Spatial distribution and diversity of plant-associated invertebrates in a lotic-lentic ecosystem (Lake Posta Fibreno, Central Italy) and water quality monitoring.

A survey on plant-associated invertebrates in Lake Fibreno was carried out for the first time in 2004 with the aim to evaluate the fauna composition, distribution and diversity, and to define the lake ecological status. The basin (surface area: 0.3 km²; maximum depth:15 m) is an original example of a lotic-lentic system, located in Central Italy in a Reserve classified as a Special Area of Conservation (SAC, European Union Directive 92/43/EEC). The lake originates from a complex of groundwater and surface karstic springs belonging to the Marsica hydrogeologic system and comprises a wide reed stand crossed by canals and the only floating island of the South Europe (diameter: abount 30 m). High flow and constant temperature (10-11 °C) characterize the lake water, which discharges in the Fibreno river at a flow of about 10 m³.sec-1. Aquatic vegetation and invertebrates were collected bimonthly with an hand dredge in six stations during one year. Ten tipically lotic/lentic plant taxa were collected, which colonized mostly the 0-3 m depth range and were distributed according to water flow regimes. High numbers of zoological groups (28) were found, comprising 94 taxa, mostly represented by insects (34) and secondarily by nematodes, oligochaetes, gastropods and crustaceans, thank to the assemblage of invertebrates typical of lentic and lotic waters. The presence of indicator taxa and the measure of some bioindices performed on the invertebrate community have indicated an oligo-mesotrophic condition, with a trophic gradient varying from oligotrophy to mesotrophy according to different plant composition and lake hydrodynamics. Although some signs of eutrophication were revelead by high phosphate contents in the water, high concentration of organic matter in the sediments, disappearance of vegetation with respect to the past years, extensive growth of the sulfumbacterium Thiopedia rosea on the bottom, the plant-associated invertebrate community still showed a good ecological status following the positive effect of dilution-removal of nutrients caused by the continuous water flow of the lake.