Invertebrate fauna, aquatic vegetation and physico-chemical characteristics of a lotic-lentic system in a Natural Reserve (Lake Fibreno, Italy).

Lake Fibreno (surface area: 0.28 km²; mean depth: 2.7 m) is a “spring lake” located in Central Italy in a Natural Reserve and originates from submerged and surface waters of a karstic hydrogeological system (Marsica Mountains, Abruzzo National Park). The surface water enters at a high flow and at a constant temperature of 10-11 °C, and discharges in the only effluent (Fibreno river) at a flow of about 10 m³.sec-1 (annual mean of the last forty years). The lake enlarges in a plain and wide common reed bed and comprises the southernmost floating island of Europe. For its characteristics Lake Fibreno represents an original example of lotic-lentic system, so physico-chemical parameters and zoobenthos associated with aquatic plants were studied with the aim to evaluate the benthic composition and to define the lake ecological status in view of its conservation. The samples (water, vegetation and invertebrate fauna) were collected bymonthly in six stations during one year (2004). From the hydrochemical point of view the freshwater system is comprised in the field of calcic-bicarbonate waters (pH: 7.1; Ca2+:143 mg/l; HCO3-: 411 mg/l, annual means). High contents of phosphate compounds (range: 82-148 µg/l) were found. As to the vegetation, remarkable is the presence of aquatic macrophyte species that characterize several Natura 2000's habitats. The river macrophyte communities are generally dominated by Sparganium and Potamogeton oligo-mesotrophic species. Close to the spring area prevail Apium nodiflorum, Berula erecta and Hippuris vulgaris, and whereas standing water occurs, macrophyte communities change again in composition, with dominance of Callitriche and Lemna species. Thirty four groups of invertebrates were found, mostly represented by insects and crustaceans, which reached altogheter 85% of the total fauna. Microcrustaceans (range: 8-62%) quantitatively prevailed, followed by chironomids (6-36%), oligochaetes (2-14%) and water mites (0.4-18%). The percentage invertebrate composition was remarkably different at the sites of sampling, according firstly to water flow regimes (lentic or lotic prevailing character) and composition and abundance of plant substrate. Faunal densities have revealed increasing mesotrophy from spring to river sites. Numbers of invertebrates, moderate coverages of submerged plants, high values of phosphates and organically enriched sediments clearly indicated a negative trend to an eutrophic state of the lake, which certainly need a recovery project.