MEIOBENTHIC COMMUNITIES ASSOCIATED WITH THE SEASONAL CYCLE OF GROWTH AND DECAY OF ULVA-RIGIDA AGARDH IN THE PALUDE-DELLA-ROSA, LAGOON OF VENICE

Eutrophication in the Palude della Rosa, in the Lagoon of Venice, results from agricultural runoff giving rise to a seasonal cycle of proliferation and subsequent decomposition of the green alga Ulva rigida during the summer months, with a dramatic fall in oxygen levels. A survey of the meiofauna at 42 stations in the Palude in December 1991 showed that there were two distinct associations of nematodes and total meiofauna (nematodes plus copepods), one associated with that part of the Palude where Ulva attains a high biomass, and another where Ulva biomass is low or absent. Species characteristic of the Ulva region include nematodes of the genus Diplolaimella and copepods of the genus Tisbe, both of which are known from elsewhere to be associated with decaying plant material and implicated in the decomposition process. In the Ulva region both diversity (as evidenced from K-dominance curves) and species composition (using non-metric MDS) varied considerably from season to season, associated with the seasonal cycle of growth and decay of Ulva, but diversity and species composition were more stable seasonally in the non-Ulva region. For the copepods alone, differences between the Ulva and non-Ulva regions were not as evident and species composition in both regions changed seasonally in the same way, although diversity was only reduced markedly in the summer at the Ulva stations. We conclude that it is not only likely that the Ulva cycle controls seasonal changes in meiobenthic community composition, but in turn the meiobenthic community has some control over the Ulva cycle. (C) 1995 Academic Press Limited