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 o 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) Kutzing 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.
La communaute benthique de la lagune de Lesina a ete suivie de juillet 2000 a juin 2001. La distribution des principales especes macrobenthiques a ete etudiee par recensement visuel a 24 stations en juillet 2000. La macrofaune des fonds durs et celle des fonds meubles a ete analysee a cinq stations a frequence saisonniere. De plus, les principaux facteurs hydrologiques ont ete suivis chaque mois. La lagune est caracterisee par la presence d'eau saumatre avec une salinite decroissant de l'ouest a l'est (moyenne annuelle = 23,3), une temperature moyenne de 19,8 o C et des teneurs en oxygene dissous moderees (moyenne annuelle = 9,9 ppm). Aucune crise dystrophique ne s'est produite durant la periode d'etude. Dans le bassin est, Zostera noltii Hornemann, Lissara et Ruppia cirrhosa (Patagna) Grande sont les principales especes du macrophytobenthos. Leur distribution est coherente avec les donnees de la litterature. Les fonds meubles du bassin ouest constituent le domaine de l'algue benthique Valonia aegagropila C. Agardh, qui, durant la derniere decennie, a remplace Cladophora prolifera (Roth) Kutzing et Gracilaria confervoides Greville, cette derniere auparavant tres abondante. Le macrozoobenthos comporte 53 especes (1 eponge, 2 cnidaires, 11 mollusques, 14 annelides, 21 crustaces, 1 insecte, 1 bryozoaire, 1 echinoderme et 1 tunicier), la plupart d'entre eux typiques des eaux saumatres. Les concentrations maximales se rencontrent dans la partie est du bassin. Certaines especes marines sont trouvees dans la partie ouest. A l'oppose du changement drastique de la flore, beaucoup d'especes animales ont deja ete rencontrees dans la lagune durant la derniere decennie. Cependant, la distribution de la macrofaune benthique est fortement influencee par la presence de Valonia aegagropila ; un modele montre que les stations sont selectionnees par la presence de l'algue plutot que par des facteurs hydrologiques. L'impact sur la communaute des facteurs biotiques, comme l'invasion par des especes exotiques, est avance comme facteur de forcage des variations temporelles du benthos.