Response of sediment composition in Jamaican reef facies to changes in environmental factors

Modern sediments from reef facies at Discovery Bay, Jamaica, were analyzed to document changes in biotic sediment composition over the past four decades and to evaluate their relationship to the impacts of hurricanes, rising temperatures, and sea-level rise. Discovery Bay is a classical Caribbean reef research site, where reef community structure has been quantitatively and systematically documented since 1977, providing a robust long-term framework for interpreting sedimentary responses to environmental and climatic stressors. New data reveal that sediments from the backreef/lagoonal zone are dominated by plates of the green alga Halimeda (35.7–54.0%) and coral fragments (29.7–48.7%). Compared to the 1982–1999 period, Halimeda has nearly doubled in relative abundance, while coralline algae, echinoid fragments, and the encrusting foraminifer Homotrema rubrum have decreased by more than 50%. Despite the present scarcity of living corals in the backreef, coral fragments remain abundant in the sediments. They are derived mainly from the adjacent reef crest, where only old, dead coral skeletons are present today. Intensified wave action associated with sea-level rise promotes erosion of the old reef framework and transport of sand-sized material into the backreef. Additional coral skeletal material is supplied by storm-induced reworking of older backreef sediments. In the fore-reef zone (<30 m water depth), samples were collected along transects across a sand channel and a reef terrace. In the sand channel, sediments are dominated by coral grains (40.7–62.0%) and Halimeda fragments (21.7–41.7%), showing little change compared to 1999, except for a slight decline in coralline algae and Homotrema. On the fore-reef terrace, coral grains and Halimeda fragments contribute similar proportions (approximately 43% and 40%, respectively). With increasing depth, coral grains generally decrease, whereas Halimeda reaches its highest abundance at depths of 10–15 m. Compared to 1999, Halimeda fragments have increased by more than 10%, while coralline algae have decreased by approximately 10%.

Although living coral cover in the fore-reef zone has declined to less than 5% over the past three decades, the contribution of coral grains to sediments has remained nearly constant. This indicates that dead coral skeletons and older reef material present in the fore-reef zone have long residence times and continue to contribute to sand-sized sediment production. Consequently, fore-reef sandy sediments represent time-averaged assemblages and have limited capacity to record short-term changes in reef biotic composition.

This study was supported by the Polish National Science Centre, grant No. 2022/45/B/ST10/00599.