Grazing damage and encrustation by an invasive bryozoan reduce the ability of kelps to withstand breakage by waves

Increased breakage of macroalgal fronds during large wave events can significantly reduce canopy cover and biomass. We examined the effects of encrustation by the invasive bryozoan Membranipora membranacea and
damage by the snail Lacuna vincta on the ability of kelp blades (Saccharina longicruris, Laminaria digitata, and Laminaria complanata) to withstand wave forces. Using standard materials testing procedures, we
documented significant reductions in the maximum stress before breakage, toughness, and extensibility of blade material following bryozoan encrustation. Histological sections of blade tissue indicated a significant
degradation of the outer layers of cells following prolonged encrustation by M. membranacea as a likely cause of weakening. Full-thickness perforations and partial-thickness grazing scars also reduced blade strength,
suggesting that grazing damage can initiate cracks that lead to blade breakage. Our findings provide a mechanistic link between the damaging effects of mesograzers and encrusting bryozoa on their algal hosts and the export of detrital material from subtidal kelp beds.