Activity in 2019

Cruise observation in FY2019

1. Western Pacific subarctic – subtropical regions

R/V Mirai (MR19-02: 2019/5/24-6/14)
“Carbon Hot Spot observation, Aerosol impacts on marine ecosystems”

Interdisciplinary observation at KEO, K2 and other stations.

2. Western Pacific subtropical regions

R/V Hakuho-maru (KH-19-4: 2019/7/20-7/30)
“Role of turbulence in ecosystem and material cycles in the Kuroshio recirculation region, western subtropical North Pacific”

Recovery / Redeployment of KEO sediment trap mooring system.


Topic 1
Comparison of vertical transport of Particulate Organic Carbon (POC) between Subarctic Eutrophic station K2 and Subtropical Oligotrophic station S1

Seasonal or time-series observation by using research vessels, sediment trap experiment, satellite data analysis and numerical simulation during “K2S1 project” revealed that primary productivity and particulate organic carbon (POC) flux upper 200 m at subtropical station S1 were comparable to or slightly higher than those at subarctic station K2 (Honda et al. JO 2017). However, POC flux at deep sea (~ 5000 m) of K2 was ~ 2 times higher than that at S1. These observations resulted in that POC flux vertical attenuation at K2 was smaller than that at S1(Fig.1). Major chemical component of sinking particle was biogenic opal at K2 while CaCO3 was major component at S1. Multiple linear regression analysis indicated that correlation coefficient between biogenic opal and POC at K2 was the highest among other ballasts (CaCO3 and lithogenic materials). Thus, biogenic opal might play an important role in effective POC vertical transport in the western North Pacific subarctic region. In addition, from the view point of metabolism, lower water temperature and dissolved oxygen concentration in the twilight zone at K2 might also support smaller POC flux vertical attenuation(Fig.2).

Fig.1 Vertical profile of POC at K2 (blue) and S1 (red). PP is primary productivity.
Fig.2 Vertical profile of Temperature (left) and DO (right) at K2 (blue) and S1 (red).

Topic 2
Study of mechanism of nutrient supply in the oligotrophic area: Time-series observation at KEO

In order to study the mechanism of nutrient supply to support primary productivity in the oligotrophic area, time-series sediment trap has been deployed at about 5000 m of station KEO (32.5°N / 144.5°E), which is time-series station of NOAA and surface buoy has been deployed, since July 2014, and seasonal variability and inter-annual variability of settling particles have been observed together with meteorology and physical oceanography (Fig.1).
Based on observation between 2014 and 2016, it was pointed out that mesoscale cyclonic eddy is one of important nutrient suppliers(Honda et al. PEPS 2018). Time-series sediment trap experiment till July 2019 revealed that the maximum mass flux was observed in April 2018. The discussion about the relation between variabilities of settling particles and these of intensity of winter mixing and frequency of typhoon around KEO is ongoing.
In July 2019, time-series sediment trap was also deployed at about 1800 m. In future, vertical change in settling particles and lateral transport of lithogenic materials will be discussed.

Fig.1 Seasonal and inter-annual variabilities in Total Mass Flux (TMF gray: black) and chemical composition (colors for respective components are same as above. Yellow is Organic matter: OCF / 0.36)

Topic 3
Impacts of eddies on phytoplankton spring bloom onsets during stable and unstable Kuroshio extension periods

Cumulative sum method estimated spring bloom onset in the Kuroshio extension (KE) region around mid-March (Fig. 1). KE region interannually experiences stable and unstable conditions characterized by more and less cyclonic eddies, respectively. South of or along the Kuroshio axis, bloom onset and winter sea surface height anomaly (SSHA) positive correlation was observed (Fig. 2) indicating earlier bloom onset associated winter low SSHA. Low SSHA can be an indication of cyclonic eddies.

Fig. 1. A cumulative sum method to define spring bloom onset. Chl-a is accumulatively summed from the time of minimum Chl-a in winter. The time when percentage of cumulative sum reaches 40% is defined as spring bloom onset.

Low SSHA during unstable KE period therefore indicates more cyclonic eddies (Fig. 3a). Cyclonic eddy-induced upwelling increases nutrient and by shoaling mixed layer depth (MLD) improve light condition for phytoplankton (Fig. 3a). Increases of nutrient and light availabilities are likely responsible for surface Chl-a increase even during the winter. Improved light condition during the winter may also be responsible for earlier bloom onset during unstable than during stable KE periods.

Fig. 2. Map of correlation between spring bloom onset and winter SSHA. Positive correlation (red) indicates a tendency of earlier bloom onset associated with low winter SSHA (cyclonic eddy). Arrows indicate sea surface current derived from the OSCAR (
Fig. 3. (a) Temporal variations of Chl-a (solid bold line), SSHA (dashed line), and MLD (solid thin line) during unstable (blue) and stable (red) periods of KE spatially averaged within a yellow box shown in Fig. 2. (b) Temporal variations of normalized Chl-a during unstable (blue) and stable (red) periods of KE.

Topic 4
Impact of atmospheric deposition on primary productivity

Aerosols originated from the east Asian continent are transported to the ocean surface as the wet and dry depositions. Previous model studies showed the positive effect of atmospheric deposition on phytoplankton growth in the oligotrophic ocean due to the rich inorganic nitrogen compounds containing in aerosols.
However, it is difficult to detect the actual impact of atmospheric deposition on phytoplankton growth in nature. We conducted the incubation experiments by the shipboard observation at the several stations of the oligotrophic ocean in the northwestern subtropical Pacific Ocean.

Fig1. Experimental locations (left panel), Relationship of the photosynthetic parameter at the sea surface (right panel).

Nutrients were almost depleted at the surface in the subtropical stations, although those were rich at the subarctic station. Oceanic conditions were similar in the subtropical stations, while the incubation experiments showed large variation among the stations. Photosynthetic parameters showed a significant linear relationship in the subtropical stations. This means that there is a factor controlling primary productivity.

Fig2.Rainfall events before the sampling and the nutrient concentrations in rain.

Remarkable rainfall event was recognized at the station S2, which showed the highest primary productivity. Rich nitrogen compounds were measured from the rain sample. Rainfall event was also recognized at the station S4. The impact of atmospheric deposition on primary productivity was detected in the oligotrophic region through the field observation.