High-resolution mapping and time-series measurements of 222 Rn 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 ( 222 Rn) concentrations in the shallow zone in a semi-enclosed sea, Obama Bay, Japan, were undertaken in 2013. The temporal and spatial variations in the 222 Rn 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 222 Rn flowing into the bay from rivers was known to be relatively high because groundwater seeps from the river bed. High- 222 Rn water was almost always present around the river mouth, and northward advection of the water affected the distribution of 222 Rn concentrations in the bay. The southward wind suppressed the advection of the high- 222 Rn water and largely controlled the temporal variations in 222 Rn concentrations at a station located on the north side of the river mouth, whereas the local SGD affected the short-term changes in the 222 Rn concentrations. The concentrations of 222 Rn 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.