Depletion of palatable algae by sea urchins and fishes in a Mediterranean subtidal community

Abstract

I studied the relative and combined effect of sea urchins and fishes on algal biomass in the Mediterranean Medes Island Marine Reserve, using tethering experiments with palatable algae in large enclosures that contained an orthogonal combination of sea urchins and fishes. Sea urchins contributed most to the decline of algae. The effect of fishes was more moderate, but nevertheless not negligible, as a decrease in algal biomass was also observed in the fish treatments at all sites. Surprisingly, the effects of fishes and sea urchins on algal biomass were not additive: the presence of fishes reduced the impact of sea urchins, and the loss of algal biomass was less in the combined treatments. These results add to the literature showing that drastic depletion of palatable algae can be caused by sea urchins, and highlight the contribution of herbivorous and omnivorous fishes to this process. Furthermore, since sea urchin densities were held constant during the experiment, my results support the concept that omnivorous fishes benefit algae in subtidal communities by altering the behavior of urchins (e.g. by inducing avoidance behavior), thus reducing their effectiveness as grazers. © Inter-Research 2006.