10:15 AM - 10:30 AM
[ACG40-06] Seasonal variation in phytoplankton primary production at a shellfish aquaculture farm in Miyazu Bay: evaluation by pulse-amplitude modulation (PAM) fluorometry
Keywords:Coastal sea, Aquaculture, Nutrients, Primary production, PAM
Trophic transfer of energy and essential elements from phytoplankton biomass supports marine organisms of higher trophic levels, including shellfish aquaculture. External nutrient supply is crucial for high phytoplankton primary production. Submarine groundwater discharge (SGD) as well as riverine nutrients are now recognized as one of the most important nutrient sources for phytoplankton production in coastal ecosystems. In Miyazu Bay located in the western part of Wakasa Bay, shellfish aquaculture is popular as a commercially important aquatic resource. The discharge of nutrient-enriched groundwater has been found from the seafloor under the aquaculture farms. Therefore, assessment of phytoplankton dynamics related to submarine groundwater discharge in the aquaculture farm is essential. However, little is known about the net primary production of phytoplankton within Miyazu Bay. In this study, we conducted monthly observations from November 2021 to December 2022 at the shellfish aquaculture farm (bottom depth, 12m) in Miyazu Bay. We took seawater samples at five layers from surface to bottom and analyze Chl-a concentration, nutrient concentration, 222Rn activity, and Radium activity. We assessed the net primary productivity (NPP) of phytoplankton using Pulse Amplitude Modulation (PAM) fluorometry.
Chl-a concentrations ranged from 0.03 to 5.80 mg/L. Higher concentrations were obtained in all layers during the winter and near the bottom layer during the summer. Vertically integrated Chl-a concentrations varied seasonally from 3.7 mg/m2 in September 2022 to 38.3 mg/m2 in February 2022. They showed a positive correlation with concentrations of dissolved inorganic nitrogen (DIN) and phosphorous (DIP). Vertically integrated NPP varied seasonally: lower in summer and higher in winter. Annual total NPP (166.71 g C/m2/y) is comparable to the estimate in the Seto Inland Sea. During the winter, NPP in the upper layer (≦ 3 m) accounted for more than 50% of that in all layers. On the other hand, NPP from spring to summer was higher in the lower layer (≧ 6 m), accounting for 37-45% of that in all layers. Vertically integrated phytoplankton biomass-specific NPP (=NPP/Chl-a) was lower in winter (11.89 g C/mg Chl-a/d in January 2022) and higher in summer (84.48 g C/mg Chl-a/d in September 2022). They showed a positive correlation with total solar radiation and a negative correlation with nutrient concentrations. Moreover, in the presentation, we will show the effects of groundwater discharge on seasonal variations in those values.
Chl-a concentrations ranged from 0.03 to 5.80 mg/L. Higher concentrations were obtained in all layers during the winter and near the bottom layer during the summer. Vertically integrated Chl-a concentrations varied seasonally from 3.7 mg/m2 in September 2022 to 38.3 mg/m2 in February 2022. They showed a positive correlation with concentrations of dissolved inorganic nitrogen (DIN) and phosphorous (DIP). Vertically integrated NPP varied seasonally: lower in summer and higher in winter. Annual total NPP (166.71 g C/m2/y) is comparable to the estimate in the Seto Inland Sea. During the winter, NPP in the upper layer (≦ 3 m) accounted for more than 50% of that in all layers. On the other hand, NPP from spring to summer was higher in the lower layer (≧ 6 m), accounting for 37-45% of that in all layers. Vertically integrated phytoplankton biomass-specific NPP (=NPP/Chl-a) was lower in winter (11.89 g C/mg Chl-a/d in January 2022) and higher in summer (84.48 g C/mg Chl-a/d in September 2022). They showed a positive correlation with total solar radiation and a negative correlation with nutrient concentrations. Moreover, in the presentation, we will show the effects of groundwater discharge on seasonal variations in those values.