| Anthropogenic eutrophication has been observed in many coastal zones for more than 50 years.In the China seas,negative events from the coastal eutrophication increased rapidly from the end of the 1970s,but have been alleviated in the recent decade with unknown cause.The coastal eutrophication is usually associated with enhanced nitrogen(N)levels in those systems.But so far,a key mechanism of the coastal N-enrichment is still unclear.On this issue,the main viewpoint is that the riverine transport plays a dominant role in the enhanced N-loads of coastal seas,and other sources,such as submarine groundwater discharges,atmospheric deposition,sewages and aquacultures,further enrich the coastal N-loads.This theoretical framework is agreed well with frequent eutrophic events in river plumes,but is fail to quantify enhanced N-loads in the continental shelf seas(distant from river plumes).For example,the eutrophic events increased in the central Bohai Sea in the 2000s,while the riverine N-discharges have been stabilized.And the Yellow Sea showed an enhanced DIN-level from the 1980s,but where is around limited rivers.Another perspective is needed for the N-enrichment in shelf seas.Aiming at the global environment problem of coastal eutrophication,this study examined dissolved inorganic nutrients in the Bohai and Yellow seas where the eutrophic events increased dramatically in the recent 40 years.Temporal and spatial variations of the nutrient structures in the two seas,as well as their controls,have been analyzed and discussed to clarify the major N source in the coastal systems.Contribution of marine N to the coastal eutrophication has been estimated.A wintertime nutrient budget in the central Bohai Sea has been established in this study.Based on historical datasets,the central Bohai Sea wintertime nutrients during the period of 1985-2018 have been budgets to inquire reasons of the enhanced N-loads in the study region.Finally,the central Bohai Sea nitrogen budget in 2019 has compared with that in 2016,suggusting that the atmospheric nitrogen manages in China after 2013 bring about a recent alleviation in the Bohai Sea eutrophication.In this study,raw data from eight field surveys in the Bohai and Yellow seas,spanning the four seasons,have been analyzed.During these surveys,excess N(i.e.,N*=DIN-16×DIP)of more than 5?mol kg-1 occurred only in the nearshore areas nearby the Yellow River mouth,the Yangtze River mouth and the Jiangsu coast.Distant from river plumes,surface N*usually ranged from-2.0 to 2.0 ?mol kg-1,and bottom N*mostly ranged from-6.0 to 0 ?mol kg-1.Combining our field data and historical literature data,we found that N*in the Bohai and Yellow seas increased in the 1990s and 2000s,but the coastal N*increases stopped in the 2010sBase on the six field datasets in 2011-2018,the N*budgets in the Bohai and Yellow seas have been built,and then the marine N contents have been calculated by other parameters such as dissolved inorganic phosphorus(DIP),salinity,water depths and so on.The results showed that the marine N contents in 71%of all collected samples were agreed well with the field-measured dissolved inorganic nitrogen(DIN).The marine N dominated DIN in the central areas of the Bohai and Yellow seas,whereas the non-marine N composition occurred mostly in limited areas with the salinity of less than 31.0,and nearby the Yellow River mouth,the Yangtze River mouth and the Jiangsu coast.During our surveys,the marine N percentages among DIN reached(51±38)%for the Bohai Sea and(67 ± 37)%for the Yellow Sea.Moreover,source apportionment of the N*-increasing rates in the Bohai and Yellow seas in 1990-2010 suggested that accumulation of atmospheric nitrogen along oceanic circulation pathways dominates the decadal evolution of coastal eutrophication.The Kuroshio N*-change accounted for 29%of the N*-increase in the Yellow Sea from 1990 to 2010.while the rest of the increase was from the accumulated effect of atmospheric deposition.In the Bohai Sea during the same period,61%of the N*-increase originated from the Yellow Sea and the rest was from local deposition of atmospheric N.In the 2010s,however,the coastal N*increases stopped,as China have managed to stabilize the atmospheric N emission and the Kuroshio N*basically tends to stableDuring the period of 2011-2016,the central Bohai Sea occurred frequently the eutrophic events,such as the brown blooms and summer bottom oxygen-depleted waters.To better understand the eutrophication characteristics of this important coastal sea,seasonal and regional nutrient levels in the Bohai Sea have been analyzed.The Bohai Sea nutrient levels were mostly subject to seasonal cycles,whereas the nutrient structures showed regional difference rather than seasonal variations.Survey-averaged DIN:DIP in estuarine and nearshore areas were 20-133.In contast,the central Bohai Sea was detected to the DIN:DIP ratios of 16.9± 3.4 during the autumn of 2011 and 16.1 ± 3.0 during the winter of 2016,which arer very cloase to the traditional N:P Redfield ratio of 16.In summer,both DIN and DIP were used up in the surface waters of the central Bohai Sea,suggesting that the biological consumption of DIN and DIP may also folloe the Redfield ratio.Based on a mass balance study,wintertime nutrient budgets of the central Bohai Sea water wete then estabilished,to avoid the strong biological activities in summer.The budgeting resultes suggest that the adjacent North Yellow Sea supplied additional DIP to the central Bohai Sea via wintertime water intrusion,balancing terrigenous excess DIN that was introduced in summer.According to our nutrient budgets,the North Yellow Sea water intrusion supplied 55%of DIN and 99%of DIP to the central Bohai Sea.Moreover,atmospheric nitrogen deposition served as a significant DIN source,accounting for 28%of DIN inputs to the central Bohai Sea.The terrestrial DIN from freshwater discharges just contributed 17%of DIN in the central Bohai Sea wintertime waters.A water-mixing simulation combining these two nutrient sources with atmospheric nitrogen deposition suggests that eutrophication in the central Bohai Sea will likely be enhanced by the large-scale accumulation of anthropogenic nitrogen in adjacent open oceans.The increased DIN input from the Yellow Sea(accounted for 71%)resulted in the enhanced eutrophication in the central Bohai Sea in 1985-2018.As the atmospheric pollution in China have been alleviated,the atmospheric N emission/deposition has been declined in the recent decade.Meanwhile,the surface chlorophyll-a in the Bohai Sea trended to decline in the 2010s,and the eutrophic events have been decreased in the recent Bohai Sea.This study found that the central Bohai Sea DIN:DIP was about 11 during the two surveys in 2019,being close to those in the early-2000s.The calculated results from the N*budgets indicated that the central Bohai Sea DIN was the marine N and less of the non-marine N during the two surveys.Meanwhile,the central Bohai Sea wintertime nutrient budgets in 2019 were compared with the condition in 2016,showing that the recent declined N-inputs via both of the oceanic circulations and the atmospheric deposition contributed to(65±35)%of the declined DIN:DIP in the central Bohai Sea in 2016-2019.In summary,the marine N contents dominated DIN in the central areas of the Bohai and Yellow seas,and the marine N changes led to the coastal eutrophication of the shelf seas.This eutrophication process in the shelf seas is different from the enhanced N-loads in river plumes.There were two factors leading to the marine N changes.One was because of a N-enrichment in the open ocean,and the other came from an increase in the atmospheric N deposition.Point-sources such as rivers and sewages have limiting impact on the coastal eutrophication in the shelf seas,especially in the central areas.Moreover,the local deposition of atmospheric N in the Yellow Sea contributed to part(43%)of the Bohai Sea eutrophication,revealing that the atmospheric N deposition not only has a local impact on the marine systems,but also has a remote control on the coastal eutrophication via oceanic circulations.Because oceanic circulations dominate hydrology and chemical dynamics in most coastal seas,our findings and new insights may improve the understanding of nutrient dynamics in the coastal eutrophic systems. |
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