Changes in Coral Morphologies and Total Coral Cover with Depth: Kingman Reef and Palmyra Atoll

By using a remotely operated vehicle to survey benthic community structure, this study investigated questions concerning coral morphological changes with depth and total coral cover changes with depth at Kingman Reef and Palmyra Atoll, two sites in the Pacific, southwest of Hawaii. It was found that the fore-reef at Kingman showed no particular trend in terms of coral morphologies at different depths. However, the data from Palmyra did show the expected trend with more vertical structures, digitate and columnar morphologies, near the surface and more horizontal, foliose morphologies at greater depths. Although it is possible that these two coral communities behave differently along a depth gradient, considering the limited number of clear images captured with the ROV, it is more likely that a few plots at Kingman skewed the results away from the previously documented trend noted at Palmyra.
The data on total coral cover was much more consistent between the two sites. Total coral cover was consistently high from 15 to 45 meters and then exhibited an abrupt decline after 50 meters. Correlating this data with the light extinction curves for each site raises the possibility that there is a very specific minimum light requirement for hard corals, below which none can grow. However, since the data set was most limited between 50 and 80 meters, it is possible that the transition to 0% cover was actually more gradual than observed in the ROV imagery.