Assembly rules vary seasonally in stable phytoplankton associations of the Gulf of Naples (Mediterranean Sea)

Coastal phytoplankton present remarkable variability because of complex patterns and relationships of hydrological parameters at the land-sea interface. We analysed a time series of 917 phytoplankton samples collected in the period 1984-2010 at the coastal station LTER-MC (Gulf of Naples, Mediterranean Sea) with the aims of determining: (i) whether recurrent species associations are recognizable in this variable coastal environment; (ii) what are their composition and seasonal patterns; (iii) what are the environmental variables driving their distributions, and (iv) to what extent those associations are functionally homogeneous. Using a cluster analysis of a dataset of the 87 more frequent and abundant species in the samples, we identified seven main groups of species. Five of them were recurrent over the years in the same seasons, while two other groups showed a bimodal occurrence or no clear seasonality, respectively. Differences in the occurrence of those assemblages were primarily associated with the season (i.e., temperature), and the coastal nature of the site (mainly salinity). Except for the winter cluster, which included mostly coccolithophores and silicoflagellates, diatoms were present in all associations, along with species belonging to diverse phylogenetic groups. The diversity of morphological features defining ecological traits varied across the different associations, with a maximum divergence in the spring association and a minimum in winter. We conclude that the stability of the associations is related to the marked phenological recurrence of their individual species and their overlap in specific periods of the year, while convergent or divergent trait compositions in the different associations indicate a variable strength of the environmental filtering across the seasonal succession.

Automated summary

Coastal phytoplankton exhibit remarkable variability, and this study analyzed phytoplankton samples from the Gulf of Naples in the Mediterranean Sea to identify species associations and understand their composition, seasonal patterns, and environmental drivers. Cluster analysis revealed seven main groups of species, with five recurring in the same seasons and two displaying bimodal occurrence or no clear seasonality. Differences in occurrence were primarily associated with temperature and salinity. Diatoms were present in all associations, along with species from diverse phylogenetic groups. The stability of associations was related to phenological recurrence and overlap, while trait compositions indicated variable environmental filtering across seasons.