3:45 PM - 4:00 PM
[AOS21-08] Influence of river water and lower shelf water on chlorophyll a distribution in the shelf area off the coast of Boso, Japan
Keywords:Shelf, Nutrient transport, Upwelling, Vertical mixing, Tone river, Kuroshio
The continental shelf areas of the Japanese archipelago are strongly influenced by the western boundary and coastal currents, and the estuarine and inner bay areas interact actively with the open ocean. Although biological production in the continental shelf region is considered to be higher than that in the open ocean, it is not clear how the nutrients supporting this production are brought to the region. Our research group is working to elucidate the circulation and mixing processes of materials related to the production in the coastal areas under JSPS Grant-in-Aid for Transformative Research Areas “Macro Coastal Oceanography”. In this study, we analyzed observation data obtained from the research cruises by the R/V Shinsei Maru off the coast of Boso, in order to clarify the effects of river water and lower shelf water masses on the distribution of water temperature, salinity, and chlorophyll a in the continental shelf areas.
The data used were underway continuous observation data obtained by a thermosalino-fluorograph during 12 cruises from 2019 to 2024, and underway CTD (UCTD) cross-sectional observation data conducted off Choshi during five cruises from 2021 to 2024. The UCTD observations were conducted from a depth of about 50 m off Choshi seaward, about 35 km from the surface to just above the seafloor, and acquired temperature, salinity, dissolved oxygen, chlorophyll a fluorescence, and turbidity profiles with a horizontal resolution of about 0.5–1.5 km. In this study, water temperature, salinity, and chlorophyll a fluorescence were analyzed.
Salinity measured by the thermosalino-fluorograph was distributed in the range of 32.0–34.7 centered at 33.7–34.6 in the shelf area between 50 and 200 m depth. Low salinity water occurred mainly during the summer and fall seasons, partly accompanied by elevated chlorophyll a. However, when compiled seasonally, there was no clear relationship between salinity and chlorophyll a. All UCTD observations captured the Kuroshio front; chlorophyll a at UCTD tended to be higher in the coastal side, but in some cases was higher in the offshore side. Satellite chlorophyll images suggest that the higher offshore chlorophyll a concentrations were caused by the entrainment of water masses with high chlorophyll concentrations from the Boso coast. Water masses with high chlorophyll a observed by the UCTD observations were not necessarily low-salinity, but in some cases were similar to the water mass properties of the Kuroshio. On the other hand, in the shelf region, the depth of the 25.0–25.5 σθ density typical in the sub-surface layer of the Kuroshio and its Extension areas varied greatly from cruise to cruise, and chlorophyll a tended to increase significantly when they were at shallower depths.
The results of the thermosalino-fluorograph and UCTD observations suggest that nutrients in river water do not directly enhance production in the continental shelf region. This upwelling is thought to excite primary production by providing nutrients from the bottom layer just above the continental shelf floor to the euphotic layer. Since the area off the coast of Boso is not a constant upwelling area, accumulation of nutrients in the bottom layer and bridging transport of nutrients from the bottom to the intermediate and the intermediate to the surface layer by vertical mixing, etc. may play a role in producing high-average production in the continental shelf area. At this JpGU2025 meeting, in addition to this study, presentations on internal tides (Masunaga et al.), large-amplitude internal solitary waves (Takahashi et al.), material leaching from pore water on the continental shelf (Nakajima et al.), and the structure and mixing of the Tone River plume (Murakami et al.; Yambe et al.) will also be given.
The data used were underway continuous observation data obtained by a thermosalino-fluorograph during 12 cruises from 2019 to 2024, and underway CTD (UCTD) cross-sectional observation data conducted off Choshi during five cruises from 2021 to 2024. The UCTD observations were conducted from a depth of about 50 m off Choshi seaward, about 35 km from the surface to just above the seafloor, and acquired temperature, salinity, dissolved oxygen, chlorophyll a fluorescence, and turbidity profiles with a horizontal resolution of about 0.5–1.5 km. In this study, water temperature, salinity, and chlorophyll a fluorescence were analyzed.
Salinity measured by the thermosalino-fluorograph was distributed in the range of 32.0–34.7 centered at 33.7–34.6 in the shelf area between 50 and 200 m depth. Low salinity water occurred mainly during the summer and fall seasons, partly accompanied by elevated chlorophyll a. However, when compiled seasonally, there was no clear relationship between salinity and chlorophyll a. All UCTD observations captured the Kuroshio front; chlorophyll a at UCTD tended to be higher in the coastal side, but in some cases was higher in the offshore side. Satellite chlorophyll images suggest that the higher offshore chlorophyll a concentrations were caused by the entrainment of water masses with high chlorophyll concentrations from the Boso coast. Water masses with high chlorophyll a observed by the UCTD observations were not necessarily low-salinity, but in some cases were similar to the water mass properties of the Kuroshio. On the other hand, in the shelf region, the depth of the 25.0–25.5 σθ density typical in the sub-surface layer of the Kuroshio and its Extension areas varied greatly from cruise to cruise, and chlorophyll a tended to increase significantly when they were at shallower depths.
The results of the thermosalino-fluorograph and UCTD observations suggest that nutrients in river water do not directly enhance production in the continental shelf region. This upwelling is thought to excite primary production by providing nutrients from the bottom layer just above the continental shelf floor to the euphotic layer. Since the area off the coast of Boso is not a constant upwelling area, accumulation of nutrients in the bottom layer and bridging transport of nutrients from the bottom to the intermediate and the intermediate to the surface layer by vertical mixing, etc. may play a role in producing high-average production in the continental shelf area. At this JpGU2025 meeting, in addition to this study, presentations on internal tides (Masunaga et al.), large-amplitude internal solitary waves (Takahashi et al.), material leaching from pore water on the continental shelf (Nakajima et al.), and the structure and mixing of the Tone River plume (Murakami et al.; Yambe et al.) will also be given.