3:00 PM - 3:15 PM
[AOS21-06] Effect of water intrusion from Bisan Straits on phytoplankton dynamics in Harima Nada in the Seto Inland Sea
Keywords: Mixed region, Stratified region, Phytoplankton dynamics, Delivers nutrient
The Seto Inland Sea (SIS) is consisted of straits and basins. Strong tidal currents facilitate vertical mixing in the straits, while stratification develops in the basins. The density of vertically mixed water in the straits often shows the intermediate between the densities of the upper and the bottom layers in the stratified basin. It is thus expected that density current occurs in the straits–basin transition, and the intermediate-density water intrudes into the middle layer of the stratified water column (Takeoka et al. 1993). It is also expected that the middle layer intrusion delivers nutrient and consequently enhance the phytoplankton production around the transitional region (Yamamoto et al. 2000). The present study examined water mass structure and water quality in Bisan Straits–Harima Nada transition in the SIS. The aim was to reveal the effect of water intrusion from the straits on the phytoplankton dynamics in the basin.
Nine sites were set along eastern Bisan Straits–western Harima Nada transition and visited once a month during April–September 2023 and May–September 2024. Temperature, salinity, and chlorophyll fluorescence were measured vertically and Secchi depth was determined. The sites were categorized as mixed or stratified, depending on the vertical differences in density. The route of water intrusion from the mixed to the stratified region was estimated by tracking the horizontal and vertical locations of isopycnal water. Seawater was taken vertically at the one mixed and two stratified sites and only from the surface at the other sites. These samples were used for the analysis of chlorophyll-a (Chl-a), dissolved inorganic nitrogen (DIN), and so on.
The vertical difference in density in Harima Nada became larger from spring to early summer both in 2023 and 2024, showing stratification developed with the time. The distribution of isopycnal water proposed that water intrusion from edge of the mixed site to the stratified sites occurred typically around 10-20 m depth within the euphotic zone. In early summer, subsurface Chl-a maximum was frequently observed at those depths in the stratified sites. The mean DIN in the mixed sites was increased from spring to early summer, and the trend was similar to the bottom DIN in the stratified sites, a supply source of DIN for the mixed region. On the other hand, the mean surface DIN in the stratification sites showed the opposite trend and decreased with the time. As a result, the former DIN was distinctly higher than the latter one in early summer. The present study suggests that co-occurrence of stratification development in Harima Nada and nutrient elevation in Bisan Straits enhances the subsurface phytoplankton production during early summer in the transitional region.
Nine sites were set along eastern Bisan Straits–western Harima Nada transition and visited once a month during April–September 2023 and May–September 2024. Temperature, salinity, and chlorophyll fluorescence were measured vertically and Secchi depth was determined. The sites were categorized as mixed or stratified, depending on the vertical differences in density. The route of water intrusion from the mixed to the stratified region was estimated by tracking the horizontal and vertical locations of isopycnal water. Seawater was taken vertically at the one mixed and two stratified sites and only from the surface at the other sites. These samples were used for the analysis of chlorophyll-a (Chl-a), dissolved inorganic nitrogen (DIN), and so on.
The vertical difference in density in Harima Nada became larger from spring to early summer both in 2023 and 2024, showing stratification developed with the time. The distribution of isopycnal water proposed that water intrusion from edge of the mixed site to the stratified sites occurred typically around 10-20 m depth within the euphotic zone. In early summer, subsurface Chl-a maximum was frequently observed at those depths in the stratified sites. The mean DIN in the mixed sites was increased from spring to early summer, and the trend was similar to the bottom DIN in the stratified sites, a supply source of DIN for the mixed region. On the other hand, the mean surface DIN in the stratification sites showed the opposite trend and decreased with the time. As a result, the former DIN was distinctly higher than the latter one in early summer. The present study suggests that co-occurrence of stratification development in Harima Nada and nutrient elevation in Bisan Straits enhances the subsurface phytoplankton production during early summer in the transitional region.