Recovery assessment in Lake Nemi (Italy) after a twenty year period (1981-2001) using plant-associated invertebrates

Lake Nemi was subjected to organic pollution in the 1970s due to domestic wastes, which led to a severe eutrophication process. Local authorities consequently planned a gradual waste diversion programme completed in 1990. Comparison between data on plant-associated invertebrates in the period of maximum eutrophication (1981/82) and twenty years after the total waste diversion (2001/02) was carried out with the aim of demonstrating the performance of this littoral community for the purpose of lake recovery assessment. Following the water clarity improvement characterizing the 2001/02 period, the macrophyte community displayed an amelioration in plant diversification, an enlargement of the colonized area, and an appearance of Charales, important bioindicators of oligotrophic conditions. In complete agreement with this new situation, the invertebrate fauna extended its colonization, and the species composition and quantitative structure changed completely. A considerable increase in species richness and diversity, and significant modifications of percentages and densities of bioindicator taxa (cladocerans, gastropods and acari) showed a very appreciable recovery, which can be defined as a phase of oligotrophication. This trophic state is closely related to nutrient reduction in the water and enhanced by a drift of organic materials from littoral to profundal bottoms. This was due to the remarkable water level reduction of about five meters which occurred in the lake in recent times.