Ecological effects of level fluctuation in Lake maggiore, Nothern Italy

Global climatic changes increased the frequency of extreme events such as drought periods followed by heavy rains. We hypothesized that the level fluctuations occurring in Lake Maggiore, an oligotrophic deep lake in Northern Italy, could cause the release of pulses of phosphorus from the littoral zone, thus favoring cyanobacterial growth. The occurrence, in summer 2005, of a bloom of Anabaena lemmermannii, a species never encountered before in the pelagic zone of Lake Maggiore, could be the consequence of the unusual fluctuations of lake level. This bloom prompted us to experimentally evaluate the dynamics of accumulation and release of organic substance and nutrients from shores undergoing a succession of drought and flood events. Inert substrates were exposed at -0.5 m depth for around 30 days, moored 50 m from the coast. At recovery, the Particulate Organic Carbon and Nitrogen and the Particulate Inorganic Phosphorus accumulated on the inert substrates were measured. The lowering or rising of lake levels was simulated leaving the substrates to dry for 30 days and successively rewetting them in MQ water. Analyses of dissolved carbon and phosphorus released during 3 days of rewetting have been successively performed. This experiment was carried out from late spring to early autumn for two subsequent years. The results obtained showed that, along the perimeter of the lake (170 km), around 2 tons of organic carbon and 10 kg of phosphorus can accumulate monthly in the productive seasons. The composition of this material changes during the season, enriching in P over the summer. The appearances of A. lemmermannii blooms are coincident with a pulse of P and of organic material with C/N < 10 from the littoral. We observed this pattern over two successive years (2010 and 2011) when A. lemmermannii bloom appeared in the summer, after a heavy rain following a dry period of low-level. These results confirm our hypothesis of a relation between lake level fluctuations and cyanobacterial blooms.