Long-term ecological studies on phytoplankton in Mediterranean reservoirs: a case study from Sardinia (Italy)

Artificial lakes are strategic water resources in the Mediterranean region but are one of the most vulnerable areas to the impacts of climate change, particularly to the expected significant decrease in water resources; therefore, enhancing the ability of water management to improve water quality is crucial. This study contributes to the knowledge of the long-term ecological features of Mediterranean artificial lakes based on a case study of Bidighinzu Lake in Sardinia, Italy, a warm, monomitic and hypereutrophic reservoir mainly used for drinking water. Among the different restoration actions carried out in Bidighinzu Lake to improve water quality is the reduction of nutrient loads from the watershed. To study multiannual trends of the trophic state, a long-term series (1988–2012) of both environmental variables and phytoplankton was analysed. Mann-Kendall tests revealed increasing trends for total nitrogen to total phosphorus ratios (TN:TP; significant in spring, summer, and winter) and decreasing trends for ammonium (winter and autumn), TP (all seasons), orthophosphate (spring, summer, and winter), reactive silica (autumn), and pH (summer). Among phytoplankton, significant increasing trends were assessed in biomass of Bacillariophyceae, Chlorophyceae, Dinophyceae, and Chrysophyceae. Cyanobacteria did not show a clear trend, although at the order level, Chroococcales significantly increased in summer and decreased in winter. Despite these major tendencies, the lake did not show significant improvement in its eutrophic state, probably due to an inherent resistance and the insufficient and/or not well calibrated or applied recovery actions.