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Map showing population density for each administrative area in Japan and MOE’s monitoring sites (yellow squares). Dark gray means population density is less than 500 persons/km², and light gray shows population density is more than 500 persons/km². The population density was based on October 2020 (Prefecture/City ,2020; https://uub.jp/rnk/c_j.html)
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Nutrient load reduction is widely used to improve coastal water quality, but it can lead to oligotrophication. This paper evaluates the current status of river water origin and the water recharge system based on isotope values and dissolved compositions recorded in 2018, and it also assesses the impact of nutrient load reduction efforts on river nu...
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Nutrient targets based on pressure-response models are essential for defining ambitions and managing eutrophication. However, the scale of biogeographical variation in these pressure-response relationships is poorly understood, which may hinder eutrophication management in regions where lake ecology is less intensively studied. In this study, we de...
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... Over the last 20 years, ZJB has mostly experienced DIN: DIP ratios below 16, indicating severe nitrogen limitation, which is consistent with China's coastal mariculture zones (DIN: DIP of approximately 7-11) (Feng et al., 2024). However, this contrasts with the long-term P-limited state in nearshore areas (Figure 8), such as the South Yellow Sea , Pearl River Estuary (Yao et al., 2022), Chinese rivers (Feng et al., 2024), and offshore central Japan (Katazakai and Zhang, 2021). These coastal waters receive a substantial amount of nutrients from land inputs. ...
... The colored dots are the seasonal investigation data, and the gray dots during 1998-2005 are yearly averaged values from references shown in Table 1. The colored lines represent DIN: DIP variations in different worldwide aquatic environments, where the red line indicates the main variation trend in ZJB; the Chinese river and coastal mariculture data are from Feng et al. (2024), the Pearl River Estuary close to Hongkong coastal area data are from Yao et al. (2022), the South Yellow Sea data are from Chen et al. (2023), the data of Central Japan are from Katazakai and Zhang (2021), and the data of the open ocean including the Pacific Ocean and India Ocean are from Olsen et al. (2016). ...
Coastal eutrophication is a major issue of marine pollution. The main factors controlling eutrophication must be identified to ensure effective marine environmental management according to the respective local conditions. Zhanjiang Bay (ZJB), located northwest of the South China Sea, is a semi-closed bay influenced by complex water flows and the development of surrounding cities. In this study, we investigated the development of nutrient concentrations and compositions in ZJB seawater over the past 20 years and the factors influencing eutrophication based on several field investigations from 2006 to 2022 and historical data. High concentrations of dissolved inorganic phosphorus (DIP) and dissolved inorganic nitrogen (DIN) were the main contributors to the severe long-term eutrophication in ZJB; however, light eutrophication was observed in the outer bay, primarily caused by chemical oxygen demand (COD) and DIP. The primary sources of COD and nutrients were riverine freshwater, sewage outfalls, mariculture and domestic effluents carried by rivers. Tidal effects diluted the nutrient concentrations in the bay with seawater from the outer bay, thereby playing a key role in nutrient redistribution. The DIN: DIP ratio of ZJB showed long-term nitrogen restriction and excess phosphorus, primarily owing to mariculture activities. Marine undertakings can exert various impacts on water quality. Eliminating illegal aquaculture and launching aquaculture tailwater treatment can improve water quality, whereas practices such as channel dredging may worsen it. This study demonstrates the intricate dynamics of the ZJB ecosystem and offers valuable insights for effective environmental management and conservation efforts.
... Restoration efforts have therefore focused on the reduction of watershed nutrient loads, including in the Chesapeake Bay (Linker et al., 2013a;Shenk and Linker, 2013), Great Lakes (Stow et al., 2020;Scavia et al., 2023), northern Gulf of Mexico (Rabalais et al., 2002;Turner et al., 2008), and Baltic Sea (Zillén et al., 2008;Meier et al., 2018). Indeed, positive responses of ecosystem conditions to nutrient load reductions have been observed in various ecosystems, including the Chesapeake Bay (Lefcheck et al., 2018;Murphy et al., 2022), Toyama Bay (Katazakai and Zhang, 2021), Bohai Sea (Xiang et al., 2023), and Danish coastal waters (Riemann et al., 2015). ...
Eutrophication has been a major environmental issue in many coastal and inland ecosystems, which is primarily attributed to excessive anthropogenic inputs of nutrients. Restoration efforts have therefore focused on the reduction of watershed nutrient loads, including in the Chesapeake Bay (USA). To facilitate watershed management, watershed models are often developed and used to assess the expected impact of scenarios of past and future management policies and practices and the impact of watershed conditions. However, the level of load reductions estimated using monitoring data often does not match with model predictions, which may cast doubt on the effectiveness of the restoration efforts, the reliability of the model, and the prospect of achieving pre-established reduction goals. To better reconcile such inconsistencies between expectation (i.e., modeling estimates) and reality (i.e., monitoring information), a watershed-wide indicator was developed for the Chesapeake Bay watershed to explicitly quantify the progress toward nutrient reduction goals in the context of the Chesapeake Bay Total Maximum Daily Load (TMDL). Results of the indicator show that since 1995 long-term progress has been made toward the TMDL planning targets for both nitrogen and phosphorus. Specifically, management practices that are implemented and realized (in monitoring data) have been increasing over time, whereas management practices that need to be implemented in the future to meet the goals have been decreasing. In addition, the progress of nutrient reduction toward meeting the goals has varied with source sectors and watershed locations: i.e., point source management has been fully or nearly fully implemented, whereas nonpoint source management has been implemented by 50%-70%. In summary, this indicator, which is largely based on monitoring data, can provide at least four benefits: (1) evaluating the validity of the modeled estimates of nutrient reductions by comparing them to monitoring information; (2) placing the monitored riverine trends into a management context; (3) comparing progress between different nutrient source sectors and watershed locations; and (4) facilitating communication of the progress to the Chesapeake Bay Program Partnership and the public. Although we focus on the indicator development and interpretation for the Chesapeake Bay watershed, the framework can be transferred to watersheds within and beyond this watershed, where similar modeling and monitoring information exists, to gauge expectations on the trajectory and pace of the progress toward meeting restoration goals.
... Although several countries have implemented water pollution treatment strategies to reduce the N and P concentration in marine sewage discharge to cope with ocean eutrophication, current sewage treatment technologies are more effective at treating P and have unexpectedly resulted in increased N:P ratios in water (Tong et al., 2020), making nutrient conditions in marine ecosystems more inclined towards P limitation. Furthermore, even though the control of river N and P pollution has achieved positive results, P is controlled more effectively relative to N but high inlet N:P ratios have led to lower marine primary productivity and reduced fishery harvests, which have subsequently impacted local economic development (Katazakai and Zhang, 2021). Although the effects of eutrophication on the components and structure of phytoplankton have been documented (Bužančić et al., 2016;Suikkanen et al., 2013;Van Meerssche et al., 2019), whether persistent disturbances in the marine physico-chemical environment would cause phytoplankton community stoichiometry to deviate from the classical Redfield ratio has not been adequately addressed. ...
... Recent studies have found that the N:P ratio in discharged sewage was also significantly higher (Tong et al., 2020). It is likely that over the past 20 years, to cope with eutrophication, managers have taken effective measures to control N and P inputs in sewage discharge (Andersen et al., 2010;Katazakai and Zhang, 2021). However, the current sewage treatment technology is more effective in treating P than N, which has resulted in higher DIN concentration, relative to SRP, in sewage discharge and thus a higher N:P ratio (Tong et al., 2020;Katazakai and Zhang, 2021). ...
... It is likely that over the past 20 years, to cope with eutrophication, managers have taken effective measures to control N and P inputs in sewage discharge (Andersen et al., 2010;Katazakai and Zhang, 2021). However, the current sewage treatment technology is more effective in treating P than N, which has resulted in higher DIN concentration, relative to SRP, in sewage discharge and thus a higher N:P ratio (Tong et al., 2020;Katazakai and Zhang, 2021). This investigation found that the molar ratio of TN:TP in the direct sea discharge from the sewage treatment plant around Xiamen Bay (Fig. 7) ranged from 130 to 565 (average, 277), far exceeded 16:1 (i. ...
Anthropogenic activities dramatically alter biogeochemical cycling and stoichiometry of carbon (C), nitrogen (N), and phosphorus (P) within coastal marine ecosystems. Long-term nutrient input has led to significant deviation of the C, N and P stoichiometry of phytoplankton (C167N28P1) and surface seawater (C1299N24P1) from the Redfield ratio (C106N16P1), and overall P-limitation in Xiamen Bay. The bio-P in phytoplankton was spatially consistent with that of inorganic P in seawater; and the phytoplankton bio-C:N:P shifted towards the same trend as the seawater under prolonged nutrient stress, as one of the cascade effects of the imbalanced seawater elemental stoichiometry on the species composition and abundance, and in turn on bio-elemental stoichiometry, of phytoplankton community. It is suggested that the significant deviations in elemental stoichiometry of primary producers might impair their capacity of nutrient supply for organisms at higher trophic-levels of the food chains in the local coastal ecosystem and thus damage its health.
... In the inner Oslofjorden (Norway), declined P concentration has contributed to the decrease in chlorophyll-a from 1980s to 1990s (Lundsør et al., 2020). In the Toyama Bay (Japan), 25 years of reduction of P load, especially from WWTPs, has led to significant reductions of riverine loads and enhanced P limitation in coastal waters (Katazakai and Zhang, 2021). ...
Many coastal ecosystems suffer from eutrophication, algal blooms, and dead zones due to excessive anthropogenic inputs of nitrogen (N) and phosphorus (P). This has led to regional restoration efforts that focus on managing watershed loads of N and P. In Chesapeake Bay, the largest estuary in the United States, dual nutrient reductions of N and P have been pursued since the 1980s. However, it remains unclear whether nutrient limitation – an indicator of restriction of algal growth by supplies of N and P – has changed in the tributaries of Chesapeake Bay following decades of reduction efforts. Toward that end, we analyzed historical data from nutrient-addition bioassay experiments and data from the Chesapeake Bay Long-term Water Quality Monitoring Program for six stations in three tidal tributaries (i.e., Patuxent, Potomac, and Choptank Rivers). Classification and regression tree (CART) models were developed using concurrent collections of water-quality parameters for each bioassay monitoring location during 1990-2003, which satisfactorily predicted the bioassay-based measures of nutrient limitation (classification accuracy = 96%). Predictions from the CART models using water-quality monitoring data showed enhanced nutrient limitation over the period of 1985-2020 at four of the six stations, including the downstream station in each of these three tidal tributaries. These results indicate detectable, long-term water-quality improvements in the tidal tributaries. Overall, this research provides a new analytical tool for detecting signs of ecosystem recovery following nutrient reductions. More broadly, the approach can be adapted to other waterbodies with long-term bioassays and water-quality data sets to detect ecosystem recovery.
... Recent results of terrestrial-derived nutrient fluxes to Toyama Bay have demonstrated that anthropogenic nutrient loads have declined in the last 25 years because of the increased adoption of wastewater treatment facilities. 61 Therefore, we can infer that riverine nutrient loads in the study area have been affected by two factors: climate change and reduced anthropogenic nutrient loading (i.e., increased wastewater treatment capacity), although it is difficult to assess each effect quantitatively. As a result, our data show that the total nutrient fluxes in the FSGDS and rivers have roughly halved from the past. ...
... 70 N/P ratios in the coastal waters of this study area have continuously increased, indicating that the coast of Toyama Bay has experienced a worsening phosphorus limitation. 61 In this region, carbon consumption per kg of water for nutrients supplied by terrestrial water (FSGDS and river runoff) was estimated to be 908 Figure 5). The amounts have nearly doubled at most in 20 years. ...
