Spatial changes in community composition and food web structure of mesozooplankton across the Adriatic basin (Mediterranean Sea)

Zooplankton are critical to the functioning of ocean food webs because of their utter abundance and vital ecosystem roles. Zooplankton communities are highly diverse and thus perform a variety of ecosystem functions; thus changes in the community or food web structure may provide evidence of ecosystem alteration. Assemblage structure and trophodynamics of mesozooplankton communities were examined across the Adriatic basin, the northernmost and most productive basin of the Mediterranean Sea. Samples were collected in June–July 2019 within the framework of the MEDIAS (MEDiterranean International Acoustic Survey) project, along coastal–offshore transects and from the surface to ca. 200m depth, covering the whole western Adriatic side; consistently environmental variables were also recorded. Results showed a clear separation between samples from the northern-central Adriatic and the southern ones, with a further segregation, although less clear, of inshore vs. offshore stations, the latter being mostly dominated in the central and southern stations by gelatinous plankton. Such patterns were mainly driven, based on the outputs of the distance-based linear model, by fluorescence (as a proxy for primary production) for northern-central stations, i.e. closer to the Po River input, and by dissolved oxygen, together explaining 44% of the total variance. Overall, at the basin level, the analysis of stable isotopes of nitrogen and carbon allowed for identifying a complex food web characterized by three trophic levels from filter feeders–herbivores to carnivores, passing through a general pattern of omnivory with varying preference towards herbivory or carnivory. Stable isotope signatures spatially varied between inshore vs. offshore communities and across sub-areas, with the northern Adriatic exhibiting greater d15N and more variable d13C than the other two sub-areas, likely attributable to the occurrence in the area of organic matter of both terrestrial and marine origin. Our results contribute to the knowledge of mesozooplankton community and trophic structure, at the basin scale across a coastal–offshore gradient, also providing a baseline for the future assessment of pelagic food webs within the European Council (EC) Marine Strategy Framework Directive.