Marine plankton community and net primary production responding to island-trapped waves in a stratified oligotrophic ecosystem

During the typical summer, the oligotrophic Adriatic Sea is characterized by low productivity determined by strong water column stratification that inhibits vertical mixing and nutrient supply to the euphotic zone. These conditions can be disrupted at times by transient physical forcing that enhances nutrient fluxes and creates localized hotspots of relatively higher net primary production (NPP). In this study, plankton abundance and diversity were investigated in relation to physical forcing and nutrient concentration in an area impacted by island-trapped waves (ITWs) near Lastovo Island (Adriatic Sea). The episodic ITW events resulted in an enhanced uplift and vertical excursion of the thermocline, marked by anomalously higher nutrient concentrations and a corresponding increase in NPP in the thermocline layer. Physicochemical properties explained 11.7% (p = 0.002) of micro- and nanophytoplankton variability, and 88.9% (p = 0.001) of the picoplankton community variability. A significant response in the plankton community composition (p = 0.001) due to the ITW phenomena was observed in bacterioplankton composition. Within identified amplicon sequence variance (ASVs), primary producers were scarce and mainly represented by cyanobacteria (Synechococcus, strain CC9902), stramenopiles (Pelagomonas) and chlorophytes (Ostreococcus). The remaining ASVs were assigned to class Copepoda, parasitic fungi (Meyerozyma spp.), mixotrophic dinoflagellates (family Peridiniales, mostly genus Blastodinium) and parasitic Ciliophora (Scuticociliata). Bacterial ecological function corresponded to chemoheterotrophy, degradation and fermentation processes, while samples collected after the most intense ITW episode also showed abundant bacteria linked to plastic degradation and parasitosis. These results highlight the ecological role of localized physical phenomena in enhancing near-shore primary productivity and fine shifts of plankton taxa in oligotrophic systems.