Distribution, persistence and change in the macrobenthos of the lagoon of Lesina (Apulia, southern Adriatic Sea)

The benthic community of the lagoon of Lesina was studied from July 2000 to June 2001. The distribution of the main macrophytobenthic species was recorded by visual census at 24 stations in July 2000. The macrofauna, from both hard and soft substrates, was analyzed at five stations with seasonal frequency; in addition, the main hydrologic parameters were recorded monthly. The lagoon was mainly characterized by brackish water, with salinity values decreasing from west to east (annual mean: 23.3 psu). It showed mild water temperatures throughout the year (annual mean: 19.8 °C) and moderately high values of dissolved oxygen (annual mean: 9.9 ppm). No dystrophic crises were observed during the period of study. In the eastern basin, Zostera noltii Hornemann, Lissara and Ruppia cirrhosa (Patagna) Grande were the most abundant species of macrophytobenthos. Their distributional pattern was coherent with literature data. Soft bottoms of the western basin were largely occupied by the benthopleustophytic alga Valonia aegagropila C. Agardh, which during the last decade had almost completely replaced Cladophora prolifera (Roth) Kützing and Gracilaria confervoides Greville, the latter of which was previously very abundant in the lagoon. Macrozoobenthos consisted of 53 species (1 sponge, 2 cnidarians, 11 molluscs, 14 annelids, 21 crustaceans, 1 insect, 1 bryozoan, 1 echinoderm, 1 tunicate), most of which are typical of brackish water. Highest abundances were found in the eastern area of the basin. Some marine species were recorded in the western area. In contrast with the dramatic changes observed in the floral assemblage, a large amount of species had already been recorded in the lagoon during the last 10 years, pointing out the persistence of the macrozoobenthic community. Nevertheless, the distribution of benthic macrofauna seemed to be greatly influenced by the presence of Valonia aegagropila, as shown in an ordination model, where station-points were segregated by the presence of the alga rather than by hydrologic parameters. The impact of biotic factors on the community, such as non-indigenous species invasions, was suggested as constraints inducing the temporal variations of benthos.