Meiobenthic Communities Associated with the Seasonal Cycle of Growth and Decay ofUlva rigidaAgardh 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 algaUlva rigidaduring 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 whereUlvaattains a high biomass, and another whereUlvabiomass is low or absent. Species characteristic of theUlvaregion include nematodes of the genusDiplolaimellaand copepods of the genusTisbe, both of which are known from elsewhere to be associated with decaying plant material and implicated in the decomposition process. In theUlvaregion both diversity (as evidenced fromk-dominance curves) and species composition (using non-metric MDS) varied considerably from season to season, associated with the seasonal cycle of growth and decay ofUlva, but diversity and species composition were more stable seasonally in the non-Ulvaregion. For the copepods alone, differences between theUlvaand non-Ulvaregions were not as evident and species composition in both regions changes seasonally in the same way, although diversity was only reduced markedly in the summer at theUlvastations. We conclude that it is not only likely that theUlvacycle controls seasonal changes in meiobenthic community composition, but in turn the meiobenthic community has some control over theUlvacycle.