The invertebrate community associated with plant substrate in Lake Bracciano (Central Italy): short-term modifications occurred after recent environmental changes (1998-2003).

Lake Bracciano is a large and deep volcanic lake (max. depth: 165m) which plays an important role as source of drinking water for the city of Rome and for fishing and recreational activities. Increasing amount of water drawn from the lake and recent climatic changes have reduced considerably in the last years the water level (0.70-1m), involving also notable seasonal fluctuactions. It suggested the advisability of undertaking an new ecological analysis of the zoobenthic community associated with aquatic plants, previously analysed (1998-’99), in order to compare new data with the past situation, and to evaluate possible changes in the littoral ecological status. Zoobenthos was collected bymonthly in four stations and at three depth intervals for a period of one year (2003-2004). The comparison of recent and previous data evidenced an almost identical qualitative composition of the plant community (8 taxa in the collected samples) and a notable colonization of Charales (present during the entire year at depths comprised from 6 to 15-20m), but plant colonization and abundance in shallow waters (0-4m) appeared dramatically reduced in comparison with the past. The invertebrate structure showed several important modifications in comparison to 1998, as a reduction in number of faunal groups (from 20 to 18), a remarkable quantitative decrease of all invertebrates strongly associated with plant substrate (mainly chironomids, oligochaetes, nematodes, water mites and gastropods) and a great dominance of microcrustaceans (cladocerans and copepods: 78-92% of the total fauna). The negative trend evidenced in the previous study appear to be verified: plant decline in shallow waters was the first consequence of recent environmental changes (intense shore management for recreational purposes, considerable water level variation, etc.) and it involved the observed negative modifications of the fauna structure. Despite a water quality approaching oligotrophy, the regressive trend of the littoral community represent a conspicuous deterioration of ecological status, mainly in term of predictible reduction of biodiversity and influence on the food chain in the lake.