Biodiversity loss in a small riverine wetland of the Ticino river (Lombardia, Northern Italy).

Wetlands are heterogeneous habitats that include various biotopes with different formations, structures and geographical positions. There are many small wetlands in the Ticino river area around Pavia (Lombardia, Northern Italy), many of which evolve naturally and originate from abandoned riverbeds, isolated river meanders, oxbow lakes or small ponds fed by terrace springs or underground waters. We decided to consider the evolution of one of these wetlands, namely the topo oxbow lake. It had previously been studied 24 years ago (1988-1990), so we compared the results with those collected during a recent one-year investigation (June 2011-June 2012) on the following aspects: dimensions, hydrometric level fluctuation and bathymetry, principal chemical-physical parameters, hygrophytic and aquatic vegetation, and zooplankton communities. The resulting geo-morphological and biotic community changes are typical of riverine wetlands, thus supporting the hypothesis that this oxbow lake is following its natural evolution. Morphological changes in the oxbow lake should influence its biotic communities: the high plant biomass that existed in the past could be considered to have been the first step towards eutrophication, but the oxbow lake remained at mesotrophic level due to high biodiversity and dissolved oxygen levels in the water. Nowadays, the lower biodiversity in aquatic plants and zooplankton suggests a trophic level shift towards eutrophication due to the endogenous evolution of the oxbow lake. However, these ideas are not supported by the physical and chemical parameters of the water which indicate that the oxbow lake is still at mesotrophic level. We would suggest planning a gentle restoration of this biotope, with the aim of rejuvenating the physical habitat to ensure the long-term ecological functioning of the aquatic environment.