10:45 〜 11:00
[ACG42-06] 海底湧水が宮津湾内のトリガイ養殖場の一次生産環境に及ぼす影響
キーワード：海底地下水湧出、栄養塩、一次生産、養殖場、沿岸海域
Trophic transfer of energy and essential biochemicals from phytoplankton biomass supports marine organisms of higher trophic levels, including shellfish aquaculture. Nutrients supplied from submarine groundwater discharge (SGD) as well as riverine nutrients are now recognized as one of the most important sources for phytoplankton production in coastal ecosystems. In Miyazu Bay, Japanese cockle Fulvia mutica aquaculture is popular as a commercially important aquatic resource. Nutrient enriched groundwater discharge has been found at the seafloor under the aquaculture farms. However, the influence of SGD on primary productivity and biomass of phytoplankton has not been still uncleared. In this study, we conducted monthly observations from June to December 2021 at the shellfish aquaculture farm (bottom depth, 12 m) in Miyazu Bay. Seawater samples were taken at five layers from the surface to the bottom to analyze Chl-a concentration, nutrients concentration, 222Rn activity, and 224Ra activity. We also measured the gross primary productivity (GPP) and net primary productivity (NPP) of phytoplankton at each layer using in situ bottle incubation method.
In the upper layer (<= 3 m), higher 224Ra activity with low salinity water showed the influence of lateral inputs of shallow groundwaters as well as river water. On the other hand, the maximum activities of 222Rn and 224Ra in the lower layer during the stratified period supported the steady inputs of SGD from the seafloor. Higher activities of 222Rn and 224Ra were found during the mixing period. These results suggest that the influence of SGD at the aquaculture farm was significant throughout the observation period. The depth impacted by SGD during the stratified period (June to September) is estimated to be 3-7 m from the seafloor by using the activity ratio of 222Rn (t1/2 = 3.8 d) to 224Ra (t1/2 = 3.7 d). Estimated NPP and GPP ranged from -1.65 to 0.83 mgC/L/d and from -0.27 to 0.87 mgC/L/d, respectively. The maximum values of NPP, GPP, and phytoplankton biomass-specific GPP (= GPP/Chl-a) were found in the SGD impacted lower layer. The mean value of vertically integrated GPP during the observation period was 3.6±1.0 gC/m2/d, and 17~59% of the integrated GPP during the stratified period were accounted for the SGD impacted depths. Therefore, we conclude that nutrients delivered via SGD across the seafloor are crucial for primary production at the shellfish aquaculture farm in Miyazu Bay.
In the upper layer (<= 3 m), higher 224Ra activity with low salinity water showed the influence of lateral inputs of shallow groundwaters as well as river water. On the other hand, the maximum activities of 222Rn and 224Ra in the lower layer during the stratified period supported the steady inputs of SGD from the seafloor. Higher activities of 222Rn and 224Ra were found during the mixing period. These results suggest that the influence of SGD at the aquaculture farm was significant throughout the observation period. The depth impacted by SGD during the stratified period (June to September) is estimated to be 3-7 m from the seafloor by using the activity ratio of 222Rn (t1/2 = 3.8 d) to 224Ra (t1/2 = 3.7 d). Estimated NPP and GPP ranged from -1.65 to 0.83 mgC/L/d and from -0.27 to 0.87 mgC/L/d, respectively. The maximum values of NPP, GPP, and phytoplankton biomass-specific GPP (= GPP/Chl-a) were found in the SGD impacted lower layer. The mean value of vertically integrated GPP during the observation period was 3.6±1.0 gC/m2/d, and 17~59% of the integrated GPP during the stratified period were accounted for the SGD impacted depths. Therefore, we conclude that nutrients delivered via SGD across the seafloor are crucial for primary production at the shellfish aquaculture farm in Miyazu Bay.