Effects of environmental stress on ascidian populations in Algeciras Bay (southern Spain). Possible marine bioindicators?

The distribution and abundance of littoral ascidians were analyzed with respect to thelr possible relationships with environmental stress.As part of a multidisciplinary research project on the benth~c communities in Algeciras Bay, southern Spain, a suite of environmental variables was measured (hydrodynamism, silting, suspended solids and organic matter).After displaying the similalities of fauna through clustering and ordination of sampling sites, the relationships between community differences and changes in the abiotic component were established based on the BIO-ENV procedure and Canonical Correspondence Analysis.Hydrodynamism and the percentage of organic matter in the silt is the variable combination that best explains (Spearman correlation of 0.82) the biotic structure.While all ascidians show a certain tolerance to diverse environmental factors, some species such as Ciona intestinalis.Diplosoma spongiforme, Phallusia mammillata, Microcosmus squamiger, Styela plicata and Synoicum argus could be considered as indicators of areas which have been subject to intense stress (substrate transformation, water stagnation and sedimentation excess) over long periods of time, whereas others such as Aplidium conicurn, Aplidium punctum.Clavelina dellavallei, Halocynthia papillosa and Stolonica socialis, which live only in natural and non-perturbed rock areas, could be categorized as species very sensitive to stress, as well as ind~cators of good conditions.