THE ROLE OF FISHES IN THE ORGANIZATION OF A MEDITERRANEAN SUBLITTORALCOMMUNITY .1. ALGAL COMMUNITIES

Observational studies and experimental manipulations of grazing intens ity were used to examine the role of herbivorous fishes in determining abundances and seasonal variations of algal species in a sublittoral algal community in the NW Mediterranean Sea. Three sites with differen t grazing pressures were compared, two with high herbivorous fish dens ities located in the Medes Islands Protected Area (NE Spain), where fi shing is prohibited, and one with low fish abundance on the unprotecte d coast opposite. The algal community at the three sites exhibited mar ked seasonal variations in biomass, attaining peak biomass levels in l ate spring. Within the Protected Area, a habitat heavily grazed by fis hes was characterized by the seasonal proliferation of the unpalatable red alga Asparagopsis armata Harvey, and by the dominance of a turf o f epiphytic and small erect thalli of articulated calcareous algae mos t of the year. In contrast, nearby unprotected habitats were character ized by the seasonal occurrence of dense stands of several fleshy macr oalgal species and by the dominance of well-developed articulated calc areous algae most of the year. Experimental reduction of fish grazing rapidly and dramatically altered patterns of algal abundance and dynam ics. After 5 weeks of reduced herbivory, several fleshy erect algal sp ecies and epiphytic algae abundance increased significantly in herbivo re exclusion areas relative to unmanipulated controls. Caging led to d ecreased abundance of A. armata but had no effect on the abundance of encrusting non-calcareous algae or articulated calcareous algae. These results suggest that fish grazing influences algal species abundance and dynamics at one investigated site within the Medes Islands Protect ed Area by reducing the populations of fleshy erect macroalgae with su perior overgrowth ability, and thereby facilitating the seasonal domin ance of a chemically-defended algal population. However, effects of fi sh grazing were not so apparent at a second site with lower abundance of fish. This preliminary study provides a basis for reevaluating the conclusion that in the Mediterranean, like in other temperate seas, fi sh grazing has negligible effects on benthic communities. (C) 1997 Els evier Science B.V.