Submarine Groundwater Discharge and its Influence on Primary Production in Japanese Coasts: Case Study in Obama Bay
We report the relationship between submarine groundwater discharge (SGD) and primary production in the nearshore coast of Obama Bay, Japan, using three approaches. First, we conducted high-resolution mapping of 222Rn and biogeochemical properties along the coast. The eastern part of the bay was strongly influenced by groundwater through several direct and indirect pathways. Lower δ15N values in seaweed collected from the eastern area were indicative of larger influences of groundwater. Second, we measured the vertical distributions of 222Rn, salinity, and chlorophyll-a (Chl-a) concentrations along two transects from onshore to offshore at two sites (Tomari and Kogasaki) located on the eastern coast of the bay. In Tomari, Chl-a concentrations were higher in the surface layer in the nearshore coastal area where 222Rn and salinity showed higher and lower values, respectively, due to terrestrial spring water and SGD in the intertidal zone. In contrast, higher 222Rn and Chl-a values were detected in the bottom layer in Kogasaki. This suggested that SGD was composed mainly of recirculated seawater discharge from the seafloor. Finally, temporal variations in multiple parameters related to SGD and phytoplankton production were recorded in Kogasaki in July and November. There was no clear relationship between tide and 222Rn concentrations in either month, but pCO2 and dissolved O2 showed clear diurnal variations. The estimated O2 production rate in July was higher than that in November. This seasonal difference may have been caused by differences in the SGD rate (7.1 cm d−1 in July and 3.7 cm d−1 in November).
KeywordsSubmarine groundwater discharge 222Rn Nutrients Primary production Coastal seas
The transect survey from onshore to offshore in two transects were made in collaboration with Wakasa High School. We are grateful to the diving club members and their teachers, Dr. Yasuyuki Kosaka and Mr. Hiroaki Hirayama, and the headmaster of Wakasa High School for their assistance. This research was financially supported by the R-08-Init Project, entitled "Human-Environmental Security in Asia-Pacific Ring of Fire: Water-Energy-Food Nexus" the Research Institute for Humanity and Nature (RIHN), Kyoto, Japan.
- Honda H, Sugimoto R, Kobayashi S, Tahara D, Tominaga O (2016) Temporal and spatial variation in primary production in Obama Bay. Bull Jpn Soc Fish Oceanogr 80:269–282. (in Japanese with English abstract)Google Scholar
- Kobayashi S, Sugimoto R, Honda H, Miyata Y, Tahara D, Tominaga O, Shoji J, Yamada M, Nakada S, Taniguchi M (2017) High-resolution mapping and time-series measurements of 222Rn concentrations and biogeochemical properties related to submarine groundwater discharge along the coast of Obama Bay, a semi-enclosed sea in Japan. Prog Earth Planet Sci. https://doi.org/10.1186/s40645-017-0124-y
- MacIntyre S, Wanninkhof R, Chanton JP (1995) Trace gas exchange across the air-sea interface in freshwater and coastal marine environments. In: Matson PA, Harris RC (eds) Biogenic trace gases: measuring emissions from soil and water. Blackwell Science, pp 52–97Google Scholar
- Obama City (2017) http://www1.city.obama.fukui.jp/category/page.asp?Page=3741
- Sasajima S, Sakamoto K (1962) Subsurface geology and groundwater of Obama plain, Fukui pref., central Japan. Part 2: groundwater of Obama plain. Memoirs of the Faculty of Liberal Arts, University of Fukui. Ser. II Natural science (in Japanese with English abstract)Google Scholar