Littoral zoobenthos associated with aquatic vegetation and water quality recovery in the volcanic Lake Nemi (Italy).

Lake Nemi is located close to Rome and have small surface area (1.8 m²) and relatively great depth (30 m). From the 70ies it was subjected to an organic pollution due to domestic wastes, which was channelled elsewhere from 1990. Here we report a comparison between data on the zoobenthos associated with aquatic macrophytes of two periods of study (1981-1982 and 2000-2001), with the aim of demonstrate the performance of this littoral community in lake recovery assessment. The first study well evidenced the heavy eutrophication of the lake. The vegetation was extremely reduced both in extention and in number of taxa (plant colonization reached only 3 m of depth and comprised 5 taxa found in the samples) and the poor quality was widely confirmed by the values of some significant faunal parameters such as low species richness (61 taxa), very low diversity (2.2), low number of cladoceran species (3), and high percentages of a bioindicator of meso-eutrophy (Chydorus sphaericus, 57.7%). The recent data, collected in the same sites of the past investigation and partly analyzed, showed clear evidences of an improvement of the lake water quality: macrophyte colonization increased until 8 m of depth and a higher number of plants was found (9 taxa), comprising also a little, but important presence of macroalgae Charales, good indicators of low trophy. Fauna composition have shown interesting signs of positive modifications such as an increase in species richness (67 taxa) and in number of cladoceran species (8) and some changes in the quantitative structure, represented especially by decrease of the oligochaete percentages, associated at increase of other faunal groups.