Enhanced nutrient loading and herbivory do not depress the resilience of subtidal canopy forests in Mediterranean oligotrophic waters

The interaction between top-down and bottom-up forces determines the recovery trajectory of macroalgal forests exposed to multiple stressors. In an oligotrophic system, we experimentally investigated how nutrient inputs affected the recovery of Cystoseira brachycarpa following physical disturbance of varying intensities, both inside forested areas and at the boundary with sea urchin barrens. Unexpectedly, Cystoseira forests were highly resilient to disturbance, as they were able to recover from any partial damage. In general, the addition of nutrients sped up the recovery of Cystoseira. Thus, only the total canopy removal, in combination with either low nutrient availability or intense grazing pressure, promoted the expansion of mat-forming algae or urchin barrens, respectively. Our study suggests that the effects of enhanced nutrient levels may vary according to the trophic characteristics of the waterbody, and hence, are likely to vary among regions of the Mediterranean basin.