EFFECTS OF ENVIRONMENTAL-STRESS ON ASCIDIAN POPULATIONS IN ALGECIRAS BAY (SOUTHERN SPAIN) - POSSIBLE MARINE BIOINDICATORS

The distribution and abundance of Littoral ascidians were analyzed wit h respect to their possible relationships with environmental stress. A s part of a multidisciplinary research project on the benthic communit ies in Algeciras Bay, southern Spain, a suite of environmental variabl es was measured (hydrodynamism, silting, suspended solids and organic matter). After displaying the similarities of fauna through clustering and ordination of sampling sites, the relationships between community differences and changes in the abiotic component were established bas ed on the BIO-ENV procedure and Canonical Correspondence Analysis. Hyd rodynamism and the percentage of organic matter in the silt is the var iable combination that best explains (Spearman correlation of 0.82) th e biotic structure. While all ascidians show a certain tolerance to di verse environmental factors, some species such as Ciona intestinalis, Diplosoma spongiforme, Phallusia mammillata, Microcosmus squamiger, St yela plicata and Synoicum argus could be considered as indicators of a reas which have been subject to intense stress (substrate transformati on, water stagnation and sedimentation excess) over long periods of ti me, whereas others such as Aplidium conicum, Aplidium punctum, Claveli na dellavallei, Halocynthia papillosa and Stolonica socialis, which li ve only in natural and non-perturbed rock areas, could be categorized as species very sensitive to stress, as well as indicators of good con ditions.