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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...

In the present study, we investigated the applicability of the Pearson-Rosenberg (P-R) conceptual model describing a generalized pattern of response of benthic communities in relation to organic 25 enrichment to a specific typology of marine systems, i.e. coastal lagoons. The study was performed with independent matching data on the structure of macrobenthic communities and total organic carbon (TOC) content of sediments obtained from 349 stations representing three of the most ecologically and economically relevant coastal lagoons in the Mediterranean Sea, i.e. the lagoons of Cabras, Orbetello, and Venice (Italy). Consistent with P-R model predictions, we 30 found two different peaks in benthic diversity and abundance at low (> 2.5-5 mg g -1 ) and high (> 25-30 mg g -1 ) TOC ranges, respectively. We identified TOC thresholds indicating that risks of reduced benthic diversity from organic loading and other associated stressors in sediments should be relatively low at TOC values < about 10 mg g -1 , high at TOC values > about 28 mg g -1 , andintermediate at values in-between. Predictive ability within these ranges was high based on 35 results of re-sampling simulation. Our results validate and support a prior independent study that identified similar TOC thresholds for assessing risks of benthic impacts in samples from seven coastal regions of the world. While not a measure of causality, it is anticipated that these TOC thresholds should serve as a general screening-level indicator for evaluating the likelihood of reduced sediment quality and associated bioeffects in such eutrophic systems of the 40 Mediterranean Sea, which also are exposed to a variety of stressors from multiple human uses.