3:30 PM - 3:45 PM
[AOS21-07] On the mechanism of pH drawdown after rainfall ivents in Shizugawa Bay
Keywords:coastal acidification, rainfall, riverine organic substances, primary production
Results of previous monitoring showed that coastal waters around Japan keep satisfactory high pH vales above the biological thresholds of coastal organisms in most of the time, while short time drawdown of pH sometimes occurs after the rainfall events (Ono et al., 2024a). To examine detailed mechanism of these short time pH drawdown, we made multi-sensor observation in Shizugawa Bay, Miyagi prefecture in Japan, during a heavy rainfall event in July 2022 (Ono 2024b). We observed significant increase of turbidity in coastal waters just after the rainfall, with successive drawdown of pH accompanied with the decrease of oxygen. Chla in coastal waters had increased several days after these events. Based on these results, we concluded that the drawdown of pH had caused by the degradation of riverine organic materials, while the organic materials produced in the bay using riverine nutrients had no contribution at this rainfall event. However, it was still unknown whether the process observed in this study is prevailing in other rainfall events in this bay and the other coastal aeras in Japan.
To evaluate the generality of the mechanism observed in recent study, we extracted peak signals of surface water temperature, salinity, oxygen, and pH across the 55 rainfall events in Shizugawa Bay, from the 5-year hydrographic data after 2020 of the time series station in near-center of Shizugawa Bay maintained by the Ocean Acidification Adaptation Project funded by Nippon Foundation. 53 rainfall events had accompanied by negative peaks of pH (i.e, pH drawdown events). The extent of pH drawdown showed statistically significant positive correlation with both the integrated rainfall and the duration of the accompanied salinity drawdown peak, suggesting that the extent of pH drawdown had controlled by both the amount and retention time of low salinity water created by the rainfall.
All rainfall event had been accompanied by simultaneous or immediate increase of turbidity, and in the 77 % of the rainfall event, the drawdown of pH had been stared after the increase of turbidity similarly to the case of previous observation in Ono (2024b). Although Chla started to increase after the beginning of pH drawdown in 62 % of the rainfall event, undeniable numbers of the rainfall events (ca, 38% of the total) showed Chla increase before initiation of the pH drawdown. When Chla had increased beforehand of pH drawdown, the created organic substances may immediately decay and act as additional source of negative pH in addition to the decomposition of riverine organic materials. In this case, the extent of pH drawdown should become larger than the case of no Chla increase beforehand of the pH drawdown. We, therefore, plotted the extent of pH drawdown in each rainfall event against the difference of the date of positive Chla peak and negative pH peak (plus value of X-axis means that the Chla increased after the pH drawdown). The result, however, showed positive correlation between the X-axis and Y-axis, meaning that the extent of pH drawdown become small when Chla had increased before the pH drawdown. This result indicate that the organic substances produced by the increased Chla just after the rainfall do not decay so fast, and hence they do not act as the source of pH drawdown after the rainfall event. Rather, the increase Chla diminish the extent of pH drawdown by retrieving oceanic DIC increase that is caused by the decomposition of riverine organic substances.
To evaluate the generality of the mechanism observed in recent study, we extracted peak signals of surface water temperature, salinity, oxygen, and pH across the 55 rainfall events in Shizugawa Bay, from the 5-year hydrographic data after 2020 of the time series station in near-center of Shizugawa Bay maintained by the Ocean Acidification Adaptation Project funded by Nippon Foundation. 53 rainfall events had accompanied by negative peaks of pH (i.e, pH drawdown events). The extent of pH drawdown showed statistically significant positive correlation with both the integrated rainfall and the duration of the accompanied salinity drawdown peak, suggesting that the extent of pH drawdown had controlled by both the amount and retention time of low salinity water created by the rainfall.
All rainfall event had been accompanied by simultaneous or immediate increase of turbidity, and in the 77 % of the rainfall event, the drawdown of pH had been stared after the increase of turbidity similarly to the case of previous observation in Ono (2024b). Although Chla started to increase after the beginning of pH drawdown in 62 % of the rainfall event, undeniable numbers of the rainfall events (ca, 38% of the total) showed Chla increase before initiation of the pH drawdown. When Chla had increased beforehand of pH drawdown, the created organic substances may immediately decay and act as additional source of negative pH in addition to the decomposition of riverine organic materials. In this case, the extent of pH drawdown should become larger than the case of no Chla increase beforehand of the pH drawdown. We, therefore, plotted the extent of pH drawdown in each rainfall event against the difference of the date of positive Chla peak and negative pH peak (plus value of X-axis means that the Chla increased after the pH drawdown). The result, however, showed positive correlation between the X-axis and Y-axis, meaning that the extent of pH drawdown become small when Chla had increased before the pH drawdown. This result indicate that the organic substances produced by the increased Chla just after the rainfall do not decay so fast, and hence they do not act as the source of pH drawdown after the rainfall event. Rather, the increase Chla diminish the extent of pH drawdown by retrieving oceanic DIC increase that is caused by the decomposition of riverine organic substances.