the physical and chemical parameters in the central Adriatic areas of Blitvenica and Jabuka at three stations during two seasons (November 2020 and May 2021). The analysis of thermohaline properties and horizontal currents showed that warmer and more saline water from the southern Adriatic flowed along the coast in November and that there was a strong freshwater influence in May. Different hydrodynamic processes in the different seasons lead to different nutrient sources, e.g. higher concentrations of phosphate and inorganic nitrogen in November were the result of the advection of warmer and saltier water from the southern Adriatic, while May was characterized by a higher concentration of organic nitrogen and the influence of the Krka River. Both situations were also reflected in the composition and size of the phytoplankton, zooplankton and bacterioplankton community. In November, picoplankton dominated the phytoplankton biomass and primary production, while in May, nanoplankton predominated. Microplankton was most strongly represented in the lower layers during both sampling season characterized by certain phytoplankton species. The development of larger phytoplankton cells in May favored the development of zooplankton, which was reflected in an increase in the number of copepods and a lower diversity compared to November. Of all the biological parameters analysed, only bacterial production (BP) was high in May at the station in Blitvenica (BL1). • Plankton communities respond to mesoscale hydrodynamic fluctuations. • Advection of water from the southern Adriatic resulted in a deeper mixed layer. • Advection of warmer and saltier water is favoured by coccolithophores. • Oligotrophic marine systems respond rapidly to episodic nutrient enrichment from river discharges. • The deep chlorophyll maximum (DCM) is composed of different sizes and taxonomic groups of phytoplankton in different seasons. • The high bacterial production (BP) in the entire water column is the result of the supply of organic nitrogen by the freshwater, but also the excretion of organic material and nutrients by certain groups of phytoplankton and zooplankton.