Eutrophication, hypoxia, and ocean acidification is biogeochemical triple treat in the world coastal ocean, effecting biodiversity loss, human health and well-being, coastal economy specifically fishery and tourism sector. This study proposes the spatiotemporal variation and risk of eutrophication, hypoxia, ocean acidification in Thailand waters include the Gulf of Thailand and Andaman Sea. We complied documents and a database of red tide evets and its relevance to hypoxia and ocean acidification in Thailand’s coastal water between 1952-2022, analyzed spatiotemporal variations based on frequency, scale, duration, causative plankton species, and its consequence results and developed a multiple dimensional hazard index. Our analysis reveals that The ecological impacts are multifaceted and irreversible without intervention. Nutrient enrichment has shifted phytoplankton communities from diatom to dinoflagellate dominance (62% of blooms post-2000), favoring harmful algal species that disrupt fisheries and coastal livelihoods. Hypoxia, first documented in the 1970s, now persistently afflicts the Tha Chin and Chao Phraya estuaries, where ammonia concentrations reach 808 µmol kg⁻¹, overwhelming microbial respiration and creating anoxic sediments. Seasonal monsoon dynamics compound these effects, with August-November stratification isolating oxygen-depleted bottom waters, while extreme hydrological events, such as the 2011 La Niña-linked floods, amplify hypoxic zones to 4,120 km². This study provides a critical insight for targeted red tide, hypoxia, and ocean acidification management and policy-making.

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