Long-term phytoplankton dynamics in a Mediterranean eutrophic lagoon (Cabras Lagoon, Italy)

The temporal variations in phytoplankton abundance and diversity were analysed from 1999 to 2009 in Cabras Lagoon, a shallow eutrophic Mediterranean lagoon (west coast of Sardinia). It is one of the aquatic ecosystems listed in the “Marine Ecosystems of Sardinia” by the Italian Network of Long-Term Ecological Research (LTER Italy). The objectives of this work were to assess the general features of phytoplankton succession and its dynamics in relation to environmental factors along the decade 1999–2009, and to identify the main forces driving phytoplankton variability in terms of the general trends in Mediterranean transitional waters. Our results highlight several important aspects: a shift in the annual peak density from the summer–autumn season, observed in 1999–2002, to the winter–spring season in subsequent years; simultaneous reductions in chlorophyll a concentrations and cell size from 1999 to 2008; and the modification of the structure of the phytoplankton assemblages, from a more differentiated class composition in 1999–2002 to the subsequent dominance of the Cyanophyceae. Moreover, ANOSIM analysis on both abiotic and phytoplanktonic data shows significant differences among the hydrological years considered. Pearson’s correlations indicates temperature, salinity and nutrients as the environmental parameters that most influenced the phytoplankton composition and dynamics.