Herbivory effects on the morphology of the brown alga Padina boergesenii (Phaeophyta)

Many coral reef benthic algae exhibit morphological plasticity enabling them to persist in diverse habitats under different environmental pressures. Morphological plasticity has been previously observed in the brown alga Padina jamaicensis, which displays a foliose morphology under low grazing pressure and a turf morphology in habitats with medium to high herbivory levels. To determine whether a different species of Padina, P. boergesenii, exhibits morphological plasticity regulated by herbivory, we excluded macroherbivores from experimental plots containing P. boergesenii thalli on a rocky reef off Santa Marta in the Colombian Caribbean. The experiment used exclusion cages to test for effects of herbivores and was carried out in two contrasting oceanographic seasons: upwelling (low water temperature and high water transparency) and rainy (higher temperature and low water transparency). The morphology of P. boergesenii was significantly affected by herbivory but only during the upwelling season. Padina boergesenii rapidly changed from a turf morphology to a fan-shaped form and increased in size when protected from herbivores in full cages in a period of less than a week. Thalli exposed to macroherbivores in open plots did not shift to a foliose morphology, and neither did thalli investigated during the rainy season, independent of herbivore treatment. Our results show that herbivory plays an important role in controlling the morphology and may affect life history processes of the alga P. boergesenii but suggest that plant response to herbivory may be a seasonal process. This suggests that there are important climatic (e.g. trade winds inducing upwelling) and oceanographic factors (e.g. water temperature) limiting algal abundance and that these factors may mediate morphological plasticity of algae. Therefore, reductions of grazing are not enough to promote morphological plasticity during the rainy season.