How do phytoplankton biomass and functional groups react to environmental constraints? A comparison in two different Mediterranean ponds

Grouping organisms into functional grouping represents a way to better understand how the environmental constrains act on these organisms through the analysis of peculiar features, usually morphological, shared by the set of species. Since groups (Coda) react to environmental constrains with an analogous response, this approach was adopted to compare the structure and the biomass of phytoplankton assemblages in two different Mediterranean ponds, one temporary and the other permanent. A phytoplankton survey, carried out monthly in the brackish and shallow Biviere di Gela in 2005-2007, revealed a deep transformation in the structure of its phytoplankton assemblage as compared with similar data collected in 1987-1988. A progressive shift from a clear water macrophyte-dominated state to a turbid cyanobacteria-dominated one was observed together with a transition from a given set of Coda to another one. In addition to the effect driven by the climate, an alteration of the original composition was induced by an anthropogenic-mediated decrease in the electrical conductivity of the water that likely opened the way to a toxic alga, Prymnesium parvum, whose allelopathyc and toxic glycosides further contributed to the transformation of the lake biota. While Biviere di Gela shows a discrete influences shaped both by human and climatic impact, the second study site, Gorgo di Rebuttone, seems to be more stable in its principal habitat characteristics.