Abstract: Compositional Changes in Reef Sediments Related to Changes in Coral Reef Community Structure

PRECHT, W. F. and R. B. ARONSON

Long-term monitoring of coral reefs reveal that they are dynamic, disturbance- driven systems. Yet despite the disequilibrial nature of the system, there has been little emphasis on the response of sediments to these changes. Here we present the results of a twenty-year monitoring program on LTS Reef, Discovery Bay, Jamaica (1978-1997) that addresses this question of comparative sedimentology.

The purpose of this study was to characterize the complexion and genesis of reef sediments, pairing sedimentary data with ecological survey data. We had no idea in our initial surveys of the dramatic changes that would occur over the next two decades. Multiple major disturbances have had a profound effect on the distributional patterns observed. The most significant change has been the reduction of stony corals (75% to 5%), and the increase of non-calcareous, fleshy macroalgae (<5 % to >65%). However, the abundance of calcareous green algae, Halimeda, did not change throughout the course of this study (8%). Evaluation of the benthic community could not have predicted changes in the composition of the sediments.

Before 1980 the sediments were principally comprised of Halimeda (>50%), with corals being subordinate (<35%). This indicates that there was high productivity and rapid turnover of Halimeda, while the coral fraction was "locked-up" in long-term, in-situ growth. By contrast, all samples since 1981 have been comprised primarily of coral fragments (>60%) with a reduction in Halimeda (<25%). This increase in coral is related to an increase in bioerosion subsequent to widespread coral mortality. These results are evidence that the composition of reefs sediments can be used as an indicator of reef health, with potential application to reefs through time and space.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky