Spatially Explicit Inventory of Sources of Nitrogen Inputs to the Yellow Sea, East China Sea, and South China Sea for the Period 1970–2010

Abstract

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...

Biotic and Abiotic Controls on Dinitrogen Production in Coastal Sediments

“…In addition, this sediment denitrification flux is proportionate to ∼7.5% of global marine benthic denitrification (124 Tg N yr −1 [Bianchi et al., 2012]). N inputs from river export, atmospheric deposition, mariculture, and submarine fresh groundwater discharge to the coastal waters of China are estimated to be 10.42 Tg year −1 , with the contribution of river export to total N inputs is in excesses 80% (Wang et al., 2020). Although the potential denitrification flux of sediments near the estuaries of three largest rivers of China was likely underestimated in our study, because we assumed N 2 production rate by denitrification was zero in winter, it was comparable to total N inputs in coastal waters of China.…”

“…Here, we leverage a ∼2,500 km continental shelf sediment transect along coastline in eastern China to explore sedimentary denitrification and anammox processes and their controlling factors (Figure 1). Anthropogenic reactive N inputs in this region have substantially increased over the past 40 years (Wang et al., 2020). A large amount of N in the rivers is delivered to the sea via terrestrial runoff (Liu et al., 2018; Tao et al., 2010), which contributes to the abnormal proliferation of algae and eutrophication of seawaters.…”

“…The Yellow River, Yangtze River, and Pearl river are three major out‐flowing rivers to Bohai Sea, Eastern China Sea, and South China Sea, respectively (Figure 1). Previous studies have analyzed variations of N export rates from rivers to the sea in those regions (Wang et al., 2020), but have not explored the fates of such N, especially gaseous N via sedimentary denitrification.…”

Assessments of Nitrate Budgets in the Yellow Sea Based on a 3D Physical-Biogeochemical Coupled Model

“…Seasonal NO 3 − budgets from the numerical model indicated that the atmospheric flux was much lower than other sources although the flux did not consider tempo-spatial variation. However, recent studies revealed that the contribution of atmospheric deposition to NO 3 − budgets in the YS continues to increase due to anthropogenic activity (Zhang et al, 2007;Luo et al, 2018;Wang et al, 2020). Thus, the impact of a recent increase in the atmospheric deposition on budgets in the YS will be investigated through regular observation and an atmospheric-ocean coupled model.…”

Exploring Seasonal and Annual Nitrogen Transfer and Ecological Response in River‐Coast Continuums Based on Spatially Explicit Models

“…Another N source to the TWS is atmospheric deposition. Atmospheric N deposition to the whole strait during 2001-2010 was estimated as 135.6 ± 12.6 Gg yr −1 based on study by Wang et al (2020), accounting for less than 5.5% of all DIN input to the TWS (see result 3.5); as atmospheric N was relatively unimportant, it was not included in the modeling. Small rivers in the region (both Chinese mainland and Taiwan) with less runoff were not considered as their influence in the TWS is small (Wang et al, 2013).…”