Despite the ecological and economical importance of seagrass beds, an increasing number of reports document the ongoing loss or regression of sea-grasses in all countries (Duarte et al., 2002; Walker et al., Chapter 23; Kenworthy et al., Chapter 25). The surface area occupied by Posidonia beds is limited by environmental factors (light, turbidity, salinity, temperature), but in some coastal regions where anthropogenic pressures (trawling, aquaculture, wastewater outfalls) are high, severe damage is observed. The loss of Posidonia beds is unfortunately not recent; it has been reported since 1930s (Boudouresque and Meinesz, 1982; Cambridge and McComb, 1984; Shepherd et al., 1989; Larkumand West, 1990; Walker and McComb, 1991). Recent mapping of the historical evolution of Pos idonia dis tributions generally shows a decline in seagrass coverage (e.g. Pasqualini et al., 2000; Kendrick et al., 2002) reflecting anthropic activities, port developments, grazer impacts, shading of seagrass leaves by excessive growth of epiphytic algae or phyto-plankton following nutrient enrichment of column water (Cambridge et al., 1986; Walker and McComb, 1992; Short and Wyllie-Echeverria, 1996; Chisholm et al., 1997). Climate change will affect seagrass distributions (Short andNeckles, 1999; Duarte et al., in press). The recolonization of altered sites is rare but does occur (e.g. Kendrick et al., 1999, 2000; Mee-han and West, 2000; Pergent-Martini and Pasqualini, 2000; Cambridge et al., 2002)—but is slow because Posidonia are long lived species with a slow rhizome spread (0.3-21 cm year-1) (Boudouresque et al., 1984; Meinesz and Lefevre, 1984; Kirkman and Kuo, 1990; Marba and Duarte, 1998; Marba and Walker, 1999; Paling and McComb, 2000). The loss of Western Australian Pos idonia meadows is caused by a reduction in light reaching the meadows, either through decreased water clarity or shading by epiphytic or unattached algae (Cambridge et al., 1986; Walker and McComb, 1992; Kendrick et al., 2002; Walker et al., Chapter 23). Some mechanical transplantation of Pos idonia species has been conducted with some success (Paling et al., 2001).
The main strategies for marine environmental management in Australia (7.7 x 106 km2 and 18 millions permanent residents) include maintenance of water quality, prohibiting or regulating destructive and unsustainable activities, zoning for particular uses to separate and control incompatible uses, protection of vulnerable and threatened species and regulation of fisheries through licenses, size limits, quotas, etc. Australia is a world leader in using Marine Protected Areas (Fogarty et al., 2000; Kenworthy et al., Chapter 25) but our understanding of the marine environment is still patchy.
The Mediterranean Sea (3 x 106 km2 and 44 million permanent residents) is known for the considerable diversity of its fauna and flora as well as for the high rate of species endemism. But generally, Mediterranean countries have coastlines that are the most intensively used in the world for tourism and related recreational activities. The coastline (including islands) receives some one hundred million visitors per year from all nationalities. As such, the Mediterranean coast and nearshore waters experience the increasing impact of tourism but also the effects of resident demographic growth. In addition, the coastal region supports agriculture and mariculture as well as wild fisheries, industry, and navigation.
Monitoring strategies have been recently proposed to control and conserve meadow habitats (e.g. Moreno et al., 2001). Seagrass meadows are a priority habitat under the European Union Species and Habitats Directive (H&SD, 92/43/EEC).
The protection and conservation is of primary importance, but the replanting of Posidonia shoots is slow and difficult. Survival of transplants is low in the Mediterranean (Molenaar and Meinesz, 1995; Molenaar et al., 2000), and only about 40% survive in southern Australia (Lord et al., 1999). Transplantation is accompanied by modifications of biometry, and modifications of C, N, and P content, which reflect weakness of the strategy (Lepoint et al., 2004a). One year after transplanting, shoots are still unable to satisfy their nutrient demand (Gobert, 2004; Lepoint et al., 2004; Vangeluwe et al., 2004).
The introduction of exotic marine organisms, from accidental release, vessel ballast water, hull fouling, and aquaculture, remains an area of concern (Boudouresque and Verlaque, 2002; Duarte et al. in press; Ralph et al. Chapter 24), particularly where the introduced species are competitors for soft bottom substratum such as the algae Caulerpa taxifolia, and Caulerpa racemosa in the Mediterranean (Meinesz et al., 1993; Deville and Verlaque, 1995; Ceccherelli and Cinelli, 1999) and the fan worm Sabella spallan-zanii in southern Australia (Lemmens et al., 1996).
Aquaculture of fish and algal biomass has been shown to produce major environmental impacts, particularly due to shading, eutrophication, and sediment deterioration through excess organic inputs (Seymour and Bergheim, 1991; Shireman and Ci-chra, 1994; Dosdatetal., 1995; Holmeretal.,2001). The effects of fish farms and other aquaculture developments are of concern as areas of productive seagrass habitats are often targeted for such developments, such as in the Mediterranean coast (Delgado et al., 1999; Pergent et al., 1999). Fish pens have been demonstrated to cause seagrass loss (Delgado et al., 1999; Pergent et al., 1999). Extensive and intensive aquaculture developments are expanding worldwide, increasing the risk of more loss.
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