Biological Invasions 2: 165–176, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. Biotic invasions in a Mediterranean Lagoon Anna Occhipinti Ambrogi Sezione Ecologia, Dipartimento Genetica e Microbiologia, Università degli Studi di Pavia, Via Sant’Epifanio, 14, I-27100, Pavia, Italy (e-mail: occhipin@unipv.it; fax: +39-0382-528496) Received 4 May 1999; accepted in revised form 3 August 2000 Key words: biotic invasions, bryozoa, environmental changes, Lagoon of Venice, Mediterranean Sea Abstract Many non-indigenous species have been found during recent decades in the Lagoon of Venice; many of these species are rapidly becoming both widespread and abundant. Reported here is the invasion of an accidentally introduced bryozoan Tricellaria inopinata, which dominated the Lagoon’s fouling community within seven years of its initial detection. Recent declines in its population as well as those of indigenous bryozoans illustrate the importance of placing invasions in the context of the hydrological, chemical and biotic changes that occur in the Lagoon environment. Introduction The loss of biodiversity and the radical changes in the abundance and ecological status of native species caused by biotic invasions are attracting increas- ing attention (Carlton 1989); the Mediterranean Sea, including the Lagoon of Venice, is not immune to these invasions (Zibrowius 1983, 1991; Galil 1993; Boudouresque et al. 1994). Any study of biotic inva- sions in the Lagoon of Venice should, however, best incorporate other aspects of this aquatic ecosystem, including changes in community composition, bio- geography, human activities and physical environment. Biotic invasions are part of the anthropogenic global change; they exemplify part of the vulnerability of the Lagoon environment and must be taken into account when evaluating the consequences of any loss in the Lagoon’s environmental equilibrium through human action. I summarize here invasions in the benthic commu- nities of the Lagoon of Venice. In addition I report in detail the accidental introduction of an aquatic inver- tebrate that rapidly dominated the Lagoon’s native fauna. Estuaries and brackish water environments are common sites of biotic invasions (Ruiz et al. 1997): vulnerability to these invasions in the Lagoon of Venice may be due to the instability of this ecosystem. Con- sequently, the main contemporaneous changes in the environment of the Lagoon are reported here in order to provide a framework for understanding the effects of these recently-introduced species. Environmental features of the Lagoon of Venice The Lagoon of Venice has a surface area of about 550 km 2 and is part of a large system of wetlands that border the northern Adriatic. The unique biological features of the Lagoon of Venice have been reported repeatedly (Vatova 1960; D’Ancona and Battaglia 1962; Sacchi 1974; Occhipinti Ambrogi and Sacchi 1992). The human-mediated biogeographic and eco- logical determinants of the community composition in this brackish water body have operated since medieval times. The Lagoon has long been managed against impoundment by diverting the estuaries of the region’s main rivers either outside the Lagoon itself (the Adige and Brenta Rivers) or in a southerly direction (the Po River). The risk of marine erosion has been lessened by an elaborate system of sand bars coupled with stone