Study On The Atmospheric Dry And Wet Depositions And Budgets Of Nutrient Elements In The Jiaozhou Bay,north China,under The Enhanced Impacts Of Human Activities

With the rapid development of economy in the world,more and more anthropogenic pollutants have been emitted into the atmosphere,and most of them can enter into the sea through atmospheric deposition.Thus,as one of the important pathways for terrigenous human-induced pollutants transported into the ocean,atmospheric deposition has received more and more attentions from the researchers and governments around the world.In this study,we focused on the Jiaozhou Bay,a typical semi-enclosed bay located in the north of China,which suffered from intense human activities,and systematically analyzed the atmospheric dry and wet depositions of the chemical components,ecological effects and the budgets of nutrient elements.Based on the aerosols and atmospheric dry and wet deposition samples collected in the Jiaozhou Bay from June 2015 to May 2016,we analyzed the atmospheric dry and wet deposition?including acid deposition?fluxes and influencing factors of the main biogenic elements?nitrogen,phosphorus,silicon and sulfur?and dissolved trace elements?aluminum,iron,manganese,cobalt,zinc,selenium,lead,cadmium and chromium?in the semi-enclosed bay under the high-intensitive human activities.Meanwhile,their sources and atmospheric transmission processes were studied by means of multi-methods such as statistical analysis,enrichment factors,principal component analysis and backward air-mass trajectory.Then,the effects of atmospheric nutrient deposition on the Jiaozhou Bay ecosystem were systematically evaluated in terms of marine primary productivity and nutrient structures.On that basis,the budgets and pattern of“source”and“sink”for nutrient elements in the Jiaozhou Bay seawater were assessed combined atmospheric deposition,river inputs,point source pollution,mariculture,the diffusion and releases processes at the sediment-water interfere and the water exchange with the South Yellow Sea.A series of new results have been obtained as follows:1)The nitrogen pollution levels in the Jiaozhou Bay were very serious,and meteorological conditions?rainfall,wind direction?,the emission intensities of local natural and anthropogenic pollutants together with the long-range transport of them constituted the main influencing factors of concentrations and deposition fluxes of atmospheric nutrients.There were obvious monthly various for dry deposition velocities of aerosol water-soluble nutrient species?nitrogen,phosphorus and silicon?.Owing to the differences of dry deposition velocities between different nutrient species,there were evident differences for the percentages of them in aerosols and dry deposition samples.The ratios between nitrogen,phosphorus and silicon in aerosols and dry and wet deposition samples were unbalanced badly,and the atmospheric depositions of organic nitrogen and phosphorus species cannot be ignored.Atmospheric dry and wet depositions,especially the heavy rainfall would greatly elevated the nutrient concentrations of surface water of Jiaozhou Bay,facilitated the growth of phytoplankton and promoted the primary productivity,as well as changed the nutrient structures of surface water and then exerted effects on the community structures and size-fractionated structures of phytoplankton.All of these would aggravate the unbalance conditions of Jiaozhou Bay ecosystem.The dry deposition fluxes of NH₄-N,NO₃-N,NO₂-N,DON,PO₄-P,DOP and DSi were 29.4,29.9,0.058,15.4,0.099,0.165 and 8.48 mmol·m^-2·yr^-1,respectively,while the wet deposition fluxes for them were 92.8,54.5,0.427,47.5,0.274,0.448 and 1.73mmol·m^-2·yr^-1,respectively.For the most of nutrient species except DSi,the wet deposition fluxes exceeded dry fluxes.The indirectly calculated dry deposition velocities for NH₄-N,NO₃-N,NO₂-N,DON,PO₄-P,DOP and DSi were 0.17±0.11?0.35±0.20?0.34±0.25?0.30±0.22?0.43±0.28?0.91±0.60?1.31±1.27 cm·s^-1respectively,with obvious seasonal variations.Inorganic nitrogen was the dominating forms of dry and wet deposition,and the percentages of DON in dry and wet depositions of nitrogen were 20.6%and 24.3%,respectively,while for the dry and wet depositions of phosphorus,DOP was the main form and their percentages were62.5%and 62.0%,respectively,implying it was negligible for the atmospheric depositions of organic nitrogen and phosphorus.The primary sources of atmospheric nitrogen,silicon and phosphorus in Jiaozhou Bay were agricultural activities,soil dusts and the mixing sources combined with anthropogenic emissions and natural source,respectively.The results from the backward air-mass analysis indicated that the precipitation air-masses originated from northwest?34.7%?and southeast?42.8%?directions have major effects on wet depositions of nutrient components,while for dry deposition,the effect of air-masses deriving from northwest direction?57.9%?was principal.Due to the phosphorus limitation of Jiaozhou Bay,according to the Redfield ratios,the new productivities supported by atmospheric phosphorus inputs in spring,summer,autumn and winter were calculated to be 1.94,1.15,1.86 and 1.15 mg C·m^-2·d^-1,respectively.Although the average new productivities induced by atmospheric phosphorus were low,the largely phosphorus inputs from short-term heavy rainfall happened in the rainy season can support a high new productivity,which can reach up to²5%of the average primary productivity in this season.In addition,the extremely high N:P ratios and extremely low Si:N ratios in dry and wet depositions together with the fact that NH₄-N was the principal nutrient species in atmospheric deposition jointly indicated that atmospheric deposition would change the nutrient structure of the surface water of Jiaozhou Bay,worsen the conditions of phosphorus and silicon limitation in Jiaozhou Bay.That,we concluded,may be one of the possible reasons for the changes of community structure and size-fractioned structure of phytoplankton as well as the succession of phytoplankton dominant species in the Jiaozhou Bay in recent years.2)The precipitation in the Jiaozhou Bay were seriously acid,together with the higher electrical conductivity and atmospheric deposition fluxes of nitrogen and sulfur,collectively suggested that Jiaozhou Bay have been undergoing severe atmospheric secondary pollution.SO₄^2-and NO₃^-were the main acidic components,while NH₄⁺and nss-Ca²⁺were the primary alkaline components.The serious lacking of Ca²⁺may be responsible for the severe acid rain condition in the Jiaozhou Bay.The principal sources of atmospheric water-soluble ions in Jiaozhou Bay were anthropogenic activities.The p H range of precipitation in the Jiaozhou Bay were from 3.84 to 6.91,the Volume-weighted Mean low to 4.77 and the frequency of acid rain reached up to71.4%.NH₄⁺,SO₄^2-,NO₃^-,Ca²⁺,Cl^-constituted the main body of atmospheric water-soluble inorganic ions with the percentages in aerosols and precipitation over80%.NH₄⁺exceeded Ca²⁺and became the dominant cations,while SO₄^2-was the most abundant anions.The atmospheric deposition fluxes of sulfur and nitrogen were20.17 and 28.92 kg·ha^-1·yr^-1,respectively,and wet deposition constituted the main body for both of them.Although the currently acid rain in Jiaozhou Bay was still sulfur acid type,it has been changing into nitric acid type with the enhancing of atmospheric NO_x and strictly controlling of SO₂ around the Jiaozhou Bay.Quantitative analysis showed that the relative contribution rates of marine,crust and anthropogenic sources were 28.7%,14.5%and 56.8%,respectively.Considered the contribution from marine phytoplankton to atmospheric SO₄^2-had been neglected,the marine sources of SO₄^2-may be underestimated in the previous studies.3)The concentrations and deposition fluxes of atmospheric dissolved trace elements in the Jiaozhou Bay were moderate on the global scale,which showed obvious monthly variations under the impacts from complex weather conditions?rainfall,wind speed,wind direction?.The dominating origins of atmospheric dissolved aluminum and iron were the crustal dusts,while the main sources of the remaining 7 trace elements were anthropogenic sources.The air-masses originating from the northwest direction exerted the greatest impacts on the concentrations and wet depositions of atmospheric dissolved trace elements in the study area.The dry deposition velocities of atmospheric manganese,zinc,cobalt were relatively high,while for the typical anthropogenic elements such as lead,cadmium and chromium have relatively low dry deposition velocities.The short-term heavy rainfall would increase the concentrations of dissolved trace elements in the surface water of Jiaozhou Bay,contaminate the aquatic environments and then exert both trophism and toxic effects on the growth of phytoplankton.The dry deposition fluxes of atmospheric dissolved trace elements were aluminum19.3 mg·m^-2·yr^-1,iron 10.2 mg·m^-2·yr^-1,manganese 21.6 mg·m^-2·yr^-1,cobalt 0.154mg·m^-2·yr^-1,zinc 101.5 mg·m^-2·yr^-1,selenium 0.862 mg·m^-2·yr^-1,lead 0.175mg·m^-2·yr^-1,cadmium 0.035 mg·m^-2·yr^-1 and chromium 0.083 mg·m^-2·yr^-1.The wet deposition fluxes of the above trace elements were 29.2,14.2,24.2,0.090,24.1,0.725,2.21,0.131 and 0.669 mg·m^-2·yr^-1,respectively.For most of these trace elements except for cobalt,zinc and selenium,the wet fluxes all exceeded the dry fluxes.Based on the dry deposition model,the indirectly calculated dry deposition velocities of aluminum,iron,manganese,cobalt,zinc,selenium,lead,cadmium and chromium were 1.29±1.60?1.27±1.66?2.33±1.48?1.67±0.94?1.98±2.22?0.51±0.29?0.12±0.13?0.15±0.12?0.17±0.10 cm·s^-1,respectively,with apparent seasonal variations.The contributions from the crust to most of atmospheric trace elements can be neglected except aluminum and iron.The air-masses came from the northwest direction have relatively high concentrations of dissolved trace elements with higher frequency of occurrence.Thus,it had the greatest effects on the concentrations and wet deposition of atmospheric trace elements in the Jiaozhou Bay.The theoretical magnitude of biological fixation of nitrogen supported by the dissolved iron in atmospheric deposition were 94.9×10⁸ g N·yr^-1,which accounted for 4.9%¹00%of the total biological fixation of nitrogen in the Jiaozhou Bay.4)Compared to riverine inputs,point source pollution and mariculture,the loadings of atmospheric depositions of nutrients and trace elements accounted for relatively low percentages,but it has increased greatly relative to previous results,indicating the status of atmospheric deposition has enhanced in recent years.The water exchange between the South Yellow Sea and Jiaozhou Bay were the main sources of DOP and chromium in the Jiaozhou Bay.Jiaozhou Bayseawater were the“sink”of most of nutrients and trace elements but Zn.Due to the relatively high N:P ratio of the reserved nutrients in the seawater of Jiaozhou Bay,the phosphorus limitation may be maintained for a long time.Quantization of the influxes/effluxes and sources/sinks of nutrients and trace elements are meaningful for assessing the eutrophication and environmental evolution trend of Jiaozhou Bay.The percentages of atmospheric deposition of nutrients in the total external loadings of Jiaozhou Bay has increased in the recent years.For individual trace elements,such as zinc,atmospheric deposition loading accounted for relatively high percentage,suggesting atmospheric deposition is an important source for trace elements.In addition,its contribution has increased in recent years with the enhancing of anthropogenic activities.Through one year's water exchange,the reserved DIN,DIP and DSi that could be assimilated by phytoplankton were 193×10⁹ mol,2.04×10⁹mol and 106×10⁹ mol with a N:P ratio of¹00 and a Si:N ratio of⁰.5.Thus,we concluded that under the premise that there is no major environmental change,the phosphorus limitation of Jiaozhou Bay would maintain for a long time,but the condition that silicon limitation may be alleviated.This study systematically analyzed atmospheric dry and wet depositions of nutrients and trace elements and their sources and ecological effects on Jiaozhou Bay ecosystem.Through this study,we reinforced the understanding of biogeochemical cycle processes of nutrient elements in the Jiaozhou Bay.In addition,this study provided essential data and theoretical foundation for further study on the responses and mechanisms of semi-enclosed bay to the growing anthropogenic activities.