Assessment of grazing effects on phytobenthic community structure at shallow rocky reefs: An experimental field study in the North Aegean Sea

Overgrazing can deplete macroalgal communities and lead to reduction of habitat complexity and species diversity. To quantify the effects of sea urchin and fish herbivory on the rocky reefs of the North Aegean Sea, a sixmonth grazer exclusion experiment was conducted at the southeastern part of Lesvos Island at three rocky reef sites at depths between 1.0 and 4.7 m. The aim was to test whether (a) algal growth is hampered by the combined grazing activity of sea urchins and fish, and (b) algal growth is suppressed by the foraging of fish alone. At each site, three replicate cage, cage-control, and control treatments were applied and sampled every two weeks using photoquadrats: (1) fully-closed cages that excluded both sea urchins and large herbivorous fish; (2) open top cages that excluded sea urchins only; and (3) control surfaces with no restrictions on herbivores. Algal biomass was estimated based on the percentage cover values of the analyzed images. Underwater surveys for the estimation of sea urchin population density (by quadrat sampling) and total fish biomass (by strip transects) were also conducted. The main grazers observed were the sea urchins Arbacia lixula and Paracentrotus lividus, and the herbivorous fish Sarpa salpa. Herbivorous fish dominated fish communities, but the overall fish biomass density was relatively low. The experimental monitoring of algal biomass showed that erect algal growth was significantly higher inside the fully-closed cages in contrast to the respective controls. Overall, algal growth was significantly higher inside both types of cages, when compared to the control surfaces. No significant differences among the two types of cages were found in terms of total algal growth, indicating that urchin grazing was the most important factor causing the hampering of algal growth and altering the structure of the macroalgal community towards a sparse vegetation of low complexity devoid of erect algae, while herbivore fish had a minor effect.