Light levels, depth, current velocity, and abundance of live Acropora 

 palmata all increase with geological time as a reef grows toward sea level. 

 As it reaches the surface, several of these trends reverse (i.e., coral dies 

 and water flow decreases). Aspects of this complex relationship between reef 

 morphogenesis and primary productivity can be seen along the south shore of St. 

 Croix. For example, productivity was greatest for the mature Robin Reef and 

 lower on the younger Isaacs and older Halfpenny Reefs (fig. 7). On Robin Reef, 

 the SAR of primarily dead Acropora palmata is very high and algal cover is 

 about 80% greater than that of Halfpenny. Thus, peak productivity probably 

 occurs soon after the transition from a live, coral -dominated reef to one of 

 equal surface area but dominated by algae. With time, productivity declines as 

 destructive forces (e.g., bioeroding sea urchins or severe storms) reduce the 

 standing dead coral to a pavement-like al gal -dominated flat. This pattern of 

 reef succession is characteristic of many reef systems. Adey, et al . 

 (1977) described a similar history for the now largely planar algal dominated 

 pavement in eastern Martinique. Over the past 500 yr the open Acropora palmata 

 reef matured and lost its structure, possibly due to a catastrophic event such 

 as a hurricane. It has remained an algal pavement, devoid of coral, ever 

 since. In the case of Martinique, excess sediment and nutrient runoff from 

 the island following the introduction of intense sugar cane farming in the 17th 

 century undoubtedly was a major factor in transforming these reefs into macroalgal 

 pavements. After the completion of this study in 1978, a similar event occurred 

 at the south shore of St. Croix when a hurricane passed off the coast in 1979. 

 Rogers, et al . (1982) documented the event and showed a significant reduction 

 in SAR and live coral following the hurricane. 



Differences in reef morphogenesis may be responsible for some of the 

 reported differences in reef productivity. Holocene sea level characteristics 

 for the Indo-Pacific differs from that of the Caribbean. Sea level rise stopped 

 over 5,000 yr ago in the Indo-Pacific (reviewed by Adey, 1978a_), resulting in the 

 characteristic old reefs of the region having emergent, broad reef flats (many 

 km wide), often with no epibenthic water flow during low tide and with a low SAR. 

 This extreme "old age" condition of Indo-Pacific reef flats, developed under 

 stable or dropping sea levels, contrasts with conditions in the Caribbean, where 

 sea level has been slowly rising until the past several centuries (e.g., Adey, 

 et al . , 1978a). This geological characteristic of reefs could be responsible 

 for the lower levels of primary production reported for the Indo-Pacific compared 

 to results presented here for the St. Croix reefs. 



CONCLUSIONS 



Fundamentally, reef productivity is controlled by its geology (reef growth 

 and morphogenesis). Coral reefs grow slowly towards sea level over time, 

 thereby changing their physical, chemical, and biological environment. Primary 

 productivity is maximal at the time when the reef surface is becoming too 

 shallow to support a large cover of live coral but still deep enough for contin- 

 uous water flow. Dead branching coral ( Acropora palmata ) in standing position 

 provides a complex and convoluted substratum on which minute algal turfs can 

 grow and herbivores can take refuge. The structurally complex reefs maintain a 

 high biomass per projected area of reef, thereby increasing the total turf 



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