11:15 AM - 11:30 AM
[ACG39-08] Major nutrient sources during the red tide expansion period in the Yatsushiro Sea, Japan
Keywords:red tide, nutrient, Yatsushiro Sea, Karenia mikimotoi
The Yatsushiro Sea, located in western Kyushu, is a closed inner bay surrounded by the Kyushu mainland and the Amakusa Islands. The northern part of the sea is influenced by the Kuma River, a first class river, and the southern part is influenced by the open ocean from the Tsushima Warm Current flowing in from the Ushibuka Nagashima Strait and Kuro Seto Strait. In the Yatsushiro Sea, fish farming is particularly important as an industry, and the growth characteristics of phytoplankton, which causes damage to fisheries, and the prediction of their occurrence have been conducted. In order to elucidate the mechanism of occurrence and maintenance of red tide plankton, it is necessary to develop an ecosystem model that takes into account both biological and physical processes in the marine ecosystem. In the Yatsushiro Sea, however, there is limited knowledge on nutrient dynamics, which is essential for the development of ecosystem models.
This study aimed to clarify the sources of nutrients during the expansion process of Karenia mikimotoi red tides that occurred in the Yatsushiro Sea from 2015 to 2017. First, a period of time was defined based on the flow rate, K. mikimotoi cell counts, and TS diagrams. Next, water temperature and dissolved inorganic nitrogen (DIN) concentrations at the Yokoishi station of the Kuma River were obtained for each period (Hydrochemical Database of Japan, Ministry of Land, Infrastructure, Transport and Tourism, http://www1.river.go.jp/). In addition to river water, surface water (high salinity and high water temperature: mainly in the coastal areas of the southern sea area) and bottom water (minimum water temperature: mainly at depths greater than 40 m around representative points in the southern sea area) were defined as end members during each period. Using these results, this study discusses the nutrient supply process during the red tide expansion period.
River flows ranged in each year from a maximum of about 1,250 m3 s-1 in 2017 to about 2,500 m3 s-1 in 2016; K. mikimotoi reached a maximum in July 2015 of about 150,000 cells ml-1 In 2015, K. mikimotoi red tides were reported to occur in the western part of the Yatsushiro Sea and spread to the central part of the Yatsushiro Sea under the influence of winds (Aoki et al. It has been reported (Aoki et al. 2023) that K. mikimotoi red tides did not occur in 2014 despite moderate flow rates, and in 2017 the maximum cell count exceeded 20,000 cells ml-1 despite the lowest flow rates, simply because of the scale and timing of K. mikimotoi red tides, indicating that K. mikimotoi red tides do not occur in accordance with the size and timing of K. mikimotoi red tides.
In 2015, when K. mikimotoi cell counts remained high for a long period of time, surface salinity was below 28 for about a month after the river runoff. Correspondingly, an increase in chlorophyll-a concentrations above 10 µg L-1 was observed from the surface to 15 m depth. The number of K. mikimotoi cells increased rapidly when the chlorophyll-a concentration decreased, and the cell density remained high until the end of August. Measured DIN concentrations were below 2 µM from the surface to 10 m for most of the period, but exceeded 4 µM at 10 m depth in late July and late August. According to the mixing model calculations, the influence of river-derived DIN on the southern part of the Yatsushiro Sea was limited to the surface layer, with a maximum concentration of about 3 µM, and only for a short period from late August to early September. On the other hand, bottom-derived DIN concentrations were as high as 2 µM even at 5 m depth during most periods, and 3-4 µM at 10 m depth during most periods, considering that the diurnal vertical migration of K. mikimotoi reaches up to 20 m (Shikata et al. 2017), bottom-derived DIN is likely to play an important role in maintaining K. mikimotoi spread by wind. N uptake, estimated by subtracting the measured DIN concentration from the supplied DIN concentration, was high at depths of 10-20 m, reaching 2-4 µM. However, such high DIN concentrations from bottom water were observed not only in 2015 but also in other years. It should be noted that this only means that the concentration of bottom-derived DIN was high when the red tide occurred, and that high bottom-derived DIN does not necessarily mean that the red tide becomes large-scale. As a future issue, it is necessary to quantitatively evaluate the impact of bottom-derived DIN on the maintenance and expansion of red tides.
This study aimed to clarify the sources of nutrients during the expansion process of Karenia mikimotoi red tides that occurred in the Yatsushiro Sea from 2015 to 2017. First, a period of time was defined based on the flow rate, K. mikimotoi cell counts, and TS diagrams. Next, water temperature and dissolved inorganic nitrogen (DIN) concentrations at the Yokoishi station of the Kuma River were obtained for each period (Hydrochemical Database of Japan, Ministry of Land, Infrastructure, Transport and Tourism, http://www1.river.go.jp/). In addition to river water, surface water (high salinity and high water temperature: mainly in the coastal areas of the southern sea area) and bottom water (minimum water temperature: mainly at depths greater than 40 m around representative points in the southern sea area) were defined as end members during each period. Using these results, this study discusses the nutrient supply process during the red tide expansion period.
River flows ranged in each year from a maximum of about 1,250 m3 s-1 in 2017 to about 2,500 m3 s-1 in 2016; K. mikimotoi reached a maximum in July 2015 of about 150,000 cells ml-1 In 2015, K. mikimotoi red tides were reported to occur in the western part of the Yatsushiro Sea and spread to the central part of the Yatsushiro Sea under the influence of winds (Aoki et al. It has been reported (Aoki et al. 2023) that K. mikimotoi red tides did not occur in 2014 despite moderate flow rates, and in 2017 the maximum cell count exceeded 20,000 cells ml-1 despite the lowest flow rates, simply because of the scale and timing of K. mikimotoi red tides, indicating that K. mikimotoi red tides do not occur in accordance with the size and timing of K. mikimotoi red tides.
In 2015, when K. mikimotoi cell counts remained high for a long period of time, surface salinity was below 28 for about a month after the river runoff. Correspondingly, an increase in chlorophyll-a concentrations above 10 µg L-1 was observed from the surface to 15 m depth. The number of K. mikimotoi cells increased rapidly when the chlorophyll-a concentration decreased, and the cell density remained high until the end of August. Measured DIN concentrations were below 2 µM from the surface to 10 m for most of the period, but exceeded 4 µM at 10 m depth in late July and late August. According to the mixing model calculations, the influence of river-derived DIN on the southern part of the Yatsushiro Sea was limited to the surface layer, with a maximum concentration of about 3 µM, and only for a short period from late August to early September. On the other hand, bottom-derived DIN concentrations were as high as 2 µM even at 5 m depth during most periods, and 3-4 µM at 10 m depth during most periods, considering that the diurnal vertical migration of K. mikimotoi reaches up to 20 m (Shikata et al. 2017), bottom-derived DIN is likely to play an important role in maintaining K. mikimotoi spread by wind. N uptake, estimated by subtracting the measured DIN concentration from the supplied DIN concentration, was high at depths of 10-20 m, reaching 2-4 µM. However, such high DIN concentrations from bottom water were observed not only in 2015 but also in other years. It should be noted that this only means that the concentration of bottom-derived DIN was high when the red tide occurred, and that high bottom-derived DIN does not necessarily mean that the red tide becomes large-scale. As a future issue, it is necessary to quantitatively evaluate the impact of bottom-derived DIN on the maintenance and expansion of red tides.