Natural and human-caused hypoxia occur when oxygen levels in water masses drop below 2 mg L⁻¹ (approximately 30% saturation). Natural processes like photosynthesis and microbial respiration, along with anthropogenic factors such as nutrient loading, contribute to hypoxia. Human activities, especially increased nutrient inputs from agriculture and urbanization, have exacerbated hypoxia in coastal areas. Global climate change further complicates hypoxia by increasing stratification and altering ocean circulation, which can worsen existing hypoxic conditions or create new ones. Hypoxia has occurred naturally in oxygen minimum zones, deep basins, upwelling systems, and fjords for geological time scales. However, human-induced hypoxia is now widespread, with over 400 documented areas in the world's coastal oceans. The consequences of hypoxia include habitat loss, reduced biodiversity, and impacts on fisheries. While hypoxia in natural systems is long-term and stable, human-induced hypoxia is more recent and often linked to eutrophication. Efforts to reduce nutrient loads can mitigate hypoxia, but global warming and climate change are expected to increase the extent and impact of hypoxia. Oxygen minimum zones (OMZs) are large, persistent areas of low oxygen in the ocean, often found in the eastern Pacific and other regions. OMZs are influenced by factors such as surface productivity, water mass age, and circulation. Upwelling systems, which bring nutrient-rich deep water to the surface, can lead to hypoxia in coastal areas. These systems are among the most productive marine ecosystems but can also contribute to hypoxia due to increased organic matter sinking and decomposing. The interaction between OMZs and upwelling systems can create intense shelf hypoxia. Human activities, such as nutrient loading from agriculture and urbanization, have significantly increased the occurrence and severity of hypoxia in coastal areas. The effects of hypoxia on marine ecosystems are significant, including reduced fish populations, altered trophic interactions, and changes in biogeochemical cycles. Efforts to reduce nutrient inputs and manage coastal systems are essential to mitigate the impacts of hypoxia.Natural and human-caused hypoxia occur when oxygen levels in water masses drop below 2 mg L⁻¹ (approximately 30% saturation). Natural processes like photosynthesis and microbial respiration, along with anthropogenic factors such as nutrient loading, contribute to hypoxia. Human activities, especially increased nutrient inputs from agriculture and urbanization, have exacerbated hypoxia in coastal areas. Global climate change further complicates hypoxia by increasing stratification and altering ocean circulation, which can worsen existing hypoxic conditions or create new ones. Hypoxia has occurred naturally in oxygen minimum zones, deep basins, upwelling systems, and fjords for geological time scales. However, human-induced hypoxia is now widespread, with over 400 documented areas in the world's coastal oceans. The consequences of hypoxia include habitat loss, reduced biodiversity, and impacts on fisheries. While hypoxia in natural systems is long-term and stable, human-induced hypoxia is more recent and often linked to eutrophication. Efforts to reduce nutrient loads can mitigate hypoxia, but global warming and climate change are expected to increase the extent and impact of hypoxia. Oxygen minimum zones (OMZs) are large, persistent areas of low oxygen in the ocean, often found in the eastern Pacific and other regions. OMZs are influenced by factors such as surface productivity, water mass age, and circulation. Upwelling systems, which bring nutrient-rich deep water to the surface, can lead to hypoxia in coastal areas. These systems are among the most productive marine ecosystems but can also contribute to hypoxia due to increased organic matter sinking and decomposing. The interaction between OMZs and upwelling systems can create intense shelf hypoxia. Human activities, such as nutrient loading from agriculture and urbanization, have significantly increased the occurrence and severity of hypoxia in coastal areas. The effects of hypoxia on marine ecosystems are significant, including reduced fish populations, altered trophic interactions, and changes in biogeochemical cycles. Efforts to reduce nutrient inputs and manage coastal systems are essential to mitigate the impacts of hypoxia.