The Impact of Diving on Rocky Sublittoral Communities: A Case Study of a Bryozoan Population

Abstract

In 1992 a diving buoy was installed within a marine reserve off the northeastern coast of Spain, where diving had not previously been permitted. We monitored the effects of diving on a colonial bryozoan ( Pentapora fascialis ) and carried out the study on two distinct benthic habitats, boulders and vertical walls. We used a "beyond before-after/control-impact" sampling design in which one potentially affected and several control locations were sampled simultaneously before and after the installation of the diving buoy. We hypothesized that diving has three 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 density of colonies of P. fascialis ( ‫ف‬ 50% from 1992 to 1993) in both communites at the impact location 1 year after the start of the diving disturbance. The effect of diving on the size of colonies was not clear. The analysis revealed no general pattern of significant impact, although the impact location suffered a general loss of large colonies. P. fascialis colonies on boulders were more numerous in more cryptic positions once diving activity began at the impact location. On walls, although there was a reduction of more exposed colonies after the start of the disturbance (from 1992 to 1993), the changes over time were not significant. The control locations showed no significant changes in the distribution of the degree of exposure of colonies over time on either community. The effects of diving differed in the two communities studied. The impact on boulders was stronger than that on the walls. This may be explained by the protection provided by the dense canopy of gorgonian ( Paramuricea clavata ), the largest organism on vertical walls. On boulders erosion was continuous and cumulative. On walls, however, once the divers had eliminated the most exposed colonies-mainly the epibionts-the arboreal structure of the gorgonians prevented divers from coming into direct contact with the communities and reduced the risk of erosion. Sublittoral benthic communities in the northwestern Mediterranean constituted by sessile organisms with fragile calcareous or corneous skeletons are not adapted to the severe disturbances caused by continuous and intense diving. Recovery after such a disturbance appears to be slow and difficult. Current diving activity may not allow disturbed communities at diving sites to recover. The results of this experiment should be considered by managers and the data used to evaluate the models on which management decisions are based.El Impacto del Buceo en Comunidades Sublitorales sobre Sustrato Rocoso: El Caso de una Población de Briozoos Resumen: En 1992 se instaló una boya para el buceo en un lugar previamente no frecuentado por los buceadores, dentro de una reserva marina en el nordeste de España donde el...

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