SEASONAL-VARIATION IN ALGAL SPECIES RICHNESS AND ABUNDANCE IN THE PRESENCE OF MOLLUSCAN HERBIVORES ON A TROPICAL ROCKY SHORE

Seasonal variation in species and percentage cover of macroalgae was investigated over a 15-month period on a semi-exposed tropical rocky shore. Cages were used to manipulate herbivore access in mid and low shore areas. Algal species richness and abundance showed a marked seasonal pattern, peaking during the winter months and decreasing in the summer. At both mid and low shore the number of species and abundance of algae was greatest in herbivore exclusion treatments and the effect was more pronounced in the low shore where there was a consistent difference between treatments throughout the year. In the mid shore there was a difference in the cover of algae between treatments in the winter months only; for the remaining period there was little difference. Algal abundance and species numbers declined dramatically in all treatments as the temperature increased with the onset of summer, although the decline was less pronounced in the lower shore treatments. The results indicate that at higher tidal levels physical factors influence algal abundance to a greater extent than lower on the shore where herbivory is important, although lower on the shore the physical extremes of a tropical summer are also strong limiting factors.