A Comparative Study Of Submarine Groundwater Discharge And Its-derived Nutrients In Typical Bays Under The Influence Of Anthropogenic Activities

The bay ecosystems,as the most affected areas by anthropological activities in the ocean,are the core regions of marine economic development.However,with the input of terrigenous materials,the bay ecosystem has been facing great challenges in recent years.Compared with the impact of traditional sources such as rivers and atmospheric deposition on the ecological environment of the bay,the terrigenous substances delivered by submarine groundwater discharge（SGD）are also noticed and cannot be overlooked.The impact of SGD-derived nutrients on bay ecosystem is particularly important under the combined effect of anthropogenic activities and climate change.Although numerous studies have been carried out on SGD in different types of bays,but fewer attention has been paid to the contributions and similarities of SGD and their related nutrients in different bays.Therefore,the Sanmen Bay（SMB）and the Dongshan Bay（DSB）,which are different climatic characteristics and anthropological activities,and the Kongsfjorden in the Arctic,which is less influenced by anthropological activities,were selected for this study.Radium（Ra）isotope tracing techniques were used to assess the potential impacts of SGD and their derived nutrients on the bay with different intensity of anthropological activities.To assess the environmental impact of SGD related nutrients in the SMB,which is dominated by industrial production,Ra and inorganic nutrients in the surface seawater and coastal groundwater of the SMB were observed.Based on the ²²⁴Ra mass balance model,the total SGD fluxes during the flood and dry seasons of the SMB were estimated to be（1.1±0.1）×10⁸ m³ d^-1(13.9±0.5 cm d^-1)and（1.1±0.2）×10⁸ m³ d^-1(14.1±3.2 cm d^-1),respectively.Meanwhile,combined with the water mass balance model,it was found that the submarine freshwater groundwater discharge（SFGD）during the flood season in the SMB accounted for approximately 9.0%of the total SGD,while the percentage in the dry season was about 10.4%,which indicated that SFGD was not only one of the important freshwater sources entering the SMB,but more importantly,it contributed 8.6-34.5%and 35.9-89.6%of the total SGD derived dissolved inorganic nitrogen（DIN）and dissolved inorganic phosphorus（DIP）fluxes,respectively.Similarly,considering all nutrient source and sinks in SMB,it was found that SGD contributed 53.9-62.8%of total DIN and 11.9-25.1%of total DIP,and was the most significant nutrient source in this mariculture bay.Apart from SGD,a large amount of mariculture and industrial wastewater discharge together lead to a significant increase of nutrients level in the SMB,so it is especially important to maintain the ecological environment of the SMB by moderate control of mariculture species and scale,strengthening sewage discharge and SGD management.The assessment of SFGD related nutrients in the SMB revealed that although the contribution of SFGD flux was small,SFGD-derived nutrient was one of the important sources of nutrients in the bay ecosystem,and the factors affecting the SFGD rates have not been discussed deeply.Therefore,in order to further evaluate the output forms of SFGD and its derived nutrients under different climatic characteristics and then to explore the crucial factors affecting SFGD rates,we also collected and analyzed the Ra isotopes and nutrient of seawater and coastal groundwater in the DSB,where mariculture and industrial production are the main anthropological activities.Based on Ra isotope quartet mass balance models and water mass balance model,we quantified RSGD rates(12.5 cm d^-1),SFGD rates(1.8 cm d^-1)and their related nutrients fluxes.The results showed that although SFGD accounted for 13-17%of the total SGD,it carried 7.4-80.5%nutrients of the total SGD,indicating that SFGD was an important source of"new"nutrients with high nitrogen to phosphorus（N/P）ratios（585）entering coastal waters.Further,the reconstructed nutrient budgets in the DSB shows that SGD and mariculture discharge are the two major sources of nutrients entering coastal waters,providing 2.8-21.2 times and 1.9-13.9 times more nutrients than local rivers input,respectively.In addition,by summarizing the study cases of SGD in all mariculture bays in China,it was found that SGD derived nutrients fluxes comparable to river inputs in these bays.It can be seen that SGD should be included in the ecosystem management of bays under the influence of mariculture activities.Under global climate change,water flow and related biogeochemistry in the Arctic are changing at an unprecedented rate,and potentially affect nutrient cycling in the Arctic Ocean.However,nutrient fluxes via submarine groundwater discharge（SGD）are potentially important yet poorly understood in the Arctic.Here we quantified that radium-derived nutrient fluxes through SGD were three orders of magnitude higher than those from the local river and constituted 95-99%of the total nutrient input into the Kongsfjorden.These large groundwater nutrient fluxes with high N/P ratio（115）may change the biomass and community structure of phytoplankton.Meanwhile,combining other SGD study cases around the Arctic region,SGD rates tend to increase over the past three decades,possibly on account of the effects of global warming.The total SGD flux to the Arctic Ocean is 0.1-8.3（median:1.6）times as much as the total river input,suggesting that SGD is a crucial source of fresh water and is also the key factor to assessing the freshwater inventory and matters balances in the Arctic Ocean.The SGD-derived nutrient flux is 0.1-2.1（median:0.4）times greater than that of rivers in the Arctic Ocean,which may significantly cause the increase of net primary productivity in the Arctic Ocean.The results will provide important basic data for land-ocean interactions in the typical fjord of the Arctic under the influence of global warming.To summarize,SGD（especially SFGD）derived nutrients significantly impact on coastal waters in bays affected by anthropological activities.With the intensification of anthropological activities,the impact of SFGD on the coastal environment becomes more and more severe.In pristine bays that are less affected by anthropological activities,the source of nutrients is relatively single,and SGD related nutrients becomes the most significant source of nutrients in the coastal waters,especially under the influence of global warming.Whether in bays strongly influenced by anthropological activities or pristine bays around the Arctic,although the expression of SGD differs,SGD-derived nutrients are of great importance to the material cycle of bays.Our study not only fills the theoretical basis for the SFGD in global bay ecosystems,but also expands the basic data for the SGD in the Arctic regions under the influence of climate warming,and greatly enriches the connotation of land-ocean interaction in coastal zones with different climatic characteristics.It also provides an effective scientific basis for the environmental protection and management of bay ecosystems under the dual pressure of global warming and increasing anthropological activities.