Seasonal Changes in Submarine Groundwater Discharge and Associated Nutrient Transport into a Tideless Semi-enclosed Embayment (Obama Bay, Japan)

Abstract

We carried out a seasonal study of fresh submarine groundwater discharge (SGD) and associated nutrient fluxes in a semi-enclosed bay along a tideless coastal zone using a 222 Rn and salinity mass balance model for a whole bay scale. The resulting SGD rates showed large intra-annual variability from 0.05×10 6 to 0.77×10 6 m 3 day −1 , which were controlled by seasonal changes in the interaction of multiple driving forces, including water table height and seawater level. The highest SGD rate in early spring was induced by heavy snow and low sea level, whereas the seasonal increase in sea level gradually suppressed fresh SGD rates. In summer, an elevated water table may induce higher SGD rates (approximately 0.4×10 6 m 3 day −1 ) regardless of high sea levels. The highest SGD fraction in total terrestrial freshwater fluxes also occurred in summer (>40 %), due to the decreasing rate of surface river discharge. The seasonally averaged SGD rate was 0.36×10 6 m 3 day −1 . This value was similar to the annual groundwater recharge rate (0.33×10 6 m 3 day −1 ) estimated by the water balance method in the basin. Nutrient fluxes from SGD were approximately 42, 65, and 33 % of all terrestrial fluxes of dissolved inorganic nitrogen, phosphorous, and silicate, respectively. The average fraction of SGD in the water fluxes including terrestrial and oceanic water was low (0.3 %), but that of nutrient fluxes increased to 20-38 %. Higher nutrient concentrations in groundwater compensated for the lower volumetric flux of groundwater. Because primary production was mostly restricted by phosphorous throughout the year, phosphorous-enriched nutrient transport via SGD would play an important role in biological production.

Groundwater dynamics in subterranean estuaries of coastal unconfined aquifers: Controls on submarine groundwater discharge and chemical inputs to the ocean

“…Recharge to CUAs often varies seasonally leading to seasonal variations in FSGD [Michael et al, 2005;Charette, 2007;Miyaoka, 2007;Moore, 2007;Loveless et al, 2008;Sugimoto et al, 2016]. Seasonal recharge patterns also cause the DZ of the saltwater wedge to oscillate landward and seaward [Michael et al, 2005].…”

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

“…The average salinity in March was 29.0 ± 2.7, much lower than outside the bay at the end of February (33.3; Sugimoto et al 2016). The average salinity was lowest in September (26.5 ± 2.6) in Rn (dpm/L), b salinity, c phosphorous (μM), and d nitrate (μM), showing the minimum and maximum values, 25th and 75th percentiles, and median values, from data obtained by monitoring the coastline from a boat in March, June, July, and September response to heavy typhoon rain.…”

“…For the data analysis, we used the 222 Rn concentrations in the river water flowing into the bay, which had been measured monthly in 2013 by Sugimoto et al (2016). Because approximately 80% of the riverine 222 Rn entering the bay is contributed by the two major rivers (Kita and Minami rivers), the mean 222 Rn concentrations in the two rivers were assumed to represent the value for the surface river waters .…”

“…Because approximately 80% of the riverine 222 Rn entering the bay is contributed by the two major rivers (Kita and Minami rivers), the mean 222 Rn concentrations in the two rivers were assumed to represent the value for the surface river waters . We also used the 222 Rn concentrations and salinity in the terrestrial groundwater measured in March 2013 by Sugimoto et al (2016).…”

“…Using this powerful tracer, the influence of SGD on benthic microalgae was evaluated in the intertidal zone of the Yellow Sea (Waska 2011;Waska and Kim 2010). The nutrient fluxes caused by SGD and their potential effects on primary production have also been demonstrated in several coastal seas around Japan using 222 Rn (Shiokawa et al 2013;Sugimoto et al 2016).…”