1:45 PM - 2:00 PM
[AOS22-01] Temporal changes in nutrient status and its potential impact on phytoplankton dynamics in Shido Bay, an active oyster farming area in the Seto Inland Sea
Nutrient concentration has recently decreased with time in some enclosed coastal areas. The phenomenon is so-called de-eutrophication. De-eutrophication may affect phytoplankton biomass and fisheries yield in the area. It is thus an important issue from the view of fisheries. Shido Bay is a shallow embayment in the Seto Inland Sea, where symptom of de-eutrophication has been reported. This small bay (ca., 20 km2) is extensively utilized for aquaculture. Cultured oyster yield in the bay occupies about half of the total yield in Kagawa Prefecture. This study aims to reveal how light and nutrient status in Shido Bay varied from 2011 to 2021. The present study also explored possible effect of recent changes in nutrient state on phytoplankton dynamics and oyster farming in the bay.
Field sampling was conducted at a site from January 2011 to December 2021. Concentrations of nutrient (dissolved inorganic nitrogen, phosphorus and silica; DIN, DIP and DSi) and chlorophyll-a (Chl-a), as well as attenuation coefficient of photosynthetically active radiation in the water column (Kd) were determined at each occasion. The data on respective parameter were gathered each year and the temporal trends were analyzed. In addition to the field sampling, incubation experiment to test the effect of DIN level on phytoplankton productivity (gC gChl-a-1 d-1) was conducted on July and October 2021; 13C-based phytoplankton productivity without (i.e. intact seawater) and with addition of DIN (+1-30 µM) were compared in the experiments. Then, DIP and DSi were enough added to avoid nutrient limitation other than DIN.
Field sampling since 2011 showed that 65% of the measured DIN were less than 1.0 µM. Molar DIN/DIP and DSi/DIN ratio in the seawater was almost always <16 and >1, indicating DIN was judged to be the most insufficient nutrient for the phytoplankton growth, irrespective of the season and year. The annual median DIN tended to decrease with time; the highest value was recorded in 2011, while the lowest was observed in 2021. The mean DIN concentration at each month after 2011 was almost always lower than that of the 2000s (2002-08) reported by Kaeriyama et al. (2011). These results suggest that DIN concentration continuously decreased at least since the 2000s. Chlorophyll-a since 2011 did not show clear temporal trend. While, artificial addition of DIN to the intact seawater obviously raised phytoplankton productivity (gC gChl-a-1 day-1) both on July and October 2021. These results may suggest that phytoplankton production in the bay (gC m-2 day-1) might decrease during a few decades in relation to changes in DIN, though the Chl-a level did not change largely. If so, recent decrease on oyster yield in Kagawa Prefecture may be at least partly explained by DIN decrease in the bay.
Field sampling was conducted at a site from January 2011 to December 2021. Concentrations of nutrient (dissolved inorganic nitrogen, phosphorus and silica; DIN, DIP and DSi) and chlorophyll-a (Chl-a), as well as attenuation coefficient of photosynthetically active radiation in the water column (Kd) were determined at each occasion. The data on respective parameter were gathered each year and the temporal trends were analyzed. In addition to the field sampling, incubation experiment to test the effect of DIN level on phytoplankton productivity (gC gChl-a-1 d-1) was conducted on July and October 2021; 13C-based phytoplankton productivity without (i.e. intact seawater) and with addition of DIN (+1-30 µM) were compared in the experiments. Then, DIP and DSi were enough added to avoid nutrient limitation other than DIN.
Field sampling since 2011 showed that 65% of the measured DIN were less than 1.0 µM. Molar DIN/DIP and DSi/DIN ratio in the seawater was almost always <16 and >1, indicating DIN was judged to be the most insufficient nutrient for the phytoplankton growth, irrespective of the season and year. The annual median DIN tended to decrease with time; the highest value was recorded in 2011, while the lowest was observed in 2021. The mean DIN concentration at each month after 2011 was almost always lower than that of the 2000s (2002-08) reported by Kaeriyama et al. (2011). These results suggest that DIN concentration continuously decreased at least since the 2000s. Chlorophyll-a since 2011 did not show clear temporal trend. While, artificial addition of DIN to the intact seawater obviously raised phytoplankton productivity (gC gChl-a-1 day-1) both on July and October 2021. These results may suggest that phytoplankton production in the bay (gC m-2 day-1) might decrease during a few decades in relation to changes in DIN, though the Chl-a level did not change largely. If so, recent decrease on oyster yield in Kagawa Prefecture may be at least partly explained by DIN decrease in the bay.