Under accelerating climate change and anthropogenic pressures, the global oceans face unprecedented threats from land-to-ocean contaminant discharge. Among these, herbicides are insidious but overlooked contaminants that endanger phytoplankton, the cornerstone of marine food webs, nutrient cycling, and carbon sequestration. Here, we synthesize three decades of observational and modelling data revealing sustained and widespread herbicide discharge into global coastal oceans, with productive enclosed and semienclosed seas exhibiting 4.9-fold higher concentrations than open coastal waters. Critically, herbicide metabolites, systematically overlooked in conventional monitoring, exceed parent compounds by 1.43-18-fold in concentration and 2-10-fold in phytoplankton toxicity, substantially underestimating true ecological risk. At environmentally relevant concentrations, PSII inhibitors impair photosynthesis at 0.03-1 μg/L in sensitive diatoms, restructuring communities with cascading effects on higher trophic levels and biogeochemical processes. Analysis of 352 globally distributedin situ measurements demonstrates that 11.9% of monitored coastal observations already exceed phytotoxic thresholds derived from phytoplankton toxicity endpoints, confirming that ecologically relevant herbicide exposure is an empirically demonstrable reality. Beyond phytoplankton, herbicides disrupt marine biogeochemical cycling of nitrogen, phosphorus, silica, and trace metals, collectively weakening coastal carbon sequestration by impairing both biological and microbial carbon pumps. Climate change is likely to exacerbate these impacts through increased herbicide influx and synergistic stressor interactions. Our synthesis identifies three interconnected knowledge gaps in monitoring coverage, mechanistic frameworks, and multistressor interactions, urging coordinated monitoring, ecosystem-scale assessments, and source reduction strategies.
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