International Journal of Marine Science, 2025, Vol.15, No.5, 255-267 http://www.aquapublisher.com/index.php/ijms 263 plankton and microbial rings dominate, and more carbon remains on the surface in the form of dissolved organic carbon or circulates through micro food rings, and less sinking into the deep sea through large particles. This reduces the efficiency of the biopump. Phosphorus restriction also has an indirect effect on carbon circulation by promoting the prosperity of nitrogen-fixing cyanobacteria and changing the system's carbon-nitrogen coupling. Nitrogen-fixing cyanobacteria (such as wool) generally has a C:P ratio above the average. When it is present in large quantities, it may change the local organic matter C:P, thereby affecting the organic carbon degradation rate and burial efficiency. 6.4 Case analysis: the relationship between red tide explosion in the East China Sea and phosphorus input China's East China Sea coast, especially the Yangtze River Estuary - Hangzhou Bay, is one of the frequent red tides in the world, and its red tide evolution is closely related to changes in nutrient input. The Yangtze River has transported a large amount of freshwater and nutrients to the East China Sea. In recent decades, nitrogen and phosphorus emissions in the Yangtze River Basin have increased significantly and have changed the nutrient structure near the Yangtze River estuary. Under the phosphorus-restricted environment, dinoflagellates that are more suitable for low phosphorus are likely to increase significantly, which triggers a red tide of dinoflagellate. In addition to the nutrient ratio, the increase in total phosphorus concentration itself will also promote the occurrence of red tides. When the discharge of phosphorus fertilizer and domestic sewage in the basin increases, the phosphorus load entering the estuary increases, which can directly stimulate the extremely rapid growth of phytoplankton (Fang et al., 2025). The study at the Pearl River Estuary pointed out that the increase in phosphorus concentration is positively correlated with the outbreak of local dinoflagellate and algae blooms. 7 Ecological and Environmental Impacts of Phosphorus 7.1 Coupling effect between phosphorus and marine food network The supply and circulation of phosphorus have an important impact on all trophic levels of the marine food network. The phosphorus condition of primary producers determines the quality and quantity of baits at secondary consumers and even higher nutritional grades. Phosphorus is an element necessary for nucleic acid and energy metabolism in zooplankton. If the food is relatively lacking in phosphorus, zooplankton will rebalance by increasing the excretion of carbon or nitrogen and retaining phosphorus, but it may still lead to growth restriction or decreased fecundity. Therefore, in low-phosphorus ecosystems, the population density of zooplankton is often low, or mainly small species that are good at low nutritional conditions, which in turn affects the supply of bait for higher trophic grades such as fish. This can partly explain why some oligotrophic ocean-sea fish yields are low because the phosphorus nutrients at the bottom of the food web restrict the transmission layer by layer, resulting in low productivity in the entire system (White and Dyhrman, 2013). The distribution of phosphorus affects the structure of the food web and the energy flow direction. In an environment where phosphorus is extremely lacking, small microorganisms are dominant, the trophic level may be extended and the energy transfer efficiency is reduced. In the case of sufficient phosphorus, the nutritional structure is simpler and direct. Large herbivorous zooplankton directly feeds on large algae, and the energy is transmitted to higher orders more quickly. 7.2 Phosphorus circulation disorder and the formation of marine hypoxic zones When the ocean phosphorus circulation is disordered, such as excessive input of exogenous phosphorus causes eutrophication or large-scale release of endogenous phosphorus changes the nutrient structure, it often leads to the formation or expansion of marine hypoxic zones. Mechanistically, this involves the amplification of phosphorus on primary production and organic carbon sedimentation and decomposition of oxygen consumption processes. Coastal eutrophication is a typical example. Human activities allow rivers to transport excessive nitrogen and phosphorus to semi-enclosed sea areas such as bays and estuaries, prompting explosive growth of phytoplankton, and then a large amount of organic matter sinks into the bottom layer. In the process of bacteria decomposing these organic matter, dissolved oxygen in the water is consumed, and in severe cases, large areas of the underlying layer are hypoxic or even anaerobic environment (Tsandev, 2010). Phosphorus plays a "promoter" role in this process: if only nitrogen increases but phosphorus does not increase, the growth of algae will be limited by phosphorus; but when nitrogen and phosphorus increase together, the algae proliferation will reach a higher level,
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