SUMMARY: Over the past hundred years, the composition of the submerged aquatic vegetation of the Lagoon of Venice has changed considerably, due to increased anthropic activities and large-scale industrialisation. Seagrasses have gradually been reduced, whereas macroalgae (Ulva rigida, Enteromorpha spp., Cladophora spp., Chaetomorpha spp.) have increased. Macroalgal overgrowths peaked between 1970 and 1990 to the extent that, in order to estimate macroalgal biomass and coverage, the Venice Magistrato alle Acque (the Lagoon water management authority) started a series of investigations including monthly in situ measurements and aerial photo surveys. In the present paper these data are compared with available information on the Lagoon of Venice, and the widespread phenomenon of macroalgal proliferation is described. At the end of the 1980s, in our study area (78 km 2 in the central part of the Lagoon) biomass values ranged from 10 to 25 kg wet weight (w.w.) m -2 (sub-areas of Lido and Sacca Sessola), with a total mean biomass of 392,000 t w.w. A slight reduction took place in 1992 and at the end of the 1990s the highest biomass values were relatively low, 5 kg w.w. m -2 , with a total mean biomass of 1,600 t w.w. Our qualitative research carried out in 1991 on 130 sampling stations in the study area showed that soft substrates had a greatly reduced floristic composition in the five sub-areas in comparison with the control area (from 18 to 6 taxa), with Chlorophyta (50-80%) prevailing over both Rhodophyta (14-38%) and Phaeophyta (0-14%), and a slight or reduced distribution of seagrasses. The trend in macroalgal reduction during the 1990s corresponded to seagrass recolonisation, mainly of Zostera marina, taking advantage of new, compacted, oxidised and stabilised sediments that were no longer covered by extensive Ulva beds. [1981][1982][1983][1984][1985][1986][1987][1988][1989][1990][1991][1992][1993][1994][1995][1996][1997][1998]. -Durante los últimos cien años la composición de la vegetación acuática sumergida de la laguna de Venecia ha cambiado considerablemente a causa del incremento de actividades antrópicas y la industrialización a gran escala. Las fanerógamas marinas han reducido su extensión y las macroalgas (Ulva rigida, Enteromorpha spp., Cladophora spp., Chaetomorpha spp.) la han aumentado. El crecimiento de macroalgas alcanzó una cota máxima entre 1970 y 1990 hasta el extremo que la Agencia Veneciana del Agua inició una serie de proyectos para estimar la biomasa y el recubrimiento algal que incluían medidas mensuales en el campo y fotografías aéreas. En este trabajo estos datos se comparan con la información disponible de la laguna de Venecia, describiéndose las proliferaciones algales. A finales de los años 80, en nuestra área de estudio (78 km2 en la parte central de la laguna) las biomasas oscilaban entre 10 y 25 kg de peso húme-do por metro cuadrado (subáreas de Lido y Sacca Sessola) con una biomasa media de 392.000 Tm. Una leve disminución tuvo lugar en 1992, y, al final de los 90, las biomasas más...
