Influence of internal island-trapped waves on plankton structure and trophic networks in stratified oligotrophic coastal waters.

During periods of water column stratification in marine oligotrophic ecosystems, physical forcings such as internal island-trapped waves (ITWs) can facilitate nutrient fluxes to surface waters and determine fine-scale changes in microbial communities. During a two-week in situ experiment at Lastovo Island, South Adriatic Sea, we conducted water-column community diversity assessments with environmental metabarcoding of plankton during and after ITWs events. Bacteria and eukaryotes communities showed significant dissimilarities between size-fraction and depths, with ITW event significantly contributing to clustering of eukaryotic communities. Major bacterial contributors were Gammaproteobacteria, Cyanobacteria, Bacteroidota and Verrucomicrobiota following ITW event. Bacterial functional profiling indicated that ureolysis, aerobic ammonia oxidation, nitrification and ectoparasitic or predatory roles were directly linked to shifting environmental parameters in the water column. Metazoan sequences (mainly Arthropoda, class Copepoda) dominated the micro size fraction, while various dinoflagellates (with high contribution of parasitic Syndiniales) dominated nano and pico size fractions. Primary producers were Mamiellophyceae, Prymnesiophyceae and Bacillariophaceae, with highest relative abundances in the deep chlorophyll maximum (DCM). Assessment of genus-level networks in surface waters and the DCM revealed that about 34 % and 39 % co-occurrences, respectively, were attributable to putative trophic interactions with protists dominating over zooplankton taxa in both communities. Important network taxa connectors were mainly identified among autotrophic protists within DCM, while in surface network connectors number generally decreased.