Evaluating Submarine Groundwater Discharge And Its Effects On Eco-environment In Estuarine And Coastal Waters By Radium Isotopes

Previous studies on evaluation of terrestrial pollutants into the coastal regions and their effects on the ecosystem,much attention was paid to the contributions from traditional sources,such as riverine input,atmospheric deposition,aquaculture and waste discharge,etc.However,there is an another“new”source-Submarine Groundwater Discharge?SGD?,which is usually easy to be ignored.SGD has recently received increased attention because of its significant environmental consequences in coastal regions.Not only it is an important component for the hydrological cycle,but it is also associated with chemical species,including nutrients,trace elements,carbon,organics and radionuclides into the coastal ecosystems,which could alter the biogeochemical cycle of some elements in the ocean and lead to changing ecological environment.But the research of SGD ecological effect in pristine ecological and large-scale regions are relatively lacking.Therefore,in this thesis,we chose Pearl River Estuary?PRE?and Changjiang River Estuary?CRE?,which are under high human activities,Krka River Estuary?KRE?which is a pristine ecological karstic estuary in Croatia;and Yellow Sea and Bohai Sea,which are two large-scale typical marginal seas in China to study the significance of SGD by radium isotopes,aiming to evaluate the eco-environmental effects of SGD-derived biogenic elements on estuaries and marginal seas.For the estuaries,we carried out field works in the PRE during October 2014?dry season?and June 2015?wet season?,in the CRE during March 2015?dry season?and in the KRE during September 2014.By the ²²⁴Ra mass balance model,the SGD flux in the PRE was estimated to be?4.5-10?×10⁸ m³ d^-1(23-50 cm d^-1)and?1.2-2.7?×10⁸m³ d^-1(6-14 cm d^-1)in the dry season and wet season,respectively,and the SGD flux was estimated to be?4.6-11?×10⁹ m³ d^-1(18-45 cm d^-1)in the CRE of the dry season.Moreover,the fluxes of SGD in the dry season were higher than those of the river flows during the same period.Whilst in the KRE,by applying ²²⁶Ra and ²²⁸Ra with multiple methods,the total SGD flux into the KRE surface layer was estimated to be?17.5-22.3?×10⁵ m³ d^-1(18.9-24 cm d^-1),which accounted for 36-46%of the Krka River discharge into the estuary.Compared these three estuaries,we found that almost all of the SGD in the PRE and CRE were derived from the recirculated saline groundwater discharge?RSGD?with a proportion of over 95%,while in the KRE the submarine fresh groundwater discharge?SFGD?ccounted for 34-60%of the total SGD,suggesting the basically equal proportion of SFGD and RSGD.In comparison with nutrient fluxes from other sources,the SGD-derived dissolved inorganic nitrogen?DIN?,phosphorous?DIP?and silicate?DSi?are the important nutrient source in these three estuaries.In the dry season,SGD-derived nutrient fluxes equal to several times the riverine inputs during the same period,even in the wet season it could reach 37-150%,indicating that SGD is a major nutrients external source in both highly human-affected estuary and pristine ecological estuary.Moverover,SGD-derived DIN/DIP ratios are much higher than the Redfield ratio,in which the SGD-derived DIN/DIP ratios are 53 and 90 in the PRE and CRE,respectively;and the ratio in the KRE is as high as 317.Specifically,SGD-derived high DIN flux and high ratios of DIN/DIP may affect productivity in the estuarine ecosystem and nearby adjacent sea,which would potentially contribute to eutrophication and the occurrence of red tides along the waters.Additionally,compared to the PRE and the CRE,the KRE locates in the typical karstic region,and net SGD-derived dissolved inorganic carbon?DIC?flux(2.10 mmol m^-22 d^-1)is much higher than those in most other types of estuaries worldwide(average of 0.85 mol m^-22 d^-1),suggesting that the DIC-enriched karst aquifers are important sources for global carbon cycle.Therefore,with further development of current global climate change trends,such as sea level rise,the effects of net SGD-derived DIC from karst aquifers on coastal seas will likely become more evident and substantial.Regarding to the ecological effect of SGD in the large-scale region,we evaluate the effects of SGD-derived nutrient on eco-environment of the Yellow Sea and Bohai Sea.Based on the ²²⁸Ra mass balance model,we estimated the average SGD flux to be approximately?1.3±0.87?×10¹² m³ yr^-1(1.15±0.77 cm d^-1)over the entire Southern Yellow Sea?SYS?,which is at least 3.3 times in comparison with those of riverine inputs the from the Changjiang River into the SYS.The SGD-derived nutrient shows that SGD is a major source of nutrients?especially DIN and DSi?,providing 66%and36%of the total input sources,respectively.The accumulation nutrient fluxes derived by SGD may play one of the most important roles in the green tide bloom that originated from the Subei Shoal zone in the SYS.High N/P ratio input through SGD was significant in the SYS and had a significant influence on the ecosystem which may lead to the green tide bloom that originating from the Subei Shoal zone.By structuring the development of green tide formation in the SYS,SGD played an important role in the whole process,especially by providing the DIN for massive green tide bloom and nutrient recovery after its degradation in the SYS.In the Bohai Sea,the masses balance of ²²⁶Ra and ²²⁸Ra suggest that the yearly SGD flux into the whole Bohai Sea is?2.0±1.3?×10¹¹ m³ yr^-1(0.72±0.46 cm d^-1),of which the percentage of SFGD to the total SGD is approximately 5.1±4.1%.However,the DIN and DSi fluxes from SFGD constitute 29%and 10%,respectively,of the total fluxes from by SGD.Moreover,the nutrient-enriched SGD dominates the input sources in the Bohai Sea and may have an important effect on the coastal ecosystem;specifically,the high SGD-derived DIN and DIP,especially high DIN/DIP ratios in SFGD may potentially lead to eutrophication and thus the occurrence of red tide in the Bohai Sea.Overall,even thought the SGD types in eutrophic estuary and pristine ecological karstic estuary were different,SGD-derived nutrient fluxs have the potential to affect the ecosystem of coastal waters in both estuaries;moreover,DIC-enriched karst aquifers are important sources for global carbon cycle,and will likely become more evident and substantial with further development of current global climate change trends,such as sea level rise.In the large-scale marginal seas,SGD also acts as an important external nutrient source,and showed a neglectable influence on the ecological environment of marginal seas.Therefore,the implementation of this thesis will provide relevant basic data for the management and protection of coastal areas under global climate change.