Assessing seagrass ecological health in Zanzibar using an integrated index of drone and in situ indicators

Seagrasses respond sensitively to both natural and anthropogenic pressures, making them effective indicators for assessing pressure-driven changes in marine ecosystem health. This study developed and validated a novel multimetric Seagrass Ecological Health Index (SEHI) by integrating drone-derived and in situ ecological metrics to assess nearshore seagrass condition in Unguja, Zanzibar. The index combines multiple parameters, including measures of abundance, taxonomic composition, eutrophication, leaf condition, and seascape fragmentation. Relationships between SEHI and key natural pressures-tidal desiccation (TEI) and wave exposure (REI_D)-as well as anthropogenic pressures such as hotel density, fishing intensity, seaweed farming, sea urchin density, and water quality, were evaluated. SEHI exhibited significant negative correlations with tidal desiccation (r = -0.70, p < 0.05), while wave exposure showed a negative but non-significant trend (r = -0.49), and sea urchin density (r = -0.70, p < 0.05), and responded strongly to phosphate, chlorophyll-a, and seaweed farming intensity. Among the component indicators, epiphytic cover showed the highest sensitivity to human pressures. Based on SEHI classification, five of the nine study sites were in moderate ecological condition, while two were in poor condition. The results demonstrate the robustness of the SEHI framework as an integrative, drone-based and field assessment tool capable of detecting seagrass ecosystem responses to multiple stressors. SEHI provides a scalable approach for monitoring and managing seagrass health in Zanzibar and other tropical, data-limited regions, improving the assessment and communication of seagrass ecological condition to managers and stakeholders involved in coastal and marine conservation.