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.