Ecological implications of removing seagrass beds (Zostera noltii) for bivalve aquaculture in southern Portugal

In the Ria Formosa lagoon (southern Portugal), Zostera noltii meadows have been cleared to make way for bivalve aquaculture. The present study aims to investigate the potential ecological impacts of Z. noltii removal using amphipods as bioindicators. Sampling was undertaken between August 2000 and August 2001 in an intertidal region of Ria Formosa at two different areas. One of the areas supports an ongoing commercial venture for the extensive culture of marine bivalves (Ruditapes decussatus), where Z. noltii beds have previously been removed and sand added, while the other area represents a natural Z. noltii habitat. At each monthly sampling date five replicates (0.02 m² each) were taken from both areas with a corer and sieved through a 0.5 mm mesh. Multivariate analysis and analyses of variance of faunal data showed significant differences between the amphipod assemblages. In general, Z. noltii beds presented higher densities, number of species, diversity and evenness values than the unvegetated area. Within Z. noltii beds the community was characterized by Microdeutopus chelifer, Melita palmata, Gammarus insensibilis and Ampelisca sp. A., while the unvegetated area presented Leptocheirus pilosus, Siphonoecetes sabatieri and Ampelisca brevicornis as the most characteristic species. Within both areas, the univariate variables analysed peaked in summer. The amphipod patterns among areas were time-dependent and this should be taken into account in future studies. Amphipoda appeared to be a potential ecological indicator group regarding seagrass degradation.