9:45 AM - 10:00 AM
[ACG42-03] Importance of spatial scale in assessing submarine groundwater discharge in the embayment
Keywords:Submarine groundwater discharge, Nutrients, Radon, Radium, Spatial scale, Wakasa Bay
Submarine groundwater discharge (SGD), which includes the discharge of fresh groundwater (fresh SGD) and saline groundwater (saline SGD), has been increasingly recognized for its role in the pathway of water and nutrients across the seafloor. Although the significance of SGD flux compared to river discharge would be different in various scales assessed, such as the nearshore scale and the whole bay scale, it is still unassessed simultaneously. Here, we assessed the contribution rates of fresh SGD and saline SGD on water and nutrient budgets in Obama Bay dividing into two different scales (i.e., the nearshore (<= 10 m) and the whole bay). In this study, we took 40 seawaters for 222Rn, 224Ra, and nutrients from the surface and bottom seawaters in Obama Bay and outside the bay in summer 2021. To quantify the fluxes of SGD and oceanic inflows, we constructed the mass balance model for water, salt, and 222Rn. Furthermore, the fluxes of fresh SGD and saline SGD in total SGD flux are identified using the 224Ra mass balance model.
The mass balance model revealed that the oceanic inflows accounted for >97% in both scales. Fresh SGD flux was 0.02×106 m3 d−1 (0.1 cm d−1) in the nearshore scale and 1.4×106 m3 d−1 (2.4 cm d−1) in the whole bay scale. Although the relative contribution of fresh SGD in meteoric waters (river discharge + fresh SGD) was <1% in the nearshore scale, that in the whole bay scale increased up to 54%. Saline SGD flux was 5.5×106 m3 d−1 (25.7 cm d−1) in the nearshore scale and 30.2×106 m3 d−1 (54.4 cm d−1) in the whole bay scale. Both fluxes accounted for >90% of total SGD flux. Of all external nutrient sources (river, fresh SGD, saline SGD, diffusion from the sediment, and oceanic water) evaluated in the study, the total SGD-derived DIN, DIP, and DSi fluxes contributed 54%, 47%, and 26% in the nearshore scale, respectively. These contributions increased to 66% for DIN, 60% for DIP, and 47% for DSi in the whole bay scale. The nutrient flux ratios of total SGD (or fresh SGD) to river discharge in the nearshore scale were 3 (<0.1) for DIN, 8 (<0.1) for DIP, and 1 (<0.1) for DSi, while those in the whole bay scale increased up to 19 (2) for DIN, 47 (4) for DIP, and 7 (1) for DSi. These results showed that the relative importance of SGD may become more significant with larger spatial scales because the total SGD flux increase with the extended bottom area in the embayment.
The mass balance model revealed that the oceanic inflows accounted for >97% in both scales. Fresh SGD flux was 0.02×106 m3 d−1 (0.1 cm d−1) in the nearshore scale and 1.4×106 m3 d−1 (2.4 cm d−1) in the whole bay scale. Although the relative contribution of fresh SGD in meteoric waters (river discharge + fresh SGD) was <1% in the nearshore scale, that in the whole bay scale increased up to 54%. Saline SGD flux was 5.5×106 m3 d−1 (25.7 cm d−1) in the nearshore scale and 30.2×106 m3 d−1 (54.4 cm d−1) in the whole bay scale. Both fluxes accounted for >90% of total SGD flux. Of all external nutrient sources (river, fresh SGD, saline SGD, diffusion from the sediment, and oceanic water) evaluated in the study, the total SGD-derived DIN, DIP, and DSi fluxes contributed 54%, 47%, and 26% in the nearshore scale, respectively. These contributions increased to 66% for DIN, 60% for DIP, and 47% for DSi in the whole bay scale. The nutrient flux ratios of total SGD (or fresh SGD) to river discharge in the nearshore scale were 3 (<0.1) for DIN, 8 (<0.1) for DIP, and 1 (<0.1) for DSi, while those in the whole bay scale increased up to 19 (2) for DIN, 47 (4) for DIP, and 7 (1) for DSi. These results showed that the relative importance of SGD may become more significant with larger spatial scales because the total SGD flux increase with the extended bottom area in the embayment.