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 thephytoplankton communities of two adjacent and connected ecosystems,one transitional and one coastal marine, in the Northern Adriatic Sea: theLagoon 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 theinner zones of the LoV and on the coastal sea up to 8 nmi offshore, an areasuitable for assessing the reciprocal influence of the lagoon and sea in terms ofphytoplankton. Our main interest is to verify (i) whether the sea still affects thelagoon phytoplankton and (ii) whether the lagoon can provide organisms to theadjacent sea. Using a matrix composed of 466 samples, we performed varioustypes of analysis to: (i) identify the prevalent features and seasonal patterns ofabiotic factors and chlorophyll a, (ii) assess and compare taxonomiccomposition at each station and (iii) identify the generalist and specialist taxa.Our findings provide evidence that the prevalent structure of the communitiesin the selected areas of the two environments clearly differ concerning (i)seasonal succession, unimodal in the LoV (only one peak in summer) and multipeakin the GoV (a succession of small peaks from spring to autumn), (ii)abundance and chlorophyll a, both much higher in the LoV (average: 6,009,593cells l-1 and 4.1 ?gl-1 respectively) than in the GoV (average 2,901,266 cells l-1and 1,5 ?gl-1 respectively), (iii) community composition, dominated by diatomsshared with benthic habitats (e.g. Thalassiosira, Nitzschia, Navicula) in thelagoon and by euplanktonic diatoms (e.g. Skeletonema, Chaetoceros,Pseudonitzschia) in the sea. The phytoplankton in the LoV appears to beaffected by the marine phytoplankton of the adjacent sea and vice versa: thetwo environments share taxa that are both generalist (e.g. Skeletonema,Chaetoceros, Cyclotella, Pseudonitzschia) and specialist. Although thedominant factors in structuring the phytoplankton communities are local,dispersal rates, while not intense enough to generate transport of species thatcould significantly affect assemblage composition, are also at play.