HIGHLY PRODUCTIVE EASTERN CARIBBEAN REEFS: SYNERGISTIC EFFECTS 

 OF BIOLOGICAL, CHEMICAL, PHYSICAL, AND GEOLOGICAL FACTORS 



Walter H. Adey 

 Marine Systems Laboratory 

 Smithsonian Institution 

 Washington, D.C. 20560 



and 



Robert S. Steneck 



Zoology Department and Oceanography Program 



University of Maine 



Darling Center Marine Laboratory 



Walpole, Maine 05473 



ABSTRACT 



Studies of reef productivity and community structure were conducted on 

 three bank barrier reefs along the south shore of St. Croix, U.S. Virgin Islands, 

 Primary productivity (rate of organic carbon fixation) was measured using 

 upstream/downstream flow respirometry techniques. Mean annual gross primary 

 productivity of the St. Croix reef ranged between 17-33 g C/m^/d. These are 

 among the highest values reported for reefs or for any biological community. 



Reef productivity corresponds with the distribution and abundance of turf 

 algae. These algae are minute filamentous forms which are highly diverse, 

 productive, and abundant. Algal turf productivity is limited primarily by 

 available solar energy and is influenced by wave surge and water flow (which 

 enhance metabolite exchange). Turf cover is limited by the total surface area 

 of standing dead coral. Nitrogen availability is increased by in situ nitrogen 

 fixing blue-green algae, as evidenced by increased concentrations of inorganic 

 nitrogen in water flowing over the most productive reef zones. Phosphorus 

 availability is enhanced by wave surge and maintained by inflowing water from 

 the open ocean. 



Over geological time, reefs grow toward sea level. The three reefs we 

 studied represent three stages in reef development. Younger reefs to the east 

 are deeper, with considerable live branching coral ( Acropora palmata ) and with 

 a low abundance of turf algae. The reverse is true for the older reefs to the 

 west, where live coral is minimal but the dominant algal turfs occupy a rather 

 smooth pavement of limited surface area. Intermediate between these two types 

 of reefs, the abundance of live coral is low but biomass of algal turf is very 

 high due to the large surface area left by the dead branching coral. The 

 greatest reef productivity was measured under the latter conditions because of 

 the synergistic effects of large surface area with abundant algal turf cover in 

 shallow, strongly lighted, and turbulent water. It is clear that, given optimum 

 conditions, coral reef algal turfs can develop primary productivities that 

 reach and perhaps exceed the accepted theoretical maximum for primary production 

 in the sea. 



163 



