Differences and similarities in the phytoplankton communities of two coupled transitional and marine ecosystems (the Lagoon of Venice and the Gulf of Venice - Northern Adriatic Sea)

The main aim of this paper is to paint an ecological picture of the phytoplankton communities of two adjacent and connected ecosystems, one transitional and one coastal marine, in the Northern Adriatic Sea: the Lagoon of Venice (LoV) and the Gulf of Venice (GoV). Based on 10 years (2011- 2020) of monthly samplings, we compare the taxonomic composition, abundance and seasonal cycles of the two ecosystems. We focus on the inner zones of the LoV and on the coastal sea up to 8 nmi offshore, an area suitable for assessing the reciprocal influence of the lagoon and sea in terms of phytoplankton. Our main interest is to verify (i) whether the sea still affects the lagoon phytoplankton and (ii) whether the lagoon can provide organisms to the adjacent sea. Using a matrix composed of 466 samples, we performed various types of analysis to: (i) identify the prevalent features and seasonal patterns of abiotic factors and chlorophyll a, (ii) assess and compare taxonomic composition at each station and (iii) identify the generalist and specialist taxa. Our findings provide evidence that the prevalent structure of the communities in the selected areas of the two environments clearly differ concerning (i) seasonal succession, unimodal in the LoV (only one peak in summer) and multipeak in the GoV (a succession of small peaks from spring to autumn), (ii) abundance and chlorophyll a, both much higher in the LoV (average: 6,009,593 cells l-1 and 4.1 ?gl-1 respectively) than in the GoV (average 2,901,266 cells l-1 and 1,5 ?gl-1 respectively), (iii) community composition, dominated by diatoms shared with benthic habitats (e.g. Thalassiosira, Nitzschia, Navicula) in the lagoon and by euplanktonic diatoms (e.g. Skeletonema, Chaetoceros, Pseudonitzschia) in the sea. The phytoplankton in the LoV appears to be affected by the marine phytoplankton of the adjacent sea and vice versa: the two environments share taxa that are both generalist (e.g. Skeletonema, Chaetoceros, Cyclotella, Pseudonitzschia) and specialist. Although the dominant factors in structuring the phytoplankton communities are local, dispersal rates, while not intense enough to generate transport of species that could significantly affect assemblage composition, are also at play.