Evaluation of Submarine Groundwater Discharge(SGD) effects on the fisheries resources based on stable & radio isotopes techniques

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Terrestrial freshwater and associated nutrient fluxes affect primary production at coastal areas. Differently from river water like Chikugo River, however, the contribution of groundwater discharge to coastal ecosystem has not been well evaluated in Ariake Bay. In this study,^<222> Rn was considered as a good indicator of groundwater, because^<222> Rn concentrations in groundwater along the west coast of Ariake Bay were one or three orders of magnitude higher than those in river waters.^<222> Rn distribution standardized based on the accumulated rainfall and water depth at the sampling location successfully indicated a spatial variation of groundwater discharge. Furthermore, total groundwater fluxes were evaluated from the^<222> Rn distributions, because spatial distributions of^<222> Rn were correlated with actual groundwater fluxes measured at 5 different locations. Nutrient fluxes through the groundwater discharge had also spatial variation ; 7. 8 to 510 kg-N day^<-1> for DIN(dissolved inorganic nitrogen), and 0. 5 to 3. 9 kg-P day^<-1> for DIP(dissolved inorganic phosphorus), with the maximum values at Shimabara area, where both groundwater fluxes and nutrient concentrations in groundwater were highest. On the other hand, the impact of groundwater-derived nutrients on marine ecosystem at offshore area was not observed from the researches usingδ^<15> N in nitrate andδ^<13> C andδ^<15> N in organic matter. Along the west coast of Ariake Bay, where there are no big rivers, groundwater can be important nutrient sources to enhance primary production, resulting in good food sources for bivalves and blooming of green macroalgae. However, the influential areas of groundwater-derived nutrients would be limited in shallow coastal areas.