International Journal of Marine Science, 2024, Vol.14, No.4, 285-294 http://www.aquapublisher.com/index.php/ijms 286 2 Mechanisms of Eutrophication 2.1 Nutrient enrichment sources Nutrient enrichment, primarily from nitrogen (N) and phosphorus (P), is a key driver of eutrophication in coastal marine ecosystems. These nutrients originate from various anthropogenic sources, including agricultural runoff, urban wastewater, industrial discharges, and atmospheric deposition. Agricultural activities contribute significantly to nutrient loading through the use of fertilizers, which are rich in N and P. These nutrients are transported via runoff into water bodies, leading to increased nutrient concentrations in coastal areas (Korpinen and Bonsdorff, 2015; Wurtsbaugh et al., 2019). Urbanization and industrialization also play crucial roles, with sewage and industrial effluents being major sources of nutrient pollution (Zhou et al., 2019; Malone and Newton, 2020). Additionally, atmospheric deposition of nitrogen compounds, resulting from fossil fuel combustion and other industrial processes, further exacerbates nutrient enrichment in coastal waters (Korpinen and Bonsdorff, 2015; Wang et al., 2021). 2.2 Biogeochemical cycles and nutrient dynamics The biogeochemical cycles of nitrogen and phosphorus are central to understanding nutrient dynamics in coastal ecosystems. Nitrogen undergoes various transformations, including nitrification, denitrification, and nitrogen fixation, which influence its availability and impact on eutrophication. Phosphorus, on the other hand, is primarily introduced through direct runoff and is less mobile than nitrogen. The interaction between these nutrients and the biotic and abiotic components of the ecosystem determines the extent and impact of eutrophication (Korpinen and Bonsdorff, 2015). For instance, the presence of nitrogen-fixing cyanobacteria can increase nitrogen availability, while denitrification processes can reduce it. The balance between these processes is crucial in regulating nutrient levels and mitigating eutrophication (Romanelli et al., 2020). 2.3 Role of human activities in accelerating eutrophication Human activities have significantly accelerated the process of eutrophication. The intensification of agriculture, urbanization, and industrialization has led to increased nutrient inputs into coastal ecosystems. The use of artificial fertilizers in agriculture has dramatically increased nutrient runoff, while urban wastewater and industrial discharges have added substantial amounts of nutrients to water bodies (Wurtsbaugh et al., 2019; Zhou et al., 2019; Malone and Newton, 2020). Additionally, climate change exacerbates eutrophication by altering hydrological patterns, increasing water temperatures, and affecting nutrient cycling processes. These changes can enhance nutrient loading and reduce the resilience of ecosystems to eutrophication impacts (Wang et al., 2021; Meerhoff et al., 2022). Effective management and mitigation strategies are essential to address the anthropogenic drivers of eutrophication and protect coastal marine ecosystems from further degradation (Zhou et al., 2019; Malone and Newton, 2020; Wang et al., 2021). 3 Physical and Chemical Processes in Eutrophication 3.1 Dissolved oxygen depletion Dissolved oxygen (DO) depletion is a critical process in eutrophication, primarily driven by the increased microbial respiration that accompanies the decomposition of organic material. This process is exacerbated by the stratification of water columns, which limits the reoxygenation of deeper waters. For instance, in the East China Sea off the Changjiang Estuary, marine-sourced organic matter formed by eutrophication-induced primary production was identified as the dominant oxygen consumer, leading to significant DO depletion (Wang et al., 2016). Similarly, in eutrophic estuaries like western Long Island Sound and Jamaica Bay, high rates of respiration in both surface and bottom waters contribute to persistent hypoxia, with DO concentrations dropping below 2 mg L-1 (Wallace and Gobler, 2021). The impact of DO depletion is profound, leading to the formation of hypoxic zones, often referred to as "dead zones," where oxygen levels are insufficient to support most marine life. This phenomenon has been observed globally, with the number and severity of dead zones increasing due to anthropogenic factors such as nutrient loading and climate change (Altieri and Díaz, 2019). In the Baltic Sea, for example, the extent of anoxic and hypoxic areas has been increasing, with record high anoxic bottom areas observed in recent years (Almroth‐Rosell et al., 2021).
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