Hisami Honda

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

High-resolution mapping along the coast and time-series measurements of the radon-222 (222Rn) concentrations in the shallow zone in a semi-enclosed sea, Obama Bay, Japan, were undertaken in 2013. The temporal and spatial variations in the 222Rn concentrations were analyzed in parallel with meteorological conditions, physical–biogeochemical characteristics, and the submarine groundwater discharge (SGD) flux measured with a seepage meter. These data indicate that the groundwater influences the water properties of the bay and that the groundwater supply pathways are not limited to the local SGD. The concentrations of 222Rn flowing into the bay from rivers was known to be relatively high because groundwater seeps from the river bed. High-222Rn water was almost always present around the river mouth, and northward advection of the water affected the distribution of 222Rn concentrations in the bay. The southward wind suppressed the advection of the high-222Rn water and largely controlled the temporal variations in 222Rn concentrations at a station located on the north side of the river mouth, whereas the local SGD affected the short-term changes in the 222Rn concentrations. The concentrations of 222Rn and chlorophyll-a, an indicator of phytoplankton biomass, show a significant positive correlation in the surface layer along the coastline in seasons when the nutrient supply was the main factor limiting primary productivity.

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).

Tradeoff Between Hot spring Use and River Ecosystem: The Case of Beppu City, Oita Prefecture, Japan

In Hirata River at Beppu, Oita prefecture, the inflow of hot spring drainage into the rivers increases the Nile tilapia, an alien species. It is a tradeoff between the hot spring use and river ecosystem. In order to understand details of the mechanisms connecting hot spring use and ecosystem, we investigated the river water quality and the water temperature from the upper reaches to the river mouth in the Hirata River. The results show that the hot spring drainage flowing in a specific section affects the estuary area. Additionally, it is revealed that the current average water temperature of hot spring drainage is very good condition for Nile tilapia in the estuary area. It is suggested that the tradeoffs between the hot spring use and the ecosystems can be resolved by suppressing high-temperature unused hot spring drainage, and/or by collecting hot spring drainage and reduce its average temperature further by using its thermal energy more extensively.