9:45 AM - 10:00 AM
[ACG53-04] Primary production of microphytobenthos enhanced by submarine groundwater discharge (SGD): comparison between muddy and sandy tidal flats
Keywords:Submarine Groundwater Discharge, Tidal flat, Microphytobenthos, Primary production, Ariake Bay
Tidal flats provide important ecosystem services to humans, such as regulating functions (Costanza et al., 1997; Barbier et al., 2011). However, due to direct losses from coastal development and climate change-induced sea level rise and coastal erosion, the area of tidal flats is decreasing by 0.55% per year (Murray et al., 2019). In order to conserve the tidal flats, it is urgent to expand our knowledge of the primary producers that form the tidal flat ecosystem.
Tidal flats are exposed to direct sunlight on the sediment surface due to the tides, and microphytobenthos (MPB) thrive on tidal flats (Yamaguchi, 2011). While the light condition on the sediment surface is suitable during exposure (Chevalier et al., 2010), there are also reports that the primary production (PP) of MPB is limited by evaporation of water and decrease in the nutrient concentration in pore water (Ichimi et al., 2008; Ichimi et al., 2013). In recent years, submarine groundwater discharge (SGD) has been attracting attention as major sources of nutrients in coastal waters (Wilson et al., 2024). Nutrients supplied by SGD strongly affect the PP of phytoplankton has been reported (Nakajima et al., 2024), however there are few studies on the relationship between MPB and SGD on tidal flats. However, the SGD is closely related to the MPB on tidal flats because the SGD has the potential to improve dry conditions and supply nutrients to tidal flats during exposure.
The survey area, the Tidal Flat at Midori River, faces the Ariake Bay and is rich in groundwater resources due to the influence of the Kumamoto Yatsushiro Plain (Misonou et al., 2012). In addition, mud flats and sandy flats are formed discontinuously from the shore to the offshore area. Since photosynthetic activity differs depending on the grain size distribution of the sediment (Underwood & Kromkamp, 1999), the relationship between SGD and the PP of MPB for each grain size distribution is necessary to evaluate.
Therefore, the purpose of this study was to clarify the effect of SGD on the PP of MPB during exposed. We hypothesized that the PP of MPB during exposure would increase significantly due to the improvement of dry conditions and the supply of nutrients by SGD. We then quantified the PP using the chamber method in areas with SGD (SGD area) and areas without SGD (non-SGD area) during the summer, when the amount of sunlight received during exposure is maximum, and compared the increase in PP over time during exposure in both muddy and sandy tidal flats.
In the case of the muddy tidal flat (Fig 1. a), the increase in PP during exposure was significantly higher in the SGD area (104 mgC m–2 h–1) than in the non-SGD area (47 mgC m–2 h–1). On the sandy tidal flat (Fig 1. b), the PP (126 mgC m–2 h–1) also increased significantly with the progress of exposed time in the SGD area, greatly exceeding the value in the non-SGD area (7 mgC m–2 h–1). This suggests that SGD significantly increases the PP during exposure on tidal flats. In the presentation, we will discuss in detail the impact of SGD on the PP during exposure, including the relationship between various environmental factors such as nutrients and salinity in pore water.
Tidal flats are exposed to direct sunlight on the sediment surface due to the tides, and microphytobenthos (MPB) thrive on tidal flats (Yamaguchi, 2011). While the light condition on the sediment surface is suitable during exposure (Chevalier et al., 2010), there are also reports that the primary production (PP) of MPB is limited by evaporation of water and decrease in the nutrient concentration in pore water (Ichimi et al., 2008; Ichimi et al., 2013). In recent years, submarine groundwater discharge (SGD) has been attracting attention as major sources of nutrients in coastal waters (Wilson et al., 2024). Nutrients supplied by SGD strongly affect the PP of phytoplankton has been reported (Nakajima et al., 2024), however there are few studies on the relationship between MPB and SGD on tidal flats. However, the SGD is closely related to the MPB on tidal flats because the SGD has the potential to improve dry conditions and supply nutrients to tidal flats during exposure.
The survey area, the Tidal Flat at Midori River, faces the Ariake Bay and is rich in groundwater resources due to the influence of the Kumamoto Yatsushiro Plain (Misonou et al., 2012). In addition, mud flats and sandy flats are formed discontinuously from the shore to the offshore area. Since photosynthetic activity differs depending on the grain size distribution of the sediment (Underwood & Kromkamp, 1999), the relationship between SGD and the PP of MPB for each grain size distribution is necessary to evaluate.
Therefore, the purpose of this study was to clarify the effect of SGD on the PP of MPB during exposed. We hypothesized that the PP of MPB during exposure would increase significantly due to the improvement of dry conditions and the supply of nutrients by SGD. We then quantified the PP using the chamber method in areas with SGD (SGD area) and areas without SGD (non-SGD area) during the summer, when the amount of sunlight received during exposure is maximum, and compared the increase in PP over time during exposure in both muddy and sandy tidal flats.
In the case of the muddy tidal flat (Fig 1. a), the increase in PP during exposure was significantly higher in the SGD area (104 mgC m–2 h–1) than in the non-SGD area (47 mgC m–2 h–1). On the sandy tidal flat (Fig 1. b), the PP (126 mgC m–2 h–1) also increased significantly with the progress of exposed time in the SGD area, greatly exceeding the value in the non-SGD area (7 mgC m–2 h–1). This suggests that SGD significantly increases the PP during exposure on tidal flats. In the presentation, we will discuss in detail the impact of SGD on the PP during exposure, including the relationship between various environmental factors such as nutrients and salinity in pore water.