International Journal of Marine Science, 2025, Vol.15, No.6, 292-302 http://www.aquapublisher.com/index.php/ijms 293 differences are obvious. In some areas, phosphorus is almost depleted, while in others, it is enriched due to geological or hydrological conditions (Zhang et al., 2025). The research on the bioavailability of Marine phosphorus has long trantioned academic discussions in significance. As a key limiting element, phosphorus almost determines the upper limit of primary productivity and also shapes the structure of ecosystems. Clarifying the utilization methods and control mechanisms of different forms of phosphorus can not only explain the community turnover of phytoplankton but also help understand the changes in Marine carbon sinks. In reality, human influence has long permeated this cycle. The use of chemical fertilizers and the discharge of sewage have led to a continuous increase in the phosphorus load of coastal water bodies, causing eutrophication and even ecological disorders (Hao et al., 2025); Meanwhile, atmospheric deposition and climate change are rewriting the phosphorus input pattern in the distant-water areas, causing some sea areas that were originally nitrogen-restricted to gradually shift towards phosphorus-restricted. For this reason, the research on the bioavailability of phosphorus is not only related to theoretical improvement, but also to Marine management and climate response. 2 Chemical Forms and Distribution Characteristics of Phosphorus in the Ocean 2.1 Main components and transformations of inorganic and organic phosphorus Phosphorus in the ocean can be divided into two major categories: inorganic phosphorus and organic phosphorus. Inorganic phosphorus mainly refers to dissolved inorganic phosphorus (DIP, that is, phosphate ions), which can be directly absorbed and utilized by phytoplankton and microorganisms. Organophosphorus includes dissolved organophosphorus (DOP) and granular organophosphorus (POP), which exist in organic molecules and decomposition products in organisms. Generally, it needs to be converted into inorganic phosphorus through enzymatic hydrolysis before it can be assimilated (Murphy et al., 2021). Marine organisms absorb DIP and construct phosphorus into their own organic matter. When organisms die or excrete, DOP and POP are produced. After microbial decomposition, phosphate is released again, achieving the re-conversion of organic phosphorus to inorganic phosphorus (Jin et al., 2024). In addition, under specific conditions, phosphates in the environment can combine with metal ions and precipitate into granular inorganic phosphorus (such as apatite), which then settles to the seabed. 2.2 Vertical distribution and spatiotemporal variation patterns of phosphorus in seawater The vertical distribution of phosphorus in Marine water bodies shows a typical feature of being low in the surface layer and high in the deep layer: the phosphorus concentration in the surface layer is often close to depletion due to the rapid absorption by phytoplankton. In the middle and deep layers, due to the decomposition of settled organic particles, inorganic phosphates are released, making the deep seawater rich in phosphorus (Duhamel, 2024). During seasonal changes, vertical mixing of seawater in winter can bring deep phosphorus to the surface layer, temporarily increasing the phosphorus content in the surface layer. In spring and summer, phytoplankton multiply vigorously and consume a large amount of phosphorus, causing the surface layer to become scarce again. In terms of spatial scale, there are significant differences in phosphorus levels among different sea areas: The surface phosphorus concentration is relatively high in coastal and estuarine areas due to river input and upwelling (Brady et al., 2022); On the contrary, in closed waters such as the subtropical circulation center of the open sea, the surface phosphorus level is extremely low throughout the year. Due to the limitations of light or iron in high-latitude seas, phytoplankton do not fully utilize phosphorus, and a certain amount of phosphate is often retained in the surface layer. 2.3 Differences in phosphorus dynamics among various ecological zones There are obvious differences in the dynamics of phosphorus cycling in different ecological regions of the ocean. Coastal and estuarine areas are strongly influenced by terrigenous substances, and the phosphorus concentration in water bodies fluctuates with the seasons and human activities: During the wet season, river runoff carries a large amount of inorganic phosphorus and particulate phosphorus, leading to a sharp increase in local phosphorus content. The input decreased during the dry season and the phosphorus concentration dropped (Zhang et al., 2025). In contrast, in oligotrophic sea areas such as oceanic circulation centers, phosphorus supply is chronically scarce and changes gently. Ecosystems can only rely on mechanisms such as microbial loops to repeatedly recycle trace
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