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As China's only continental sea, the Bohai Sea is a relatively closed environment and is vulnerable to natural changes and human activities. In this paper, the long-term variations in nutrients in the Bohai Sea and the potential influencing factors were analyzed based on historical summer and winter data from 1978 to 2016. The results showed that the concentrations of dissolved inorganic nitrogen (DIN) in the Bohai Sea continuously increased from 1990 and rapidly increased after 2002, the phosphate (PO 4 -P) concentration exhibited a decreasing trend, and the silicate (SiO 3 -Si) concentration decreased from 1978 to 1987 and increased from 1987 to 2008. The nutrient concentrations were lower in summer than in winter, and the bottom concentrations were higher than the surface concentrations in summer, whereas the vertical differences were insignificant in winter. The dominant factor determining the long-term variations in DIN were atmospheric deposition and nonpoint sources due to various human activities; the primary factors affecting PO 4 -P were riverine inputs and nonpoint sources related to natural changes and human activities; the primary factors afftecting SiO 3 -Si were riverine inputs. The N/P ratio followed the DIN variation, the Si/P ratio followed the SiO 3 -Si variation, and the Si/N ratio decreased. The nutrient ratios were lower at the bottom than at the surface and were lower in winter than in summer. The nutrient limitation changed from nitrogen limitation to phosphorus and silicon limitations. The long-term nutrient variations in the Bohai Sea have potential ecological impacts on the local red tide features.Plain Language Summary Nutrients are important to primary productivity and thus important to marine ecosystems. Nutrient variations in coastal oceans may potentially impact the occurrence of ecological disasters. As the only continental sea in China, the Bohai Sea is a relatively closed environment and is vulnerable to natural and anthropogenic impacts. We found that the concentration of dissolved inorganic nitrogen in the Bohai Sea increased from 1990 to 2006; the phosphate concentration decreased from 1978 to 2016, and the silicate concentration decreased before 1987 and increased after 1987. We also found that N/P increased and Si/N decreased. The nutrient concentrations were higher, whereas the nutrient ratios were lower in winter than in summer. In summer, the bottom nutrient concentrations were higher than the surface concentrations, whereas the bottom ratios were lower than the surface ratios; in winter, the vertical differences were insignificant. The primary factors driving the silicate variations were riverine inputs related to natural changes and human activities; the primary factors in the phosphate variations could be riverine inputs and nonpoint sources related to natural changes and human activities; the dominant factor in the dissolved inorganic nitrogen variations could be atmospheric deposition and nonpoint sources due to various human activities. The red tide features in the...

Under the combined impacts of natural changes and human activities, the past, current, and future marine heatwaves (MHWs) in China's marginal seas and adjacent offshore waters (CMSOW) need a comprehensive understanding. This study provides a systematic analysis of the spatiotemporal variations using daily sea surface temperature data and simulates the future trend using 12 climate models. During 1982–2018, the mean annual total days, duration, frequency, and mean intensity of the MHWs in the CMSOW increased by 20–30 days/decade, 5–9 days/decade, 1–2 decade−1, and 0.1–0.3°C/decade, respectively (p <0.01). The maximum sea surface temperature anomalies in the Bohai Sea was over 6–8°C, and the MHW's frequency, duration, and mean intensity were higher than twice the global average, which could have impacted fishery resources and occurrence of harmful algal blooms. The variations of the MHWs in the CMSOW result from the robust ocean surface warming, which is caused by increased solar radiation due to reduced cloud cover, reduced ocean heat loss from weaker wind speed, weakening but warmer Kuroshio, and strong El Niño. In the future, the areas with longer total days and duration will increase; the spatial pattern of frequency has a negative relationship with that of duration while that of mean intensity is mostly unchanged. Year 2040 is a key node for the future changes of MHW under different Representative Concentration Pathways. The trend of total days increases from fast to slow, and frequency shows an opposite trend; the duration and mean intensity rise faster after 2040.

As Chinese aquaculture production accounts for over half of the global aquaculture production and has increased by 50% since 2006, there is growing concern about eutrophication caused by aquaculture in China. This paper presents a model-based estimate of nutrient flows in China’s aquaculture system during 2006–2017 using provincial scale data, to spatially distribute nutrient loads with a 0.5° resolution. The results indicate that with the increase in fish and shellfish production from 30 to 47 million tonnes (Mt) during 2006–2017, the nitrogen (N) release increased from 1.0 to 1.6 Mt/year and that of phosphorus (P) from 0.1 to 0.2 Mt/year. Nutrient release from freshwater aquaculture was concentrated in Guangdong, Jiangsu, and Hubei, and that from mariculture in Shandong, Fujian, and Guangdong. Aquaculture is an important strongly concentrated nutrient source in both freshwater and marine environments. Its nutrient release is >20% of total nutrient inputs to freshwater environments in some provinces, and nutrients from mariculture are comparable to river nutrient export to Chinese coastal seas. Aquaculture production and nutrient excretions are now comparable to those of livestock production systems in China and need to be accounted for when analyzing causes of eutrophication and harmful algal blooms and possible mitigation strategies.

Symptoms of eutrophication (including biodiversity loss, harmful algal blooms, and hypoxia) are an increasing problem in Chinese seas. Nutrient enrichment is primarily caused by accelerated human activities that cause nutrient pollution of the aquatic environment. In this study, the Integrated Model to Assess the Global Environment-Global Nutrient Model (IMAGE-GNM) was used to estimate nitrogen inputs from river discharge, submarine fresh groundwater discharge, and mariculture, and TM5-FAst Scenario Screening Tool (TM5-FASST) for atmospheric nitrogen deposition to the three Large Marine Ecosystems (LMEs, i.e., Yellow Sea/Bohai Sea, YS/BS; East China Sea, ECS; South China Sea, SCS) bordered by China and several other countries for the period 1970-2010. China's river nitrogen export was the largest nitrogen source in YS/BS and ECS. In SCS, however, China and other countries contributed equally and although decreasing, the proportion of natural sources remain considerable. The total nitrogen inputs to YS/BS (1.0 to 4.1 Tg year −1), ECS (1.3 to 5.5 Tg year −1), and SCS (2.1 to 5.8 Tg year −1) increased rapidly during 1970-2010. River export is dominated by agriculture; nitrogen inputs from atmospheric deposition and mariculture have been increasing rapidly in recent years. Considering only the coastal zone of the three LMEs, our results show that the total nitrogen inputs are strongly concentrated spatially in areas close to river mouths and those confined regions with mariculture production. To sustain the food production and economic growth in the coming decades, nitrogen inputs may increase further, depending on future eutrophication mitigation policies. Plain Language Summary Nitrogen is a limiting nutrient for plant production. Excessive nitrogen use in agriculture and discharge from wastewater is the primary causes of eutrophication in aquatic environments. Symptoms of eutrophication (including biodiversity loss, harmful algal blooms (HABs), and hypoxia) are an increasing problem in Chinese seas. Here we quantified the nitrogen inputs from river export, atmospheric deposition, submarine fresh groundwater discharge, and mariculture to the Yellow Sea/Bohai Sea (YS/BS), East China Sea (ECS), and South China Sea (SCS) bordered by China and other countries for the period 1970-2010. Nitrogen inputs increased rapidly, mainly due to increasing land-based sources (river export and atmospheric deposition), while nitrogen from mariculture started to increase recently. River export is dominated by agriculture with growing proportions of sewage and freshwater aquaculture. Nitrogen inputs were spatially concentrated and increased faster in Chinese coastal waters than in other countries. China's contribution to nitrogen pollution exceeded that of other countries in YS/BS and ECS, while China and other countries contributed equally in SCS. Nitrogen concentrations and HAB frequency in the seas seem to be correlated as both increased substantially since 1970s. Mitigation of nitrogen pollution is therefore urgent because human ac...

The three large marine ecosystems (LMEs) bordering China (Yellow Sea/Bohai Sea, East China Sea, and South China Sea) have received excess nitrogen (N) and phosphorus (P) in the past decades with detrimental consequences for ecosystem functioning, such as increased productivity, loss of biodiversity, and proliferation of harmful algal blooms (HABs). N loading increased much faster than that of P. Here, we show that HABs in the three LMEs started to proliferate after the N:P molar ratio exceeded the threshold value of 25 in the 1980s. The mismatch of N and P inputs is not only related to differences in loads but also inherent to the differences in their biogeochemical cycles which more efficiently filter P than N in land- and waterscapes. Future Shared Socioeconomic Pathways show that high N:P ratios will persist for decades to come, even worsening in a future oriented toward sustainability, and indicate that HABs may be a persisting problem in China’s coastal waters. While efforts in agricultural systems are governed by the agronomic crop requirements and are not easy to manage with respect to N:P ratios, the separate collection of urine in urban and rural areas could contribute to decreases in both total nutrient loads and N:P ratios.

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Rivers play an important role in the global carbon (C) cycle. However, it remains unknown how long-term river C fluxes change because of climate, land-use, and other environmental changes. Here, we investigated the spatiotemporal variations in global freshwater C cycling in the 20th century using the mechanistic IMAGE-Dynamic Global Nutrient Model extended with the Dynamic In-Stream Chemistry Carbon module (DISC-CARBON) that couples river basin hydrology, environmental conditions, and C delivery with C flows from headwaters to mouths. The results show heterogeneous spatial distribution of dissolved inorganic carbon (DIC) concentrations in global inland waters with the lowest concentrations in the tropics and highest concentrations in the Arctic and semiarid and arid regions. Dissolved organic carbon (DOC) concentrations are less than 10 mg C/L in most global inland waters and are generally high in high-latitude basins. Increasing global C inputs, burial, and CO 2 emissions reported in the literature are confirmed by DISC-CARBON. Global river C export to oceans has been stable around 0.9 Pg yr –1 . The long-term changes and spatial patterns of concentrations and fluxes of different C forms in the global river network unfold the combined influence of the lithology, climate, and hydrology of river basins, terrestrial and biological C sources, in-stream C transformations, and human interferences such as damming.