Comparing Fish and Urchin Grazing in Shallow and Deeper Coral Reef Algal Communities

Abstract

Algal—herbivore (fish and sea urchin) interactions in shallow (<10 m) and deeper (>15 m) coral forereef communities off Discovery Bay, Jamaica, were investigated and compared. Algal functional form groups exhibited different bathymetric distribution patterns. Crustose corallines, the most abundant group in shallow water, decreased in abundance with increasing depth, whereas erect macroalgae increased in abundance with depth, predominating in deeper water. This distribution pattern was correlated with a decline in herbivory over depth. The hypothesis that herbivory affects algal abundance and distribution was tested using grazer exclusions and algal transplants. Five 1—m2 exclusion fences, which excluded the sea urchin Diadema antillarum but not fish, were erected in both shallow (7—8 m) and deeper (17—18 m) water. Removal of Diadema resulted in a pronounced change in the shallow algal community; erect and filamentous algae rapidly overgrew crustose corallines in 2 mo. In contrast, Diadema exclusion had little effect on the deeper algal community. In another experiment, which prevented both fish and urchin grazing, unaltered natural substrates with attached algae were suspended on racks in the water column 30—40 m from the reef at depths corresponding to the fences. Abundances of erect and filamentous algae increased on both shallow and deeper racks. The erect species that increased in abundance in the fences were resistant to fish, whereas only fish—susceptible species increased on the racks. Transplanting the most abundant erect species from the reef crest (Laurencia obtusa, Caulerpa racemosa) and deeper forereef (Lobophora variegata, Dictyota divaricata, Halimeda goreauii) to the shallow forereef revealed that all, except L. obtusa, were readily consumed by Diadema, but all exhibited low susceptibility to fishes. The reef crest and deeper reef are spatial refuges from urchin predation for erect macrophytes. In summer 1983, the Diadema population experienced >95% mortality. On the shallow reef, erect and filamentous algae and fish grazing intensity increased substantially following urchin death, while crustose corallines declined. Conversely, Diadema mortality had little effect on the deeper reef. Algal responses were similar to those observed in the urchin exclusion experiments. The erect species that increased most after mortality are resistant to fishes via chemical defenses. These findings demonstrate that herbivory is largely responsible for the algal distribution pattern on the reef. The results suggest that Diadema plays a major role in structuring the shallow community, but only a minor role relative to fishes in structuring the deeper community. They also indicate that different herbivores can have a differential effect on erect macroalgal composition.