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)

Abstract

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 multi-peak 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 µgl-1 respectively) than in the GoV (average 2,901,266 cells l-1 and 1,5 µgl-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.

Phytoplankton Size Structure and Diversity in the Transitional System of the Aquatina Lagoon (Southern Adriatic Sea, Mediterranean)

“…Moreover, these studies suggested meso-oligotrophic conditions for the system by considering the rate at which nutrients were renewed in the water column [ 22 , 43 ]. Our data confirmed these findings and demonstrated that the Aquatina Lagoon is still a highly productive ecosystem today, like other transitional Mediterranean systems [ 4 , 9 , 10 , 11 ].…”

“…The seasonal trend of phytoplankton showed large fluctuations on an annual cycle, driven both by the seasonal rhythm and by the pulse of nutrients (mainly nitrite plus nitrate). The maximum of the total autotrophic biomass was recorded in summer, as has already been described for other temperate transitional systems, such as the Venice Lagoon [ 10 , 11 ], which are characterized by shallow depths and permanently high nutrient levels [ 44 , 45 ]. On the contrary, the phytoplankton dynamics in the Aquatina Lagoon differed from those observed in other Apulian brackish environments.…”

“…In the 2007–2008 sampling period, diatoms were detected in the winter, as in the previous period, but also in the summer. Their presence in the summer is typical of temperate lagoons [ 48 , 56 ], characterized by persistently productive and turbulent conditions [ 2 , 10 , 11 ]. Some genera detected in the Aquatina Lagoon (e.g., Thalassiosira and Chaetoceros ) produce blooms in transitional waters globally [ 57 ].…”

“…Phytoplankton of transitional water ecosystems include a wide variety of prokaryotic and eukaryotic photosynthetic organisms belonging to various taxonomic groups, such as Cyanophyta, Bacillariophyta, Dinophyta, Chlorophyta, Euglenophyta, Chrysophyta, Cryptophyta, and Xanthophyta [ 8 ]. Studies on phytoplankton’s response to the environmental heterogeneity of coastal lagoons have demonstrated the direct effects of abiotic, hydrological, and geomorphological factors on biodiversity and the spatio-temporal dynamics of the phytoplankton community structure [ 4 , 9 , 10 , 11 ]. These studies analyzed patterns of variation of phytoplankton at taxonomic as well as morphological and functional levels, and demonstrated that phytoplankton’s growth is controlled by a combination of the temperature and the availability of light and nutrients [ 12 ].…”

Patterns and Predictors of Phytoplankton Assemblage Structure in a Coastal Lagoon: Species-Specific Analysis Needed to Disentangle Anthropogenic Pressures from Ocean Processes

“…In coastal lagoons, the combination of local and regional anthropogenic and natural forcings, land-ocean connectivity, strong benthic-pelagic coupling, relative confinement, complex hydrodynamics, and high habitat diversity increases the intricacy of the forces acting on phytoplankton assemblages [16,32]. This intrinsic complexity of coastal lagoons promotes variable patterns of phytoplankton change over time [33][34][35] and high spatial heterogeneity, both across [36][37][38] and within lagoons [10,21,[39][40][41].…”