Picophytoplankton in Mediterranean Lagoons: new study cases from Sardinia (Calich, Corru S’Ittiri and Santa Giusta Lagoons)

"The cell size distribution within the phytoplankton community plays an essential role in determining the direction and the magnitude of carbon and energy fluxes in aquatic ecosystems. A trend of cell size reduction in phytoplankton composition has been signaled in a wide range of aquatic environments in the last decades, including brackish and hypereutrophic ecosystems in the Mediterranean region, suggesting that it can be one of the phytoplankton’s responses to global climate change. Careful investigations are therefore necessary to understand which environmental, climatic and\/or biological factors could control the dynamics of phytoplankton of small cell size and the relative taxonomic composition. On the other hand, because of its small size (0.2-2µm), the picoplanktonic component remained unknown for a long time. During the last decades the autotrophic picoplankton or picophytoplankton proved to be ubiquitous in the pelagic ecosystems and important for the substantial contribution to the total phytoplanktonic biomass and production in both marine and freshwater environments. In particular, there is growing evidence of the importance of the ecological role of picophytoplankton in estuarine and coastal ecosystems. Lagoons have always had a large economic relevance in relation to activities of fisheries and aquaculture and their monitoring and management to preserve them in a good ecological status have always been of primary importance. On these bases, a first research to study temporal and spatial dynamics of picophytoplankton during an annual cycle (June 2011 - May 2012) was carried out in three eutrophic Sardinian Lagoons: Calich (Gulf of Alghero), Corru S’Ittiri and Santa Giusta (Gulf of Oristano). Moreover, dynamics of the main trophic descriptors were also investigated to assess which environmental conditions favored picophytoplankton development. Samples were collected at different stations in each site (3 in Calich and Corru S’Ittiri, 5 in S. Giusta).. Results of statistical analysis (one-way analysis of Similarity - ANOSIM) revealed not significant differences among sampling stations of Calich and Corru S’Ittiri Lagoons, the two smaller systems. Instead, in S. Giusta Lagoon, significant differences resulted among the station nearest to the sea and those located in proximity of the main inputs of freshwater, considering both environmental and total picophytoplankton density.. In Calich and S. Giusta Lagoons, picophytoplankton reached major mean densities and biomasses in summer (in September 2011, respectively of 73 x 106 cells l-1 and 36.19 µg C l-1 and of 58 x 106 cells l-1 and 14.88 µg C l-1). In Corru S’Ittiri Lagoon, picophytoplankton reached the highest densities and biomasses in winter-spring season, with a maximum mean peak of 124 x 106 cells l-1 and 23.88 µg C l-1 in December 2011. In all the considered lagoons, picophytoplankton was principally represented by prokaryotes, which contributed with values > 90% of the total density and biomass in the longer part of the study period and in all the sampling stations. However, eukaryotes were also important in Corru S’Ittiri and S. Giusta Lagoons, contributing with values > 90% to the total density and biomass in December 2011, January and April 2012 in the first and in December 2011 in the second. On the contrary, eukaryotes contributed with values > 10% to the total density and biomass only in June 2011 in Calich Lagoon, indicating important differences in respect to the other sites.. "