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

Shiho Kobayashi,Yoji Miyata,Makoto Yamada,Jun Shoji,Ryo Sugimoto,Makoto Taniguchi,Hisami Honda,Daisuke Tahara,Satoshi Nakada,Osamu Tominaga

doi:10.1186/s40645-017-0124-y

Abstract

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.

Highlights

It is challenging to obtain evidence of the effects of submarine groundwater discharges (SGDs) on marine ecosystems, partly because groundwater is an invisible source of the freshwater supplied to the sea
Flux, and biogeochemical properties Figure 8 shows temporal variations in the 222Rn concentration measured with a Radon detector; the SGD flux measured with a seepage meter, the wind velocity, and salinity levels inside and outside the seepage chamber at the point at Figure 9 shows the fast Fourier transforms (FFTs) power spectra for the SGD flux, the sea level at the station, the concentration of 222Rn, and the wind velocity and salinity inside and outside the chamber
The results of our various observations, including those made with seepage meters, the continuous monitoring of 222Rn concentrations at a single station, and the monitoring survey of 222Rn concentrations from a boat, together with measurements of the biogeochemical properties, suggest that the advection of high-222Rn water plays an important role in controlling the temporal changes in the 222Rn concentration at the station in the zone between B and C, whereas the local SGD might be another source of 222Rn in the period when significant SGD flux was observed

Summary

Introduction

It is challenging to obtain evidence of the effects of submarine groundwater discharges (SGDs) on marine ecosystems, partly because groundwater is an invisible source of the freshwater supplied to the sea.

Results

Conclusion