Steady-state assemblages in a Mediterranean hypertrophic reservoir.: The role of Microcystis ecomorphological variability in maintaining an apparent equilibrium

Lake Arancio is a hypertrophic Mediterranean man-made lake, located on the southern coast of Sicily. Its artificial origin and the climate make it a very dynamic environment, strongly characterised by very wide water-level fluctuations. These vertical water movements interfere with the thermal stability of the water body often causing the breaking of the thermocline in mid-summer. In addition, the summer level-decrease influences the nutrient dynamics and modifies the z(mix)/z(eu) ratio. All these modifications were observed to support a high environmental variability, which was reflected by the richness of its phytoplankton composition and by its dynamics. Nevertheless, an investigation carried out from March 2001 to March 2002 showed that the assemblage was strongly dominated by a few species, one by one. In particular, two different Microcystis morphotypes dominated the assemblage from mid-April till the beginning of October. The prolonged dominance of these 'species' should suggest that a steady state condition took place in Lake Arancio during spring and summer 2001. This is in contrast with previous investigations, which showed high diversity values especially occurring in the period of strong environmental instability when the continuous dewatering caused the breaking of the thermocline in the middle of summer. Nevertheless, this dominant species showed a very wide morphological variability and alternated among 'more S', 'S', and 'R' (sensu Reynolds) ecotypes. The ever-changing morphological features suggest a different ecological behaviour of the species involved. They seem to confirm that the environmental variability of Mediterranean reservoirs sustains high diversity values, even though this diversity has to be sought in the amplitude of morphological plasticity of one or a few species, rather than in the coexistence of a variety of species.