International Journal of Marine Science, 2025, Vol.15, No.6, 292-302 http://www.aquapublisher.com/index.php/ijms 298 7 Case Studies 7.1 The Mediterranean Sea: phytoplankton succession and community restructuring under strong phosphorus limitation The Mediterranean Sea, especially the eastern waters, has long been in a state of severe phosphorus limitation. Due to the high input of land-based nitrogen, the nitrogen-phosphorus ratio in seawater far exceeds the Red Field ratio, with phosphorus becoming the main limiting nutrient. Vertical mixing at the end of winter and the beginning of spring brings a small amount of deep phosphorus to the surface, triggering a brief spring algal proliferation. Species such as small diatoms and dinoflagella, which are good at seizing phosphorus pulses, are dominant (Yuan et al., 2018). However, after the surface phosphorus is rapidly depleted, the system enters a state of phosphorus starvation. Large algae decline due to phosphorus deficiency, while small low-phosphorus tolerant species (such as microcyanobacteria, etc.) rise and dominate the summer and autumn communities. Phosphorus deficiency forces local phytoplankton to generally increase phosphatase activity and use DOP to maintain growth (Browning et al., 2017). The limitation of primary production leads to a low productivity of the entire food web. Occasionally, when additional phosphorus is injected into the Eastern Mediterranean by Sahara dust and the like, certain species that are usually restricted (such as nitrogen-fixing cyanobacteria and macrodiatoms) take the opportunity to break out and reshape the community in the short term (Mi et al., 2023). This indicates that in extreme phosphorus-constrained environments, plankton succession is highly sensitive to external nutrient input. 7.2 The South China Sea: impacts of terrestrial inputs and human activities on phosphorus fluxes The South China Sea is a typical marginal sea and cannot do without the "gifts" of the land. Every year, major rivers such as the Pearl River and the Mekong River carry sediment and nutrients into the ocean, spreading a thick layer of nutritional background along the banks. Especially in the Pearl River Estuary area, agricultural runoff and urban sewage discharge have led to a continuous increase in the input of phosphorus year after year, and the load on water bodies has become increasingly heavy. The results are also obvious - frequent algal blooms and hypoxia at the bottom layer, these eutrophication signals have become more prominent in recent years (Carrillo et al., 2015). In contrast, in the open sea, the central part of the South China Sea was originally nutrient-poor. It is precisely these land-based inputs that have made up for some of its shortcomings. However, this kind of "supply" seems to have become unbalanced in recent decades. As the amount of phosphorus discharged by human activities continues to rise, the nitrogen-phosphorus ratio in some sea areas has been pushed up, and some areas that were originally nitrogen-restricted have instead become phosphorus-restricted. Phytoplankton also responded - their phosphorusobtaining enzyme activity was significantly enhanced, showing characteristics adapted to phosphorus-poor environments (Figure 2) (Mi et al., 2023; Jin et al., 2024). From this example, it can be seen that the dual effects of terrestrial materials and human activities are reshaping the phosphorus cycle and nutrient structure in the South China Sea, and also posing new challenges to the stability of coastal ecosystems. 7.3 The North Atlantic: alterations in phosphorus cycling induced by climate warming and atmospheric deposition The North Atlantic has somewhat "changed" in recent years. The impact of warming is first reflected in the intensification of stratification - the mixture of seawater from top to bottom is no longer as thorough, making it difficult for deep phosphorus to be brought up, and thus the supply from the surface is tightened. In winter and spring, water bodies that should be rich in phosphorus become "thin", and primary productivity is also affected accordingly. Meanwhile, a large amount of nitrogen-containing compounds released by human activities are deposited into the North Atlantic through the atmosphere. The input of nitrogen is much higher than that of phosphorus, causing the nitrogen-phosphorus ratio of surface water to rise continuously. Some once typical nitrogen-restricted sea areas have now become phosphorium-scarce zones - there is still a considerable amount of nitrate left, but phosphate is almost depleted. This is precisely the side effect of excessive exogenous nitrogen (Trombetta et al., 2019). This change reminds us that climate warming and atmospheric deposition are not acting independently but are jointly driving the North Atlantic to shift from nitrogen limitation to phosphorus limitation. This case not only reveals the fragile balance of the Marine nutrient structure, but also provides a realistic reference for understanding the coupling effect of global warming and human pollution.
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