International Journal of Marine Science, 2025, Vol.15, No.5, 255-267 http://www.aquapublisher.com/index.php/ijms 256 marine eutrophication challenges. This study discusses the source, morphological transformation, circulation mechanism, bioavailability and ecological impact of phosphorus, and reviews relevant domestic and foreign research progress in recent years, aiming to provide a scientific basis for a comprehensive understanding of the marine phosphorus circulation and its ecological role. 2 The Main Sources of Marine Phosphorus 2.1 River input and land-source material contribution Rivers are one of the important sources of ocean phosphorus. The phosphorus released by land weathering enters the estuary and nearshore waters through river runoff, and is transported to the ocean in the form of particulate phosphorus and dissolved inorganic phosphorus. Studies have shown that the total phosphorus delivered to the oceans by global rivers each year is about millions of tons, of which about half are settled or buried in the estuaries and continental shelf in the form of particulate phosphorus (Wallmann, 2010), and only partly enters the open ocean. The high sedimentation rate of the estuary and the complex physical and chemical environment cause the conversion of phosphorus during the sea entry: about 30% of the particulate phosphorus and a certain proportion of dissolved phosphorus will be intercepted in the estuary-near-shore area and become part of the phosphorus reservoir of the surface sediment. However, the impact of river input on offshore phosphorus supply varies by region: In high-nutrient estuaries such as the Yangtze River Estuary and the Pearl River Estuary, although the concentration of inorganic phosphorus (PO4 3-) carried by river water is not as good as that of nitrogen salt, it still significantly increases the basal phosphorus level in the estuary and adjacent sea areas (Liu et al., 2022; Lan et al., 2024). Studies have found that the N/P ratio of water bodies in the estuary of the Yangtze River and the Pearl River Estuary is significantly higher, indicating that these areas are mainly restricted by phosphorus. In the estuary, sufficient freshwater phosphorus supply can promote the luxuriance of phytoplankton, but excessive N/P ratio may also trigger changes in the composition of plankton communities and potential ecological risks. 2.2 Atmospheric settlement (dust and phosphorus in aerosols) Atmospheric transport is an important exogenous supply pathway for phosphorus away from land-sourced sea areas. Dust particles and aerosols contain a certain amount of soluble phosphorus, which can provide "airborne phosphorus reduction" to the surface seawater when they settle to the sea surface. It is reported that the phosphorus flux input to the ocean through atmospheric sedimentation worldwide accounts for about 10% of river input every year, but this proportion can reach even higher in ocean oligotrophic areas such as the mid-Atlantic Ocean. Increased soluble atmospheric phosphorus settlement may alleviate the demand for phosphorus by broad oceanic phytoplankton to a certain extent. However, atmospheric sedimentation is often accompanied by the infusion of large amounts of nitrogen nutrients, which will aggravate the excess of nitrogen and phosphorus in the surface seawater, thereby indirectly promoting the utilization of organophosphorus. Peripheral experimental research found that even in the case of relatively lack of phosphorus, the simulated aerosol settlement and nitrogen carried by river water input will further increase the N/P ratio of the water body and strengthen phosphorus restriction, thereby inducing phytoplankton to accelerate secretion and use alkaline phosphatase to decompose dissolved organophosphorus (DOP), making DOP the main source of phosphorus in phytoplankton (Figure 1) (Wang et al., 2022; Jin et al., 2024). 2.3 Seabed geological processes and phosphorus recharge from volcanic eruptions In addition to land source input, geological processes inside the ocean are also one of the important sources of phosphorus. Subsea volcanic eruptions, hydrothermal activity and geological tectonic movements at continental edges can release phosphorus-containing fluids or promote phosphorus redissolution in sediments, replenishing nutrients to the upper water (Sai and Kakegawa, 2019; Rasmussen et al., 2021). At the hydrothermal vent of the Zhongyang ridge, although the concentration of phosphorus is not high, the hydrothermal fluid can carry certain reducing chemicals, which converts the phosphorus form in the local environment and releases usable phosphorus. Other coastal submarine groundwater leakage (SGD) will also bring phosphorus from land seepage into nearshore waters. Research shows that the dissolved phosphorus flux carried by SGD globally can be comparable to river flux in some areas, especially in densely populated and nutrient-rich coastal areas (Figure 2) (Kreuzburg et al.,
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