SUBLITTORAL BENTHIC VEGETATION OF THE EASTERN BASQUE COAST (N. SPAIN)- STRUCTURE AND ENVIRONMENTAL-FACTORS

The sublittoral benthic vegetation of the eastern Basque coast was stu died along 55 km of coast. Information concerning the abundance of mac rophytes and the physical environmental characteristics (type and slop e of substratum, level of sedimentation, sediment nature and depth) wa s obtained from a network of 376 quadrats. The predominance of a habit at type (slightly sloping regular bedrock, little-moderate sand sedime ntation levels and a depth range of 4-9 m) permits the development of a very homogeneous vegetation constituted mainly of Gelidium sesquiped ale beds. Sedimentation, probably in relation to wave exposure, was th e environmental factor showing the widest range of variation, and appe ared to determine to the vegetation gradient detected by the correspon dence analysis which shows algal distribution as a continuum. Along an increasing sedimentation gradient, the following trends have been det ected: (1) Algal cover decrease due to the vertical layering reduction . (2) Changes in the relative abundance among species: Gelidium sesqui pedale and Mesophyllum lichenoides were negatively associated with sed iment; Pterosiphonia complanata and Cystoseira baccata were well adapt ed to sedimentation, showing an optimum development at moderate to hig h levels; Halopitys incurvus and Chondracanthus acicularis achieved th eir optimum development at high to very high sedimentation levels in s hallow waters. (3) Vegetation heterogeneity, diversity and epiphytism increase. In addition, pollution, causing an increase of allochthonous sediments and turbidity, was the other environmental factor affecting the structure and composition of the vegetation. This fact was reflec ted by the presence of a residual Gelidium sesquipedale vegetation smo thered by silt close to an urban effluent, and a vegetation of Cladost ephus spongiosus - Chondria coerulescens - Pterosiphonia pennata in th e vicinity of several ports. Multivariate analyses have proven to be a useful tool to detect marine vegetation structure, showing that in th e study area discrete communities are not distinguishable.