Influence of climate on phytoplankton dynamics in Mediterranean water bodies

Climate is among the factors that influence the hydrology of water bodies and their mixing/stratification patterns. As a consequence, it may affect the ecomorphological selection of phytoplankton. Global climatic patterns, driven by Sun’s magnetic field, were shown to promote cyclic cyanobacteria bloom during the years of drought caused by El Niño. Cyclic droughts,with a periodicity of about 11 years, also occur in the Mediterranean Basin where they produce analogous responses in phytoplankton composition. Some evidences exist that climate change can promote pseudo-eutrophication events and, by altering thehydraulic balance in shallow lakes, cause a shift between a clear macrophyte-dominated state and a turbid algae-dominated one. This is likely to occur in Mediterranean shallow lakes. A phytoplankton survey, carried outmonthly in two shallow Sicilian lakes, in 2005–2007, revealed a transformation in the structure of their phytoplankton assemblages as compared with similar data collected in 1987–1988, which cannot be explained by any increase in nutrient loading. An analysis of the trends followed by precipitation and temperature over the last 40 years, showed reduced water inflows in both the lakes, due to increased air temperature and evapotranspiration rather than to a decrease in the amount of precipitation. This reduction in water level disrupted the littoral zone of the lakes and transformedthem from clear, macrophyte-dominated environments to turbid ones characterized by huge summer blooms of cyanobacteria. The research carried out on climate change effects have shown that small changes in thephysical characteristics of a lake can have adisproportionate effect on its chemistry (e.g. an intensified recycling of phosphorus, alteration in the ratio between mixing depth and euphotic depth) andbiology (e.g. disruption of the littoral zone, enhanced cyanobacterial blooms). Thus, they need to be taken into consideration by lake managers when setting restoration targets and/or limits for nutrient loading.