Rates of overgrowth by macroalgae and attack by sea anemones are greater for live coral than dead coral under conditions of nutrient enrichment

Abstract

A mesocosm experiment was conducted to determine the effects of nutrient enrichment on competitive interactions between a hard coral, a green alga, and a sea anemone. In the low-nutrient controls, abundances of the green alga, Codium edule, and a sea anemone, Mesactinia genesis, remained low, and they coexisted with the live or dead scleractinian coral, Acropora muricata. Combined nitrogen and phosphorus additions markedly increased the photosynthetic efficiencies of zooxanthellae in A. muricata, the coverage of C. edule, and the asexual reproduction by M. genesis. After 35 d of nutrient addition, C. edule had begun to overgrow live A. muricata, but not dead coral. A. muricata finally died after 105 d, after being totally overgrown by C. edule. Within a few days of contact with live A. muricata, M. genesis was observed for the first time to have induced inflation of modified marginal aggressive organs known as acrorhagi tentacles, which it uses to attack neighboring coral. Nevertheless, M. genesis was not observed to attack C. edule, but moved away from it in the nutrient-enriched tanks. The hierarchy of competitive superiority under nutrient enrichment was in the order of C. edule . M. genesis . A. muricata. From this experiment, it was evident that nutrient enrichment inhibits corals' ability to compete with sea anemones and algae in Nanwan Bay, southern Taiwan.