Phytoplankton dynamics in two reservoirs with different trophic state (Lake Rosamarina and Lake Arancio, Sicily, Italy)

Two man-made Sicilian lakes of differing trophic status were sampled weekly for a year, in order to compare their phytoplankton dynamics and to identify their driving factors. The water bodies store comparable volumes but the eutrophic Lake Arancio may be considered to be a shallow lake whereas the mesotrophic Lake Rosamarina is deep. The dynamics of their principal physical, chemical and biological features were analysed using, among others, Canonical Community Ordination. The results obtained suggest that increased nutrient availability causes an increase of phytoplankton biomass but no community change nor any alteration to the typical assemblages supported. The major influence in the more eutrophic lake was exerted by physical variates and, in particular, by the underwater light climate regime, whereas a significant influence of nutrients was observed in the mesotrophic one. Moreover, an assemblage of species able to regulate their buoyancy, or adapted to frequent dark/light cycles was characteristic of the eutrophic lake. We hypothesize that the physical characteristics of a water body and secondary modifications induced by increased nutrient availability, e.g. higher phytoplankton biomass, contraction of the euphotic depth, selective grazing, may drive the seasonal succession of phytoplankton in the upper part of the trophic spectrum.