Seagrass composition shifts and declines associated with reduced light, high phosphate, and elevated water temperatures in Turneffe Atoll Marine Reserve, Belize

Seagrasses are foundation species that provide essential ecological services to coastal ecosystems, yet they are increasingly threatened by anthropogenic and climate-driven stressors. This study uses the SeagrassNet global monitoring protocol to assess environmental and biological indicators of seagrass condition at two back-reef sites in Turneffe Atoll Marine Reserve, Belize, the largest coral atoll in the Mesoamerican Barrier Reef System, between 2022 and 2025. Over the study period, we documented a steady decline in Thalassia testudinum and a shift in species dominance toward Syringodium filiforme. By 2025, seagrass had disappeared entirely from the shallow transects at both sites. Declines in abundance and the shift in species composition were associated with low light availability, elevated phosphate concentrations, and frequent exposure to temperatures exceeding the thermal optimum for these species. The observed loss of seagrass habitat indicates that even protected seagrass meadows within marine reserves remain vulnerable to shifting environmental conditions and raises broader concerns about the status and resilience of meadows throughout the Mesoamerican Barrier Reef System. Moreover, it underscores the broader ecological implications, as seagrass health is closely linked to the resilience of other coastal habitats including adjacent coral reef systems.