129-33 Beyond Wave Exposure: Contrasting Sediment Texture, Composition, and Benthic Habitats across Ambergris Caye, Belize

Abstract:

Carbonate sediment texture and composition reflect interactions among benthic carbonate production, hydrodynamic energy, and sediment redistribution, yet the relative importance of these controls may vary across shallow-water environments. This study compares sediments from six transects across two contrasting lagoons on Ambergris Caye, Belize: the reef-facing windward lagoon (transects A–C) and the protected leeward lagoon at Secret Beach (transects D–F). Field observations documented water depth, substrate, and benthic habitats, whereas dry-sieve granulometry and petrographic point counting were used to characterize sediment texture and grain composition. Both lagoonal settings contain polymodal, poorly sorted medium sand, but the leeward sediment is slightly coarser and contains a substantially larger gravel fraction. Mean grain size is 306 μm leeward and 291 μm windward, whereas gravel constitutes 5.4% and 1.7%, respectively. Both sediments are poorly sorted, although the leeward sediment is slightly more poorly sorted than its windward counterpart. Grain composition shows a pronounced but contrasting pattern. Peloids dominate both assemblages but are more abundant leeward (~82%) than windward (~69%). Windward sediment contains a larger and more diverse skeletal fraction, including approximately 19% molluscan grains and 4% Halimeda, compared with approximately 6% mollusks and negligible Halimeda leeward. Foraminifera are somewhat more abundant leeward (~8%) than windward (~4–5%). These compositional differences are consistent with field observations of dense seagrass, abundant Halimeda, and diverse benthic communities in the windward lagoon, in contrast to the predominantly open sandy substrate and more restricted benthic habitats leeward. Despite greater wave exposure windward, the leeward sediment is coarser and more gravel-rich, whereas the windward sediment contains the larger and more diverse skeletal fraction. This opposing pattern indicates that compositional diversity does not necessarily correspond to coarser sediment or greater hydrodynamic exposure. The windward–leeward contrast demonstrates that grain texture and composition may record partly independent aspects of sediment production and redistribution within a single carbonate-lagoon system.

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