Nutrient–Carbon Interactions and Monsoon-Driven Harmful Algal Bloom Dynamics in the Bosaso Coastal Waters of Puntland, Somalia (2023–2025)

Harmful algal blooms (HABs) pose a growing ecological and economic risk in tropical shelf seas, yet their drivers remain poorly constrained along the Somali coast, one of the least monitored upwelling systems of the western Indian Ocean. This study quantifies the spatio-temporal dynamics of chlorophyll-a and associated biogeochemical variables in the Bosaso coastal waters (48.5°–51.9° E; 8.8°–12.7° N) from January 2023 to October 2025 using Copernicus Marine Environment Monitoring Service (CMEMS) reanalysis data. Daily surface observations of chlorophyll-a, nitrate (NO₃), phosphate (PO₄), silicate (Si), and surface partial pressure of CO₂ (spCO₂) were harmonized, regridded, and merged into a unified dataset. Bloom events were identified using a 90th-percentile chlorophyll-a threshold (1.293 mg m⁻³), while a logistic regression model evaluated bloom probability from nutrient–carbon interactions. Results reveal strong seasonal and interannual variability linked to monsoon-driven upwelling, with elevated nutrients and reduced spCO₂ coinciding with bloom peaks. Chlorophyll-a exhibited significant correlations with NO₃ (r = 0.85), PO₄ (r = 0.73), and Si (r = 0.62), confirming nutrient enrichment as the primary driver of bloom initiation. The logistic model achieved high predictive performance (AUC = 0.97), indicating that simple nutrient and CO₂ metrics can serve as effective early-warning indicators. These findings provide the first quantitative HAB baseline for Puntland’s coastal waters and demonstrate a reproducible computational workflow for regional HAB monitoring and management.