Artificial wounding decreases plant biomass and shoot strength of the brown seaweed Ascophyllum nodosum (Fucales, Phaeophyceae)

Abstract

Brown seaweeds can serve as habitat and food for a number of mesoherbivore species, but the effect of mesoherbivore grazing on the fitness of individual seaweed plants is not well documented. Here we investigate how mechanical wounding, which mimics the grazing damage made by the herbivorous gastropod Littorina obtusata, affects the biomass change, apical growth, and individual shoot strength of the fucoid seaweed species Ascophyllum nodosum both in natural field populations and in the laboratory. We did not detect a significant effect of wounding on apical shoot growth in the laboratory, but wounding decreased the plant biomass in half of the studied A. nodosum populations in the field. Furthermore, we found that the strength of individual A. nodosum shoots was significantly lower when compared with unwounded control shoots initially after wounding. However, after 11 days incubation in the laboratory, the strength of the wounded shoots had increased almost to the level of the control shoots, although there was still a significant difference in the force required to break the shoots. These results indicate that wounding can increase the tissue loss (and thereby decrease the fitness) of A. nodosum plants in natural populations, probably through weakening of individual shoots. However, A. nodosum appears to have a mechanism to strengthen the shoots after mechanical wounding, and therefore, the timing of damage (e.g. prior to a storm) probably determines the amount of plant tissue lost through wave-induced forces.