Submarine Groundwater Discharge In Mangroves, Salt Marshes, Sandy Beaches And Karst Ecosystems Of Typical Coastal Zones

Submarine groundwater discharge（SGD）,an important component of the global water and biogenic element（e.g.,nitrogen,phosphorus,silicon and carbon）sources,has been considered as a significant pathway for material exchange at the land-sea interface of coastal ecosystems.According to the reported data,the nutrients and carbons transported into the ocean by SGD are comparable to or possibly higher than those transported by rivers and atmospheric depositions,whether in small-scale estuaries,bays and lagoons or large-scale marginal seas.Current studies on SGD in subterranean estuaries mainly focused on estimating the SGD-derived material fluxes and assessing the potential environmental effects,and most of the studies were ecosystems such as bays,estuaries and shelves.However,there are relatively few studies on SGD in mangroves,salt marshes,sandy beaches and karst ecosystems.Therefore,in this dissertation,four typical coastal ecosystems（mangrove,salt marsh,sandy beach and karst system）were selected to study the nutrients（dissolved inorganic nitrogen（DIN）,phosphorus（DIP）and silicon（DSi））and carbons（dissolved inorganic carbon（DIC）and dissolved organic carbon（DOC））budgets.（1）Mangrove ecosystem:Maowei Sea is the largest natural oyster growing area and the largest mangrove island in China.Based on a ²²²Rn mass balance model,the SGD-derived DIN,DIP and DSi in Maowei Sea were estimated to be（6.5±10.2）×10^–2,（1.0±2.1）×10^–33 and（6.4±7.4）×10^–22 mol m^–22 d^–1,respectively,which were more than1.9,0.9 and 3.6 times the amounts from the local river input.The N/P ratios in the SGD around Maowei Sea were high（mean:64）,and these ratios likely exceeded the environmental self-purification capacity,thereby changing the nutrient composition and phytoplankton community structure.Therefore,SGD processes with derived nutrients may affect the biogeochemical cycles and marine ecological environment in Maowei Sea.The DIC and DOC fluxes through SGD were calculated to be 0.70±0.82and 0.31±0.30 mol m^-22 d^-11 in the wet season.Meanwhile,those in the dry season were 0.25±0.24 and 0.25±0.23 mol m^-22 d^-1.The DIC and DOC fluxes via SGD were11 times and 2.1 times the amounts of the local river input during the wet season,and5.1 times and 6.0 times the amounts in the local river input during the dry season.By combining our new flux estimates with literature data,the estimated SGD-derived DIC and DOC from global mangrove systems would be 29–48%of the carbon flux from global rivers.This demonstrates that SGD should be included in the blue carbon assessments of mangrove ecosystem.（2）Salt marsh ecosystem:Salt marsh ecosystem is not only an important carbon storage,but also an important component of coastal blue carbon.As a silting coastal salt marsh wetland,Chongming Dongtan is a typical area for studying the spatial and temporal distribution of carbon and nitrogen reservoirs.We found that the NO₃^–and NH₄⁺concentrations of pore water in the salt marsh ecosystem of Chongming Dongtan were 1.48±0.98 and 282±251μmol L^–1,respectively,while the NO₃^–and NH₄⁺concentrations in other coastal ecosystems（including small-scale bays,lagoons,coral reefs and seagrass beds,or large-scale estuaries,shelves and marginal seas）were45-1640μmol L^–1(average:310μmol L^–1)and 0.89-38.1μmol L^–1(mean:8.8μmol L^–1).The NO₃^–concentration of pore water in the salt marsh ecosystem of Chongming Dongtan was much smaller than that of other ecosystems,while NH₄⁺concentration was much larger than other ecosystems.In addition,the NH₄⁺,DSi and DIC concentrations of pore water in the salt marsh ecosystem of Chongming Dongtan were significantly higher than those in coastal waters,while the NO₂^–,NO₃^–and DIP concentrations were lower than those in coastal waters,especially NO₃^–concentration.Thus we only estimated the NH₄⁺,DSi and DIC fluxes by pore water exchange.It should be noted that the NH₄⁺flux by pore water exchange can be approximately equal to the DIN flux by pore water exchange.Based on a ²²²Rn mass balance model,we estimated the pore water exchange rate in the salt marsh ecosystem of Chongming Dongtan was 37±35 cm d^-1.The DIN,DSi and DIC fluxes via pore water exchange in the salt marsh ecosystem of Chongming Dongtan were 0.10,0.08 and 3.20 mol m^–2d^–1,respectively.We found that the DIN and DSi fluxes via pore water exchange in the salt marsh ecosystem of Chongming Dongtan were in the ranges of DIN(0-0.68mol m^–22 d^–1)and DSi(0-1.4 mol m^–22 d^–1)in other ecosystems excluding salt marsh ecosystem,and DIC flux were significantly higher than those from SGD in other study areas excluding salt marsh ecosystem(0.13-2.0 mol m^–22 d^–1).The results indicate that the carbon fluxes from pore water exchange in the salt marsh ecosystem may be an important output of coastal blue carbon.（3）Sandy beach ecosystem:Pore water discharge represents a significant pathway for releasing chemical solutes into coastal zones,particularly in highly permeable aquifers such as sandy beach.Sandy beach is the main component of ice-free coastline,and its aquifer has high permeability.Based on a ²²²Rn advection-diffusion model,pore water discharge was estimated to range from 7.4 to25.8（mean:12.9±5.8）cm d^–1.Furthermore,the estimated pore water-derived DIN,DIP and DSi were（1.7±1.4）×10^–2,（2.1±1.1）×10^–44 and（1.5±1.3）×10^–22 mol m^-22 d^-1,respectively.The pore water Si/N ratio（⁰.92）in the sandy beaches of Shengsi Island and the groundwater Si/N ratio（2.18）along the Yangtze River Estuary were all significantly higher than those in the surface water of the Yangtze River Estuary（⁰.68）.Combining literature data,nutrient fluxes from SGD were the major sources of nutrients in the Yangtze River Estuary.Thus the pore water（or groundwater）with higher Si/N ratio may affect the nearshore ecosystem by changing the composition of the phytoplankton community.By comparing the SGD-derived nutrient fluxes worldwide,this study suggests that Si flux through pore water discharge（or SGD）may strongly influence the Si budget and cycling because such pore water/SGD-derived Si can compensate for the dwindling Si flux from riverine sources due to human activity（i.e.,dam construction,reservoirs）.Our results are expected to increase our understanding of not only biogenic elements cycling processes but also eco-environment processes such as the occurrences of harmful algal blooms along river-influenced coasts with human activities.（4）Karst ecosystem:Anchialine caves are common in Mediterranean karstic shorelines and elsewhere,delivering point-source fresh groundwater and nutrients to the coastal ocean.Here,we first quantified submarine groundwater discharge（SGD）in a typical karstic ecosystem（Zaton Bay,Croatia）affected by anchialine caves using a ²²²Rn mass balance model.We then combined our new estimates with the literature to provide a Mediterranean-scale estimate of karstic fresh SGD nutrient fluxes.We found that SGD and related nutrient fluxes in the upper brackish layer were much higher than those in the underlying layer.In the upper brackish layer,both SGD(m d^-1)and associated nutrient fluxes(mmol m^-22 d^-1)in the wet season（SGD:0.29-0.40;DIN:52;DIP:0.27）were significantly higher than those in the dry season（SGD:0.15;DIN:22;DIP:0.08）.Red tides were observed in the wet season but not in the dry season.Nutrient budgets imply that SGD accounted for more than 98%of the total dissolved inorganic nitrogen（DIN）and phosphorous（DIP）sources into Zaton Bay.These large SGD nutrient fluxes with high N/P ratios（190-320）likely trigger and sustain local red tide outbreaks.Combining our results with other previous studies in the Mediterranean revealed that point-source DIN and DIP fluxes via karstic fresh SGD may account for 11-32%and 1-6%,respectively,of riverine inputs in the Mediterranean Sea.Therefore,this study demonstrates the importance of karstic SGD as a source of“new”nutrients to the Mediterranean Sea and emphasizes its relevance for coastal biogeochemical cycles.In summary,this dissertation quantitatively estimated the SGD or pore water exchange rate in typical coastal mangrove,salt marsh,sandy beach and karst ecosystem and the associated nutrient and carbon fluxes in these ecosystems by using²²²Rn natural radionuclide tracer technique,and then evaluated the nutrient and carbon budgets of these ecosystems.Because of the high carbon sequestration rate（capacity）of mangrove and salt marsh ecosystems,SGD result shows a very high carbon input to coastal ocean.Therefore,the carbon inputs（such as DIC and DOC）via SGD in mangrove and salt marsh ecosystems are more significant than other ecosystems and contribute significantly to coastal blue carbon budget.Furthermore,the results show that SGD plays an important role in the nutrient budgets of these four ecosystems,and also have an important impact on the local ecological environment.