S-Space College of Natural Sciences (자연과학대학) Dept. of Earth and Environmental Sciences (지구환경과학부) Theses (Ph.D. / Sc.D._지구환경과학부)
Linking red tides and submarine groundwater discharge (SGD) in the coastal zone using a Ra tracer
연안역에서 라듐 추적자를 이용한 해저 지하수 유출과 적조발생의 연관성 연구
- Issue Date
- 서울대학교 대학원
- 해저 지하수; Submarine Groundwater Discharge (SGD); 라듐; Ra isotopes; 적조; Red tide; 부영양화; Eutrophication; 남해; Southern sea of Korea; 마산만; Masan Bay
- Thesis(doctors)--서울대학교 대학원 :지구환경과학부,2008.8.
- In order to determine the role of submarine groundwater discharge (SGD) associated nutrient fluxes on the outbreaks of red tides, radium isotopes (223Ra, 224Ra, and 226Ra) and dissolved inorganic and organic nutrients were measured in red-tide regions off the southern coast of Korea. The environmental and biological conditions of red-tide regions were characterized by measuring photosynthetic pigments and other environmental parameters. In addition, a real-time monitoring of biogeochemical parameters was conducted at a fixed station in order to look at the physical and chemical factors controlling the outbreak of red tides. Good correlations were observed among dissolved inorganic nitrogen (DIN), silicate (DSi), and groundwater-borne 224Ra (half-life = 3.66 days) in the red-tide region. This suggests that the main source of nutrients fueling red tides is coastal groundwater, rather than other previously suggested sources (such as Yangtze-River diluted water or Kuroshio warm currents) in the southern sea of Korea. Although the conditions of inorganic-nutrient levels for the outbreak of red tides are different for different regions and different years, it is consistent that either dissolved inorganic nitrogen (DIN) or phosphorus (DIP) is depleted in the red-tide areas along with elevated levels of dissolved organic nutrients, favorable conditions for the growth of dinoflagellates in competition with diatoms. This is also confirmed by the real-time monitoring data since bioluminescence, which may indicate the presence of bioluminescent dinoflagellate species, showed a sudden increase under the lowest concentrations of dissolved inorganic nutrients. The intensity of red tides correlates well with the activity of 224Ra in seawater over daily or yearly time scales. The chemically conservative 224Ra traces groundwater-borne nutrients which are utilized by marine biota in this red-tide region. Thus, our results suggest that the intensity of red tides is related to the magnitude of nutrient-enriched groundwater supply to the offshore red-tide region. The magnitude of SGD and the associated nutrient fluxes were measured in Masan Bay on the southern coast of Korea, where diatom red tides occur every year from April to October. Based on a 226Ra mass balance model, the submarine inputs of coastal groundwater were estimated to be 4.8×106 and 5.7×106 m3 d-1 (61 and 71 L m-2 d-1) in May and August 2006, respectively, which were approximately eight- (for May) and five- (for August) fold higher than the surface water discharge into the bay. The fluxes of DIP and DSi via SGD were two- to three-fold higher than those via stream water, while DIN flux was comparable to that from surface waters during both sampling periods. This study shows that the large and continuous supply of inorganic nutrients through SGD may play an important role in eutrophication and in the occurrence of red tides in Masan Bay, which should be taken into consideration in the environmental management of the bay.
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