International Journal of Marine Science, 2024, Vol.14, No.4, 285-294 http://www.aquapublisher.com/index.php/ijms 290 Advanced wastewater treatment technologies have been pivotal in reducing nutrient loads from urban areas, but agricultural sources require more complex solutions. Best management practices (BMPs) targeting nonpoint source (NPS) pollution have been implemented in various regions, with mixed results. For instance, BMPs in Roberts Bay, Florida, and Newport Bay, California, significantly reduced nutrient concentrations and harmful algal blooms over approximately 20 years, despite concurrent human population growth (Green et al., 2021). Conversely, in the Peconic Estuary, New York, the dominance of nitrogen inputs from groundwater and atmospheric sources posed greater technical and financial challenges, highlighting the need for targeted approaches based on the dominant nutrient sources (Green et al., 2021). 6.2 Restoration of affected areas Restoration of ecosystems affected by eutrophication involves both immediate and long-term strategies. Immediate actions often include eco-remediation techniques, such as the reduction of phosphorus followed by nitrogen, which has been effective in regions like Shenzhen Bay, China (Zhou et al., 2019). The recovery period for such interventions can be substantial, often taking at least five years to observe significant improvements (Zhou et al., 2019). Long-term restoration requires sustained efforts and monitoring. A global meta-analysis of recovery from eutrophication indicated that lakes and coastal marine areas achieved only 24%~34% of baseline conditions decades after nutrient reduction efforts, suggesting that full recovery may not always be possible (McCrackin et al., 2017; Wang, 2024). This underscores the importance of long-term monitoring and adaptive management to understand recovery trajectories and adjust strategies accordingly (McCrackin et al., 2017). 6.3 Policy and regulatory approaches Policy and regulatory frameworks play a crucial role in managing eutrophication. Effective governance requires the engagement of high-level officials, clear communication of risks and benefits to the public, and binding requirements for nutrient load reductions (Boesch, 2019). Voluntary actions alone are often insufficient; thus, regulations must be enforceable and accompanied by public subsidies based on performance (Boesch, 2019). In Europe, the Marine Strategy Framework Directive (MSFD) has been instrumental in guiding nutrient management. However, projections suggest that proposed nutrient reduction measures may not significantly impact the structure and function of marine ecosystems, indicating the need for more rigorous policies (Piroddi et al., 2021). The Baltic Sea Action Plan is an example of a policy that addresses both nutrient load reductions and climate change impacts, emphasizing the necessity of integrated approaches (Wåhlström et al., 2020). 7 Case Studies of Eutrophication in Coastal Regions 7.1 Examples from developed countries In developed countries, eutrophication has been extensively studied and managed with varying degrees of success. For instance, the Chesapeake Bay in the United States is one of the most impacted coastal ecosystems due to anthropogenic nutrient inputs, leading to severe eutrophication and the development of "dead zones" (Malone and Newton, 2020). Through comparative analysis of seven coastal regions including the Chesapeake Bay, the Baltic Sea, and the Northern Adriatic Sea, the study revealed how nutrient inputs driven by human activities have exacerbated ecological degradation in these areas, particularly the formation of hypoxic zones and the loss of biodiversity (Figure 2). Similarly, the Baltic Sea and the Northern Adriatic Sea have experienced significant eutrophication, resulting in loss of biologically engineered habitats and toxic phytoplankton events (Malone and Newton, 2020). In Europe, the implementation of the Marine Strategy Framework Directive (MSFD) has led to various nutrient management scenarios aimed at reducing eutrophication. However, the impact on higher trophic levels has been minimal, with only slight improvements observed in the Baltic Sea (Piroddi et al., 2021). Additionally, the Mar Menor lagoon in Spain demonstrated a sudden eutrophication break followed by a relatively rapid recovery, highlighting the resilience of some coastal ecosystems (Pérez-Ruzafa et al., 2019).
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