Sea Urchin Mass-Mortality: Effects on Reef Algal Abundance, Species Composition, and Metabolism and Other Coral Reef Herbivores

Grazing by Diadema anti l larum has been demonstrated to c o n t r o l the biomass, s p e c i e s composit ion, and most r e c e n t l y , the metabol ism of many coral r e e f algal turf communit ies. The m a j o r i t y of these data have been obta ined us ing i n c l u s i o n e x c l u s i o n cage exper iments. The r e c e n t mass-mortal i ty of t h i s sea urchin in the Car ibbean prov ided a natural t e s t of many of the experimental lygenerated hypotheses. In S t . Cro ix , twoyears o f p r e m o r t a l i t y data showed that a l g a l t u r f s grazed by D. anti l larum had higher biomassspec i f i c product ion (production/algal biomass/time), and production/area/time not less than, higher biomass t u r f s not grazed by urchins. The natural removal of 957. of the D. anti l larum ind iv idua ls produced s i m i l a r patterns in the turf biomass and product ion. F i v e days a f t e r the m o r t a l i t y event, a lga l b ianass had increased by 277. whi le b i i n a s s s p e c i f i c product ion decreased by 607. . In a d d i t i o n , algal spec ies composi t ion began changing w i t h i n f i v e days of the d i e o f f . Larger macroalgal species such as Laurencia obtusa and Acanthophora s p i c i f e r a increased in abundance whi le smal ler algal f i lamentous s p e c i e s , although increas ing in s ize , decreased in overa l l abundance. These patterns have p e r s i s t e d f o r ten months and are most pronounced in shal low r e e f zones. Recent s tudies have suggested that e x p l o i t a t i v e competit ion f o r a l g a l resources may be occurr ing between herbivorous f i s h e s and sea urchins. Pre l iminary data suggest that such competit ion was occurring p r i o r to the Diadema d i e o f f . The near removal of this spec i e s r e s u l t e d in an immediate functional response by herbivorous f i s h e s , mainly s c a r i d s . F ish grazing i n t e n s i t y , as measured by t ime lapse photography, increased 3807 . one week a f t e r the sea urchin mass-mortality. The p o s s i b i l i t y of a longterm numerical response by herbivorous f i s h populat ions is be ing invest igated. The r e s u l t i n g s h i f t s in a l g a l abundance, spec i e s composit ion, and metabol ism as a r e s u l t of the removal of D. antil larum, s t r o n g l y support prev ious exper imental r e s u l t s and r e a f f i r m the concept that this sea urch in spec ies i s instrumental in s t r u c t u r i n g many Caribbean coral reef a lgal communit ies.