Spatial variability and human disturbance in shallow subtidal hard substrate assemblages: a regional approach

Quantitative information about spatial patterns in subtidal hard substrate assemblages is scant. Such information is necessary to understand the respo nses to anthropogenic disturbances in these habitats. Along the coast of Ap ulia (Southern Italy), the collection of the European date mussel Lithophag a lithophaga is a strong source of disturbance: harvesting is carried out b y demolition of the rocky substrate and causes epibiota disappearance. A hi erarchical sampling design was used to quantify the spatial variability of subtidal epibenthic assemblages and the extent of rock damage due to L. Lit hophaga harvesting along 360 km of rocky coasts in Apulia. The surveyed coa st was divided into 8 adjacent sectors, and replicate samples were taken by visual inspection at each of the 3 sites nested in each sector. Multivaria te analyses indicated that assemblages differed consistently with spatial s cale, variability being higher at the largest scale. However, variability a mong sites within each sector was also detected. Patchiness (i.e., average similarity among quadrats) was consistent among sectors. Some species were identified as 'important' in characterising and/or differentiating sectors. The pattern of distribution of these species as well as total cover and nu mber of species were analysed by analysis of variance. Results recorded a c onsiderable source of variation at site level. Damage by L. Lithophaga fish ing was shown to be extremely widespread. A humped relationship between pat chiness and disturbances by L. Lithophaga fisheries was obtained. In partic ular, patchiness at a small scale was highest at 'intermediate' levels of d amage, because disturbance produces patches of different size and/or age, l eading to 'mosaic' landscapes of epibenthic assemblages.