Macroalgal blooms caused by marine nutrient changes resulting from human activities.
Macroalgal blooms (green tides) are occurring more frequently in many regions of the world, leading to significant impacts on marine ecology and economies. Although many studies and hypotheses have been proposed, the exact mechanism of green tide formation remains unclear. The world's largest green tides recur in the Yellow Sea and this area is representative for studying the origin and mechanism of green tide formation. We conducted a meta-analysis of studies related to green tides and associated hypotheses for their formation. Rapid industrialization/urbanization and environmental protection actions in coastal zones have led to an increase in nitrate (nitrite) and decline in ammonium (ammonia), and this has resulted in increasing levels of dissolved inorganic nitrogen in seawater. We found that presence of appropriate attachment substrates for Ulva including laver culture rafts and certain hydrological conditions are supplementary factors to green tide formation in the Yellow Sea. Changes in marine nutrient levels promote the production of nitric oxide, which is essential for Ulva sporulation. Consequently, numerous spores are created and attach to substrates including laver culture rafts and parent Ulva thalli. These spores then develop into new thalli that are capable of producing further spores. As a result, Ulva thalli rapidly occupy vast areas of the sea and develop into green tides. Synthesis and applications. Our synthesis identifies that escalating marine nitric oxide generation caused by various factors and its essential role in Ulva sporulation are underlying and significant components in the mechanism of green tide formation. We provide solutions such as the improvement of comprehensive management of aquaculture and strict restriction of marine nitrate pollution, to prevent green tides. Furthermore, we propose an innovative wastewater treatment programme combined with Ulva application to fulfil the standard of sustainable development and circular economy.