The effect of Ruppia cirrhosa features on macroalgae and suspended matter in a Mediterranean shallow system

The importance of shallow waters as a reservoir of biodiversity has been evidenced everywhere in the world both at temperate and tropical latitudes (Gray 1997; Duarte 2000; Raffaelli 2000; Pusceddu et al. 2003). Shallow waters function as a buffer system of interchange between open sea and land, and aquatic macrophytes represent key species regulating fluxes of energy and matter (Wetzel 1975). Dominant species in these systems are often seagrasses, which markedly colonize shallow habitats with extended meadows or beds (Short & Coles 2001). The multi-niche habitats provided by seagrasses play an important role in regulating the biological cycle of aquatic organisms both as trophic pabulum and nurseries for resident fauna and migrating organisms. In the Mediterranean Sea, most of the studies on seagrass meadows are mainly focused on Posidonia oceanica (L.) Delile and Cymodocea nodosa (Ucria) Ascherson (Boudouresque et al. 1984, 1989; Pergent 1987; Pergent & Pergent-Martini 1988; Peduzzi & Vukovic 1990; Buia & Mazzella 1991; Rismondo et al. 1997; Sfriso & Ghetti 1998; Buia et al. 2000; Cancemi et al. 2002; Vizzini et al. 2002) while there is less information on Zostera marina L. (Caniglia et al. 1992; Buia & Mazzocchi 1995; Sfriso & Ghetti 1998) and only a few are dealing with Ruppia cirrhosa (Petagna) Grande (Calado & Duarte 2000; Menéndez 2002). This species is widespread in Mediterranean shallow systems with salinities higher than in marine environment, as saltwork basins or some marginal secondary shallow environments. Emergent from the water for long periods, R. cirrhosa creates a multidimensional substrate for macro and microalgae. The latter are food sources, either directly or indirectly (through detritus), for a range of invertebrates (both marine and terrestrial) and small fish that represent food for migratory birds. Indeed, organic matter produced by seagrass and associated photosynthetic organisms, such as epiphytes and benthic algae, represents a direct trophic mechanism able to affect food availability to secondary consumers (Thom & Albright 1990; Mazzella & Zupo 1995). Nevertheless, R. cirrhosa is very common