Coastal plain retention modulates flashflood chlorophylla anomalies in the Red Sea: a CPRI based multisite analysis

Mohammed A Hakami, Maged Marghany

The eastern Red Sea coastal margin is among the most oligotrophic large marine ecosystems on Earth, where episodic nutrient pulses from flash floods can transiently fuel phytoplankton blooms. Yet, the extent to which upstream coastal-plain geomorphology modulates the magnitude of these flood-driven chlorophylla (Chla) responses remains poorly quantified. We tested the hypothesis that narrower coastal plains amplify postflood Chla anomalies by shortening runoff residence time and limiting terrestrial nutrient retention. Using Google Earth Engine, we integrated CHIRPS daily rainfall, MODISAqua Chla (4 km), SRTMderived basin areas and plain widths, and HydroSHEDS drainage networks across five Saudi Arabian Red Sea sites spanning a 7.4–28 km coastalplain gradient (Wajh, Yanbu, Jeddah, Qunfudhah, Jazan). The dimensionless Coastal Plain Retention Index, CPRI  were ranged from 0.71 (Wajh, narrowest) to 2.09 (Jazan, widest). Across 42 floodbloom events from 2015 to 2025, sitelevel median postflood Chla anomalies decreased monotonically with increasing CPRI (Spearman ρ = −0.90, n = 5 sites). The narrowestplain site, Wajh, exhibited the largest median (+17.4 %) and bootstrapmean (+36.3 %, 95 % CI −4.5 to +89.4 %) responses, while the midplain site Yanbu reached statistical significance at α = 0.05 (mean +29.3 %, 95 % CI +10.1 to +47.7 %, n = 10). The southernmost site, Jazan, diverged from the CPRIresponse gradient (median −0.73 %) owing to a high anthropogenic baseline from Tihama agriculture and aquaculture, compounded by a single monsooncoincident extreme event on 30 September 2018 (+290 %), treated as a sensitivity outlier. Robustness of the inverse CPRI–bloom relationship is demonstrated by the convergence of three analytical lenses on ρ = −0.90, using both median and trimmedmean anomaly metrics. In conclusion, these findings establish coastalplain geomorphology as a firstorder, remotely sensible control on flooddriven nearshore productivity, with direct implications for reef management and harmful algal bloom earlywarning systems along the Red Sea.