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

摘要

Coastal phytoplankton present remarkable variability because of complex patternsand relationships of hydrological parameters at the land–seainterface. We analyseda time series of 917 phytoplankton samples collected in the period 1984–2010atthe coastal station LTER-MC(Gulf of Naples, Mediterranean Sea) with the aims ofdetermining: (i) whether recurrent species associations are recognizable in this variablecoastal environment; (ii) what are their composition and seasonal patterns; (iii)what are the environmental variables driving their distributions, and (iv) to what extentthose associations are functionally homogeneous. Using a cluster analysis of adataset of the 87 more frequent and abundant species in the samples, we identifiedseven main groups of species. Five of them were recurrent over the years in the sameseasons, while two other groups showed a bimodal occurrence or no clear seasonality,respectively. Differences in the occurrence of those assemblages were primarily associatedwith the season (i.e., temperature), and the coastal nature of the site (mainlysalinity). Except for the winter cluster, which included mostly coccolithophores andsilicoflagellates, diatoms were present in all associations, along with species belongingto diverse phylogenetic groups. The diversity of morphological features definingecological traits varied across the different associations, with a maximum divergencein the spring association and a minimum in winter. We conclude that the stability ofthe associations is related to the marked phenological recurrence of their individualspecies and their overlap in specific periods of the year, while convergent or divergenttrait compositions in the different associations indicate a variable strength of the environmentalfiltering across the seasonal succession.