| Among the changes in environmental factors,the enhanced nutrients(i.e.nitrogen(N),phosphorus(P)and silicon(Si))play a key role in driving the evolution of the marine environment,which are the key issue of the field of marine and environment science.Due to the intensification of human activities and intervention in the natural environment,the marine environment has undergone significant changes compared to the recent past;In particular,the increase in the input of land-based nutrient has triggered a series of marine ecological problems,i.e.,the intensification of eutrophication,hypoxia,acidification,and the frequent occurrences of red tide,brown tide and green tide,which pose a serious threat to the marine ecological environment.In most near-shore waters,increasing concentrations of nitrogen and phosphorus have exacerbated eutrophication levels,and nutrient structures in some areas deviate from the Redfield-Brzezinski ratio(N:P:Si=16:1:16).As a result,and phosphorus and silicon limitations occur in the coastal ocean.Changes in the hydrochemical environment in offshore waters,especially changes in nutrient concentration and structure,have thus significant effects on phytoplankton population structure,thereby changing the structure and function of marine ecosystems.Therefore,it is necessary to clarify the nutrient distribution and structure and related ecological environment,which is the major concern of current offshore marine environmental management.Based on the comprehensive observation of multiple voyages in 2019～2022,in this study the temporal and spatial distribution and composition structure of nutrients in the East China Sea were analyzed based on the comprehensive observation of multiple voyages in 2019～2022.Key processes of river input,atmospheric deposition,bottom interface exchange and transcontinental shelf transport were quantified,and a mass balance model of nutrient budget of the East China Sea was constructed.Combined with historical data,the changes of nutrient concentration and ratio structure were analyzed over a long period of time in the lower reach of the Changjiang,the Changjiang estuary and the East China Sea,respectively.And then,the differences of nutrient in these three areas were compared,as well as the reasons.Based on the Global-NEWS2 model,the trend of nutrient in rivers entering the East China Sea was predicted,and the potential impact of river transport on nutrient variation in the East China Sea was further analyzed.The results of this study would provide a scientific basis for the governance of offshore ecological environment.The following conclusions were obtained:(1)In the East China Sea nutrient had significant seasonal differences,nutrient concentration in spring are lower than that in autumn,which was associated with the rapid phytoplankton growth.And as a lateral instance,the higher levels of dissolved oxygen(DO)and chlorophyll a(Chl-a)also reflect high primary production in spring.In terms of horizontal distribution,the nutrient concentration and ratio showed the overall distribution characteristics were high in nearshore and low in far-shore,and there was obvious phosphorus restriction phenomenon in nearshore seas.As a result,the nutrient restriction factor was not obvious in far-shore seas.In the vertical distribution,in the surface water the nutrient concentration was lower than that in the bottom waters,while in the surface water the DO and Chl-a contents were higher than the one in bottom waters.Based on the multivariate statistical analysis and the topographic distribution of the East China Sea,the continental shelf of the East China Sea was divided into the inner shelf and the outer shelf,respectively.And the nutrient in the inner shelf area was mainly affected by the combined action of the Changjiang Diluted Water and Zhe-Min Coastal Current,and the relatively high land-based nitrogen input could aggravate the enhance of phosphorus limitation.The nutrient in the surface water in the outer shelf sea was mainly affected by the Kuroshio surface water and the regarding biological utilization,resulting in a fairly lower nutrient concentration in the outer shelf area.The upsurge of Kuroshio associated with the high DIP from south to north in the bottom layer effectively alleviates the phosphorus restriction in the East China Sea.(2)The results of nutrient balance showed that Kuroshio and Taiwan Warm Current were the main exogenous sources of nutrient in the East China Sea,contributing 38.6% of DIN,69.3% of DIP and 65.2% of DSi which inflowed the study area,followed by sediment bottom interface diffusion,which accounted for31.2%,21.4% and 7.03% of the DIN,DIP and DSi sources,respectively.River transport contributed less to the exogenous DIP inflow,which was at around 4.75%.Primary production of phytoplankton was the main process of nutrient consumption in the East China Sea.The nutrient contributed by exogenous input were insufficient to support the primary production in the East China Sea,indicating that most of the dissolved nutrient that maintained the primary production in the East China Sea was come from the transformation of nutrients and the degradation of organic nutrient.In recent years,the primary productivity of the East China Sea has shown a gradual upward trend,but the species diversity in the inner shelf area has gradually decreased,which was inseparable from the continuous increase of DIN concentration and N/P ratio in the river and the decreasing concentration of DSi.In the East China Sea eutrophication has strengthened or increased due to the enhanced nitrogen and phosphorus transport fluxes,which would have a certain impact on the offshore environment.Preliminary estimates of cross-shelf transport of nutrient in the East China Sea showed that the net transport fluxes of DIN,DIP and DSi were 339±215、28.6±7.94 and 415±405Gmol/a,respectively.(3)In the past four years,the overall distribution of nutrient concentration showed the distribution trend of high nearshore and low far shore,but there were obvious differences in the same season of different years,the nutrient concentration in the inner shelf sea area changed greatly,and the range of the high nutrient value area was mainly related to the runoff of the Changjiang,while the nutrient concentration in the outer shelf sea area was relatively stable.In the long-term scale(1960～2020),the concentration of DIN in the East China Sea showed a significant upward trend,the concentration of DIP showed a trend of decreasing first and then increasing later,and the concentration of DSi showed a downward trend with time.With the increases in the DIN concentration and N/P ratio and the decline in the DSi concentration,the dominant species of red tide in inner shelf waters gradually changed from diatoms to non-diatoms or both co-existences based on the previous studies.(4)The change in the nutrient concentration and ratio in the Changjiang Estuary and the lower reaches of the Changjiang was roughly the same,indicating that the Changjiang Estuary was mainly affected by the Changjiang input.The change in nutrient in the East China Sea was significantly different from that in the Changjiang,indicating that nutrient in the East China Sea was also affected by other factors in addition to land-based import.Although the nutrient input of the Changjiang has little impact on the entire East China Sea shelf area in comparison with the contribution of Kuroshio,the Changjiang is still an important influencing factor for the change of nutrient concentration and ratio for the inner shelf area.(5)Based on the Global-NEWS2 model,the nutrient concentration and ratio in the coastal rivers of the East China Sea in the future were predicted.The results show that nutrient fluxes increase in both selected scenarios,but nutrient ratios would change with the higher increase of phosphorus input in the future in comparison with nitrogen,which may lead to uncertain development of ecosystems.Based on the observation and modelling,the controlling factor of nutrient vitiation and major processes of nutrient budget were figured out,and the cross-transport of nutrient of the East China Sea was assessed.This study would provide basis for river pollution control and offshore environmental management. |
