THE IMPACT OF DIVING ON ROCKY SUBLITTORAL COMMUNITIES - A CASE-STUDY OF A BRYOZOAN POPULATION

In 1992 a diving buoy was installed within a marine reserve off the no rtheastern coast of Spain, where diving had not previously been permit ted. We monitored the effects of diving on a colonial bryozoan (Pentap ora fascialis) and carried out the study on two distinct between habit ats, boulders and vertical walls. We used a ''beyond before-after/cont rol-impact'' sampling design in which one potentially affected and sev eral control locations were sampled simultaneously before and after th e installation of the diving buoy. We hypothesized that diving has thr ee major impacts: (1) it decreases the density of P. fascialis, (2) it reduces the mean size of P. fascialis colonies, and (3) it restricts P. fascialis colonies to cryptic positions. Surveys of diving activity , conducted from 1992 to 1995 showed that diving increased 60-fold at the impact location. Analysis revealed a significant decrease in densi ty of colonies of P. fascialis (similar to 50% from 1992 to 1993) in b oth communities at the impact location 1 year after the start of the d iving disturbance. The effect of diving on the size of colonies was no t clear. The analysis revealed no general pattern of significant impac t, although the impact location suffered a general loss of large colon ies P. fascialis colonies on boulders were more numerous in more crypt ic positions once diving activity began at the impact location. On wal ls, although there was a reduction of more exposed colonies after the start of the disturbance (from 1992 to 1993), the changes over time we re not significant. The control locations showed no significant change s in the distribution of the degree of exposure of colonies over time on either community. The effects of diving differed in the two communi ties studied. he impact on boulders was stronger than that on the wall s. This may be explained by the protection provided by the dense canop y of gorgonian (Paramuricea clavata), the largest organism on vertical walls. On boulders erosion was continuous and cumulative. On walls, h owever, once the divers had eliminated the most exposed colonies-mainl y the epibionts-the arboreal structure of the gorgonians prevented div ers from coming into direct contact with the communities and reduced t he risk of erosion. Sublittoral benthic communities in the Northwester n Mediterranean constituted by sessile organisms with fragile calcareo us or corneous skeletons are not adapted to the severe disturbances ca used by continuous and intense diving. Recovery after such a disturban ce appears to be slow and difficult. Current diving activity may not a llowed disturbed communities at diving sites to recover. The results o f this experiment should be considered by managers and the data used t o evaluate the models on which management decisions are based.