DISTURBANCE AND THE SIZE OF ACROPORA PALMATA ON LATE PLEISTOCENE JAMAICAN REEFS

Work on modern Caribbean reefs suggests that the maximum size attained by the elk horn coral, A. palmata, may be largely determined by the frequency of major storms that kill or severely damage branched coral. Also, evidence indicates that the number of years between major storm disturbance act as a control on the development of the entire A. palmata reef crest zone. With this in mind, studies of colony size in Late Pleistocene elk horn coral facies should provide insight into the frequency and degree of storm disturbance on these ancient reefs and should shed light on their applicability as analogs for modern Caribbean coral communities.

The present work focuses on reef deposits preserved in the lowest coastal terraces (late Pleistocene, Falmouth and Port Morant Formations) found at sites along the north and south coast of Jamaica. Each study site included multiple exposures composed of A. palmata dominated assemblages. These sites included in place colonies as well as beds of transported coral rubble. Transect sampling was used to assess species composition, the size distributions of dominant coral species and the extent to which colonies were in growth position or transported. Measurements were made of the maximum branch thickness and extent for over 400 colonies of A. palmata. Comparative data were also collected from modern elk horn coral including recent coral rubble beds produced by large storms that have hit the south coast of Jamaica over the last 3 years.

Based on transect data, there is little difference between the size distributions of transported corals in the different fossil sites or between fossil material and the sampled modern transported A. palmata. This suggests that the wave intensities that formed these deposits were comparable. There were, however, significant differences in mean branch thickness for in place colonies between fossil sites and between fossil and modern examples. At some Late Pleistocene sites, elk horn branches were significantly larger and presumably older than that seen in common modern examples. This suggests that the frequency of major disturbance on these fossil reefs was considerably lower than that seen on most modern Jamaican reefs.