Submarine Groundwater Discharge Borne Nutrients In The Coastal Aquaculture Ecosystem And Its Environmental Effects

Marine aquaculture has been increasingly acknowledged as an important pathway for acquiring high-quality animal protein.The development of the marine economy has become a new growth point and cornerstone that are necessary for maintaining and improving Chinese economy.However,the occurrence of various environmental problems and socioeconomic losses has gradually increased,those not only endanger the marine environment but also trigger a potential risk to marine cultured seafood,which are attributable to the high dependency and destructiveness of the marine economy for environmental resources.Previous studies only focused on the water flux and terrestrial solutes from the traditional sources（i.e.riverine input,atmospheric deposition,etc.）delivered to the sea.Submarine groundwater discharge（SGD）and associated nutrients have received insufficient attention,with only a few studies.But as an emerging and comprehensive source with the complexity and concealment of process in the coastal aquifer,it was identified that SGD can’t be ignorable in the coastal environments due to its large flux and the corresponding concentrated constituents compared with traditional sources.It has been widely recognized as a significant vector for land-sea interactions in the coastal zone（LOICZ）,and it has been shown to play a critical role in water transport and terrestrial solute fluxes in global coastal regions.Although a few scientific studies have assessed SGD and its corresponding solute fluxes in the different coastal environments around the world,related studies in coastal aquaculture ecosystems are still scarce.The healthy development of marine aquaculture ecosystems plays a profound and potential role in both the natural resources intake and the sustainable development of Chinese economy,which has become an important issue and also aroused concerns of the government.Therefore,using the geo-chemical tracer radium（Ra）radioisotope,the environmental impacts of the volumetric fluxes and associated nutrients（inorganic and/or organic）derived from SGD were evaluated in two typical geomorphological regions-an open aquaculture estuary-Aojiang River Estuary in Fujian Province and a narrow aquaculture bay-Xiangshan Bay in Zhejiang Province,which was characterized by the strong dramatic aquaculture perturbations.In addition,the reasonability and applicability in SGD evaluation in different regional area was discussed based on the sensitivity analysis of Ra quartet(²²³Ra,²²⁴Ra,²²⁶Ra,and²²⁸Ra)mass balance models.（1）Based on the high-resolution field observations of Ra isotopes quartet through three cruises during July and October 2019,and April 2021,this thesis revealed that the importance of SGD and associated nutrients in the typical coastal aquaculture region of Aojiang River Estuary.The calculated SGD rates using the Ra isotope mass balance model were 15.9±9.4 cm d^-1,18.1±8.5 cm d^-1,and 23.0±13.7 cm d^-1 for July and October 2019,and April 2021,respectively,which corresponded to approximately 25±15%,75±35%and 81±48%those of the local river discharge.We found that SGD was insensitive to the precipitation alternation according to the seasonal difference of the SGD rates,and was mainly characterized and regulated by ocean forcing stimulating by tidal power pumps,such as the SGD rates increased with the increasing tidal fluctuation.Moreover,the recirculated saline groundwater discharge（RSGD）was the dominant component of SGD in Aojiang River Estuary.By combining the nutrient concentrations in the coastal groundwater,the SGD-derived nutrient rates during three seasons were in the same order of magnitude with those of river.However,the rates of SGD-derived different nutrient speciation had large variabilities,for example,the SGD-derived NO_X-N（the sum of NO₂^-and NO₃^-）rates in Aojiang River Estuary during July2019 were one order of magnitude higher than those in other seasons,reflecting that the significant effect of aquaculture activities（i.e.,shellfish culture）as substantial nutrients were released by the shellfish than those of nori.It is noteworthy that shellfish and nori are the dominant culture species in Aojiang River Estuary during July and October 2019 but no existing effect of aquaculture activities during April 2021.Furthermore,the nutrient budgets implied that the SGD-derived nutrient loadings were the important contributor for the total nutrient sources,accounting for approximately23-62%and 45-79%of the total dissolved inorganic nitrogen and phosphorus sources,respectively.In addition,the contrasting conditions between groundwater-derived nutrient fluxes and the nutrient responses in estuarine waters,indicating the significance of seasonal alternations in the biogeochemical transformation mechanisms and aquaculture perturbations with the different important application for modifying inorganic/organic nitrogen and phosphorus distribution,speciation and their cyclings.Our results demonstrated that the nutrient speciation dynamics were primarily controlled by the extent of biogeochemical transformation and the aquaculture perturbations,highlighting the critical role of nutrient dynamics mechanism within receiving waters under the specific aquaculture perturbations.Therefore,the results from our studies had the important implications for the productivity and functioning of the coastal environment as well as the management of eutrophication in the high-density aquaculture estuarine region.（2）SGD and associated nutrients in Xiangshan Bay（XSB）were quantified during November 2019 and August 2020 using the geotracer-Ra mass balance model.The SGD rates during November 2019 varied significantly,with an average of 11.0±5.5cm d^-1 in the XSB Channel and 3.6±1.0-5.7±1.7 cm d^-1 in XSB embayments.Moreover,the SGD rates during August 2020 were also highly variable,with an average of 14.9±11.9 cm d^-1 in XSB Channel and 2.2±1.3-4.5±1.1 cm d^-1 in XSB embayments.In addition,RSGD predominated the SGD component,and the significant spatial variabilities in the SGD rates were observed in the different XSB districts,reflecting the influence of ocean dynamics coupled with tidal effects at the different scales.Furthermore,the estimated higher SGD rates in XSB were in proximity to the ocean,while relatively lower rates were observed in the inner bays.For example,we observed that significantly greater tidal fluctuations（5.7 m）occurring near the ocean during November 2019 led to a higher SGD rate(11.0±5.5 cm d^-1)in the lower XSB,on the contrary,the relatively lower tidal fluctuations（3.4 m）occurring near the XSB inner embayment led to a lower SGD rate(3.6±1.0 cm d^-1).As a major nutrient source in XSB,the SGD-derived nutrients have been proven approximately 1-2 orders of magnitude higher than those of river fluxes.In addition,the SGD rates in XSB Channel exceeded those in small embayments by factors of approximately 2-7,but the SGD-derived nutrient rates in XSB Channel were comparable with those in the three embayments and significantly lower than that for NO_X-N in XSB inner embayment during November 2019.In addition,higher groundwater-borne nutrient concentrations were observed in the oyster culture area than in the fish culture area.A general understanding is that the enriched nutrient loadings from coastal wastewater originating from industrial and domestic wastewater subsequently input into the coastal aquifer under the water exchange influence,which in turn affected and changed the nutrient concentrations in the shallow groundwater.As an extraneous nonpoint source,the SGD-derived nutrients with a relatively higher nitrogen/phosphorous ratio may strongly affect the primary production and nutrient structure as well as regulate the growth of phytoplankton in the coastal ecosystems,triggering or dampening algal blooms.（3）In order to compare the reasonability and applicability of multiple tracers regarding radioactive Ra quartet(²²³Ra,²²⁴Ra,²²⁶Ra,and ²²⁸Ra)mass balance models for SGD evaluation in three typical geomorphological Chinese regions,namely Aojiang River Estuary,XSB,and the eastern of Hainan Lagoon-Xiaohai Lagoon,respectively.The SGD fluxes estimated by four Ra isotopes mass balance models in three regions had large variabilities,with the factor of 1.7-4.7,1.5-1.6,and 8.The results demonstrated that the SGD fluxes were expected to change according to the different spatial and temporal scales.The sensitivity analysis of Ra sources and sinks in the Ra quartet mass balance model demonstrated that the mixing with open sea,radioactive decay,and the benthic sediment diffusion can induce and ultimately result in the different extent and scope for the uncertainty.In brief,a 10%change in the mixing fluxes can result in uncertainties at 2.3-46.0%for the SGD fluxes estimated by four Ra isotopes mass balance models in Aojiang River Estuary,XSB,and Xiaohai Lagoon;a10%change in decay fluxes can result in uncertainties at 0.9-18.7%for the SGD fluxes estimated by short-lived ²²³Ra and ²²⁴Ra mass balance models in three regions;a 10%change in the sediment diffusion fluxes can result in uncertainties at 1.7-43.4%for the SGD fluxes estimated by short-lived ²²³Ra and ²²⁴Ra mass balance models in Aojiang River Estuary and Xiaohai Lagoon.Ra isotopes quartet were influenced by the different source and sink terms,reflecting the time-scale of specific process with respect to the variable Ra isotopes half-lives.Therefore,this study provided a new perspective that the long-lived ²²⁶Ra and ²²⁸Ra mass balance models in the coastal region with relatively short time scale（1.61-32.1 days）can shed an insight on the accurate and reliable quantification of SGD.Furthermore,coastal aquaculture ecosystems were more sensitive to the intensive anthropogenic activities,especially the enormous nutrient loadings from aquaculture perturbations.The large amounts of enriched nutrients from groundwater influence the dynamics and structure of microbial communities,and can therefore result in the increasing environmental issues in estuarine/coastal ecosystems,simultaneously complicating and altering the nutrient speciation,behavior,and biogeochemical cycling in coastal waterbodies worldwide.More importantly,the healthy development and management of aquaculture industry are essential to the development of China’s economic structure.Overall,this study revealed that the spatial and temporal characteristics of SGD in different aquaculture regions based on the geo-tracer Ra,clarifying the importance of SGD,which is a significant but often overlooked nutrient source and have the significant potential ramifications for the ecology in the broader range of habitats such as estuary,bay,and lagoon.Moreover,the accurate and reasonable quantification of SGD using different Ra isotopes in different regions are essential to evaluate SGD and the corresponding nutrient.Eventually,these results not only provide the light on the groundwater dominated various environment at the different scales,but also the knowledge and guidance are proposed to the management and development of coastal aquaculture ecosystems.