Microbial Ecology of the Southern Adriatic Sea: Convection and Water-Mass Dynamics

The southern Adriatic Sea is an oligotrophic basin influenced by seasonal stratification, winter convection, inflow of Levantine Intermediate Water (LIW), and spreading of North Adriatic Dense Water (NAdDW). This review synthesizes knowledge on planktonic microbial distribution, composition and activity and evaluates their relationships with these hydrographic processes. During stratification, microbial communities show vertical differentiation. During convection, phototrophic cells were detected in aphotic waters, in several cases at depths consistent with documented mixing, while deep water bacterial groups occurred within the mixed layer. These observations provide the strongest, although indirect, evidence for microbial redistribution. Picocyanobacterial distributions were associated with LIW-influenced waters, while bacterial composition and abundance differed among NAdDW-influenced water masses along the western southern Adriatic margin. Prokaryotic production increased after the 2012 convection event and differed among deep water masses during NAdDW spreading. Extracellular enzymatic activities varied with depth and water masses, with higher activities in NAdDW-influenced waters. However, evidence for a direct response to convection remains limited. Overall, microbial distribution and activity were associated with hydrographic conditions, but existing studies cannot reliably distinguish physical transport from biological responses or quantify the contributions of individual hydrographic processes.