Red sky at night cyanobacteria delight: the role of climate in structuring phytoplankton assemblage in a shallow, Mediterranean lake (Biviere di Gela, southeastern Sicily)

The hypothesis that climate changes may strongly interfere with the peculiar hydrological patterns in the Mediterranean basin and alter the structure of the aquatic biota was tested in a shallow Sicilian lake. A phytoplankton survey, carried out monthly in the brackish and shallow Biviere di Gela, in 2005-2007, revealed a transformation in the structure of its phytoplankton assemblage as compared with similar data collected in 1987-1988. An analysis of the trends followed by precipitation and temperature over the last 40 years, showed reduced water inflows, due to increased air temperature and evapotranspiration rather than to a decrease in the amount of precipitation. A consequent reduction in water level occurred, which almost caused the disappearing of the lake at the beginning of the century. This reduction in water level disrupted the littoral zone of the lake and transformed it from a clear, macrophyte-dominated environment to a turbid one characterized by huge summer blooms of the cyanobacteria Aphanizomenon ovalisporum, Cylindrospermopsis raciborskii, and Pseudanabaena limnetica. The addition of freshwater from a nearby dam reservoir, to restore water levels, likely opened the way to another alga, Prymnesium parvum, whose allelopathic and toxic glycosides further contributed to the transformation of the lake biota. On the whole, the Biviere di Gela is an example of how climate changes, coupled with inadequate management procedures, may negatively impact Mediterranean aquatic ecosystems.