Causes and effects of the remarkable regression of the macroalga Ulva rigida C. Ag. in the central part of the Venice lagoon since 1990 are reported. Climatic changes triggered the progressive reduction of Ulva coverage and production (-80% in 1993; -95% in 1995) until its almost complete disappearance in 1996. Grazers, especially Gammaridae, controlled 70% of the current biomass production, on a yearly basis, and largely overcame the biomass production in summer Concurrently, the sediment resuspension increased by a factor of 6-10 in the lagoon areas where the biomass disappeared contributing significantly to control of the growth of the residual biomass by light limitation, especially by settling of a microlayer of sediment over the laminar thalli. Finally, biomass harvesting, now effective, and the heavy disturbance caused by fishing boats equipped with hydraulic dredges to catch Tapes philippinarum, a bivalve species recently introduced into the lagoon and spread in the areas previously covered by Ulva, currently play an important synergistic role in Ulva control. Conversely, the overall availability of nutrients did not appear to have any significant influence on the observed macroalgal decline. Copyright (C) 1997 Elsevier Science Ltd.