Presentation Information
[AOS26-P11]Elucidation of Primary Production Processes of Microphytobenthos During Emersion Enhanced by Submarine Groundwater Discharge
*Tatsuya Ozaki1, Rikuto Honda1, Risa Hayashi1, Ryo Sugimoto2, Sousuke Otani3, Tomohiro Komorita1 (1.Prefectural University of Kumamoto, 2.Fukui Prefectural University, 3.Mukogawa Women's University)
Keywords:
Submarine Groundwater Discharge,Tidal flat,Microphytobenthos,Primary production,Ariake Bay,Nutrient
Tidal flats are exposed to direct sunlight on the sediment surface due to tidal cycles, and microphytobenthos (MPB) thrive in these environments (Yamaguchi, 2011). While light conditions on the sediment surface are suitable during emersion (Chevalier et al., 2010), primary production (PP) of MPB has been reported to be limited by decreases in nutrient concentrations in pore water (Ichimi et al., 2008; Ichimi et al., 2013). In recent years, submarine groundwater discharge (SGD) has attracted attention as a major source of nutrients in coastal waters (Wilson et al., 2024). Nutrient supply by SGD has been reported to strongly influence phytoplankton PP (Nakajima et al., 2024); however, few studies have examined the relationship between MPB and SGD on tidal flats. Because SGD has the potential to supply nutrients to tidal flats during emersion, it is likely to be closely related to MPB production.
The study area, the Tidal Flat at the Midori River, faces Ariake Bay and is rich in groundwater resources due to the influence of the Kumamoto Plain (Misonou et al., 2012). In the previous year, we quantified primary production during emersion in areas with confirmed SGD and compared these values with those from areas minimally influenced by SGD. The results demonstrated that SGD enhances primary production in tidal flats (Ozaki et al., 2025, presented at this conference). Moreover, nutrient inputs from SGD during emersion were suggested to contribute to the observed enhancement of primary production.
In SGD-rich areas, three potential nutrient supply pathways exist during emersion: (1) horizontal advection driven by tidal head differences, (2) horizontal advection driven by SGD, and (3) vertical diffusion driven by concentration gradients. Although it is difficult to directly quantify nutrient fluxes associated with horizontal advection, if nutrient uptake by primary production and nutrient fluxes driven by vertical diffusion are measured in situ, and if vertical diffusive fluxes are negligibly small, then horizontal advective nutrient fluxes can be approximated from the nutrient uptake associated with primary production. This approach enables evaluation of the relative importance of nutrient supply by SGD.
Accordingly, the objective of this study was to elucidate the processes by which SGD enhances MPB primary production during emersion. Primary production during emersion was quantified using a chamber method at two sites, one seaward and one landward, within an area where SGD was confirmed. Nutrient fluxes driven by vertical diffusion were also measured, and nutrient uptake calculated from primary production was compared with the vertical diffusive nutrient fluxes.
Primary production during emersion was 2.1 mg C m-2 min-1 at the seaward site and 0.30 mg C m-2 min-1 at the landward site. The corresponding NH4-N uptake rates calculated from primary production were 26 µmol m-2 min-1 at the seaward site and 3.8 µmol m-2 min-1 at the landward site. In contrast, NH4-N fluxes driven by vertical diffusion were 0.014 µmol m-2 min-1 at the seaward site and 0.032 µmol m-2 min-1 at the landward site. The contribution of vertical diffusive fluxes to nutrient uptake associated with primary production was 0.054 percent at the seaward site and 0.83 percent at the landward site, indicating that vertical diffusion was negligibly small. These results suggest that nutrient supply by horizontal advection, including SGD, plays a key role in enhancing primary production during emersion.
The study area, the Tidal Flat at the Midori River, faces Ariake Bay and is rich in groundwater resources due to the influence of the Kumamoto Plain (Misonou et al., 2012). In the previous year, we quantified primary production during emersion in areas with confirmed SGD and compared these values with those from areas minimally influenced by SGD. The results demonstrated that SGD enhances primary production in tidal flats (Ozaki et al., 2025, presented at this conference). Moreover, nutrient inputs from SGD during emersion were suggested to contribute to the observed enhancement of primary production.
In SGD-rich areas, three potential nutrient supply pathways exist during emersion: (1) horizontal advection driven by tidal head differences, (2) horizontal advection driven by SGD, and (3) vertical diffusion driven by concentration gradients. Although it is difficult to directly quantify nutrient fluxes associated with horizontal advection, if nutrient uptake by primary production and nutrient fluxes driven by vertical diffusion are measured in situ, and if vertical diffusive fluxes are negligibly small, then horizontal advective nutrient fluxes can be approximated from the nutrient uptake associated with primary production. This approach enables evaluation of the relative importance of nutrient supply by SGD.
Accordingly, the objective of this study was to elucidate the processes by which SGD enhances MPB primary production during emersion. Primary production during emersion was quantified using a chamber method at two sites, one seaward and one landward, within an area where SGD was confirmed. Nutrient fluxes driven by vertical diffusion were also measured, and nutrient uptake calculated from primary production was compared with the vertical diffusive nutrient fluxes.
Primary production during emersion was 2.1 mg C m-2 min-1 at the seaward site and 0.30 mg C m-2 min-1 at the landward site. The corresponding NH4-N uptake rates calculated from primary production were 26 µmol m-2 min-1 at the seaward site and 3.8 µmol m-2 min-1 at the landward site. In contrast, NH4-N fluxes driven by vertical diffusion were 0.014 µmol m-2 min-1 at the seaward site and 0.032 µmol m-2 min-1 at the landward site. The contribution of vertical diffusive fluxes to nutrient uptake associated with primary production was 0.054 percent at the seaward site and 0.83 percent at the landward site, indicating that vertical diffusion was negligibly small. These results suggest that nutrient supply by horizontal advection, including SGD, plays a key role in enhancing primary production during emersion.