Phytoplankton abundance, community structure and diversity in the eutrophicated Orbetello lagoon (Tuscany) from 1995 to 2001

The Orbetello lagoon consists of two coastal ponds, Laguna di Ponente and Laguna di Levante, characterized by high nutrient concentrations, very poor exchanges with the sea and subjected to recurrent eutrophication crisis mainly in the past decades. To assess its current trophic status, from 1995 to 2001 the temporal evolution and spatial distribution of environmental factors, phytoplankton biomass and composition have been investigated. In this paper, the mean annual cycles of phytoplankton abundance, community structure and diversity are discussed. The seasonal cycles of micro- and nanophytoplankton densities are characterized by high frequency oscillations with fast increasing and decreasing blooms, but generally the lowest densities occur in winter, except for the peaks of December, and the highest ones during spring and summer. Despite the variability at short time scale of phytoplankton density, composition and diversity, a recurrent pattern was evidenced. Nanoplanktonic flagellates were the dominant fraction, mainly cryptophyceans during winter and early spring, while diatoms showed high summer blooms of nanoplanktonic centric genera and were very scarce during the other seasons, mainly with pennate genera. The phytoplankton community structure appeared to be more diversified from early summer to fall with the variable contribution of diatoms, dinoflagellates and other flagellates. Phytoplankton diversity, computed by means of the Shannon index, was relatively low (mean of 1.96 bit per cell) and two phases, common to the whole lagoon, have been evidenced: a first phase of lower diversity during winter and early spring, followed by an increasing trend that reached the highest diversity in late summer-fall. The pattern of nutrients' (DIN and DIP) availability and depletion leads to a seasonal change of the N/P ratio that seems to play a role as regards the phytoplankton community succession and then diversity, favoring or limiting the growth of the different groups.