Nutrient addition increases consumption rates of tropical algae with different initial palatabilities

Abstract

While nutrient enrichment increases herbivory rates on highly palatable species of tropical algae, it is unclear how nutrient enrichment affects species with differing initial palatabil- ities. We determined that there was a wide range in relative palatability among 5 perennial and common species of Caribbean coral reef algae in a cafeteria-style choice experiment, with overall preference for Acanthophora spicifera the highest, Dictyota dichotoma and Caulerpa sertulari- oides intermediate, and Turbinaria turbinata and Sargassum polyceratium the lowest, although there were significant differences in grazing rates among algal species between the 2 fringing reef sites tested. We then tested the hypothesis that increased nutrient content in algae increases palatability using choice experiments between enriched and ambient algal samples for each of 3 species along this palatability gradient: A. spicifera, D. dichotoma, and S. polyceratium. For all species, herbivores selected enriched over ambient samples across the 2 sites despite contrasting herbivore guilds (i.e. dominated by sea urchin Diadema antillarum versus herbivorous fishes). However, despite increased palatability of enriched S. polyceratium, overall herbivory on this spe- cies was still relatively low compared to other studies, suggesting that S. polyceratium in this sys- tem may have stronger chemical defenses, or that a specialist herbivore guild may be missing on this Caribbean reef. Regardless of the mechanism, our study implies that anthropogenic or natural events that result in high supplies of nutrients in coral reef seawater will increase overall seaweed palatability, even of unpalatable species.