DECLINE OF ULVA GROWTH IN THE LAGOON OF VENICE

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, contr olled 70% of the current biomass production, on a yearly basis, and la rgely overcame the biomass production in summer Concurrently, the sedi ment resuspension increased by a factor of 6-10 in the lagoon areas wh ere the biomass disappeared contributing significantly to control of t he growth of the residual biomass by light limitation, especially by s ettling 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 philippi narum, a bivalve species recently introduced into the lagoon and sprea d in the areas previously covered by Ulva, currently play an important synergistic role in Ulva control. Conversely, the overall availabilit y of nutrients did not appear to have any significant influence on the observed macroalgal decline. Copyright (C) 1997 Elsevier Science Ltd.