Pinpoint Nutrient Upwelling around Seafloor Topography Supports Biological Production over a Large Area
–A case study off Cape Shiriya, Tsugaru Strait, Japan–

Schematic representation of the findings of this study: The area near the topography offshore of the Shimokita Peninsula (Shiriya Spur) was found to be an important region, like a “well in the desert,” where nutrient upwelling points are located (indicated by warm color bars). The upwelling supports stable phytoplankton production (shown by yellow and orange contours) across a wide range of surface layers within the "Tsugaru Gyre". The colored bars represent observed turbulence intensity, while the colored contours represent surface chlorophyll-α concentration distributions observed by satellite during the summer months (August and September), normalized by the standard deviation of each grid (chlorophyll-α data period: 2014–2022).

Background and Summary:
We found that a very narrow area around a distinctive topography located at the outlet of the Tsugaru Strait, Japan, plays a crucial role in supporting the surface chlorophyll-α distribution over a wide area, much like a “hidden well in the desert".

It has been qualitatively noted that significant turbulent mixing and remarkable vertical nutrient transport occur when strong currents pass near characteristic bottom topographies, such as seamounts. However, there have been no observational reports from the eastern Tsugaru Straits. Furthermore, since turbulence intensity fluctuates significantly and continuous observation in strong currents using vessels is challenging, much remains unknown about how nutrient transport, associated with turbulent vertical mixing, affects phytoplankton production in the downstream region. The water transported from the entrance of the Tsugaru Strait by the eastward Tsugaru Warm Current originates from subtropical waters with relatively low nutrient content. Understanding how nutrients are transported into this water is also crucial for comprehending the mechanism behind the formation of rich fishing grounds on the Pacific side.

In this study, we analyzed high-resolution, long-term sea surface current monitoring data from High Frequency Radar (HFR), in combination with ocean data assimilation model output and shipboard observations. The HFR sea surface current information was used as an indicator of nutrient supply. This indicator was then analyzed alongside long-term, wide-area data on surface phytoplankton distribution obtained from satellite observations to investigate the effects of vertical turbulent nutrient transport.

As a result, we discovered that a very narrow area of about 10 km2, where the Tsugaru Warm Current overcomes the topography, is essential for chlorophyll-α distribution within the large anticyclonic eddy known as the Tsugaru Gyre, which forms from summer to autumn on the Pacific Ocean side. Specifically, we found that the strength of current "divergence" at the sea surface in this area has a significant positive correlation with the surface chlorophyll-α distribution in the Tsugaru Gyre. This indicates that nutrient-rich water from the deep sea upwells to the surface in the turbulent mixing area, and is widely dispersed in the downstream area by the Tsugaru Warm Current. We were also able to quantitatively confirm that the observed nutrient transport near the topography is sufficient to support biological production downstream by tracing the time and extent to which surface water can flow downstream from this topography, based on the output of an ocean data assimilation model.

This study provides an example of how vertical nutrient transport, continuously induced in a narrow area by topography and strong currents, can sustain biological production over a larger area. It also highlights which key areas require special attention for ongoing marine environmental monitoring to understand the ecosystem in the waters surrounding the Tsugaru Gyre. Additionally, we demonstrated that biological production within the Tsugaru Gyre may be a unique system maintained by a stable nutrient supply. The existence of this unique ecosystem may also contribute to the formation of rich fishing grounds on the Pacific side. It is expected that future vessel-based observations will focus on this key area, leading to more efficient research development.

The results will be published in Nature Communications on April 22(Japan Standard Time) under the title “Topographically Driven Vigorous Vertical Mixing Supports Mesoscale Biological Production in the Tsugaru Gyre.” (DOI: 10.1038/s41467-025-56917-4)