Activity in 2017

Cruise observation in FY2017

1. Western Pacific Subtropical region

(1)R/V BlueFin (2017/7/13-7/18)

Recovery and redeployment of NOAA KEO buoy (Study of Nutrient supply mechanism)

(2)R/V Hakuho-maru (KH1-17-05: 2017/11/14-11/30)

Recovery and redeployment of sediment trap mooring system (Study of Nutrient supply mechanism)

(3)R/V Yokosuka (YK17-E01: 2017/12/17-12/25)

Emergency recovery and redeployment of “adrift” NOAA KEO buoy (Study of Nutrient supply mechanism)

2. Japan Sea

(1)R/V Nagasaki-maru (# 458:2017/5/16-5/19)

Primary productivity observation (Global warming impacts on biogeochemical change in the Japan Sea)

(2)R/V Oshoro-maru (C040-Leg.1: 2017/6/5-6/10)

Primary productivity observation (Global warming impacts on biogeochemical change in the Japan Sea)

3. East Indian Ocean tropical region

(1)R/V Mirai(MR17-08-Leg.1: 2017/11/21-2018/1/4)

Primary productivity observation (Study of Air-Sea Interaction in the East Indian Ocean Upwelling region)


(1)Nutrient Supply Mechanism in the western Pacific Subtropical region

The Western Pacific Subtropical region is oligotrophic and is called “Ocean desert”. However a recent paper (Honda et al. 2017) reported that the primary productivity in this region is comparable to or slightly higher than that in the subarctic eutrophic region. In order to study the nutrient supply mechanism to support primary productivity, time-series observation with sediment trap was initiated in 2014 at station KEO, where is the Western Pacific subtropical time-series station of NOAA and surface buoy mooring equipped with meteorological / physical oceanographic sensors has been deployed. Based on past observation and model study, the followings are strong candidates as nutrient supply mechanism.

a. Mesoscale Eddy:Pass of Cyclonic eddy and Increase of sinking particles

Satellite-based Sea Surface Height Anomaly (SSHA) revealed that cyclonic eddies (CE) passed over KEO in late July and November 2014. During these periods, KEO buoy observed that subsurface cold and nutrient-rich water were uplifted by around 100 m, which depth was the bottom of the euphotic layer. On the other hand, sinking particles total mass flux at 5000 m increased about one – two months after CE pass. Based on numerical simulation, it was suspected that CE-induced nutrient supply could increase primary productivity resulting in increase of total mass flux.

b. Typhoon:Occurrence of Near-inertial internal wave and Turbulent flux

After typhoon #T1414 passed near KEO on 8 September 2014. a near-inertial internal wave took place at around 50 m, which depth is in the climatological nitracline, for about 10 days. Based on numerical simulation, diffusion coefficient (Kp) at around 50 m after typhoon pass could temporally become two – three orders higher than initial Kp. Simulated turbulent nutrient supply could also support increase of primary productivity and total mass flux. However, its contribution is probably much lower than that of CE.

(Honda et al. submitted to PEPPS)

(2)Changes of primary productivity by rain in the eastern equatorial Indian Ocean

The eastern equatorial Indian Ocean is an oligotrophic ocean, but it is also one of the most rainy region in the Ocean. In order to investigate the effects for marine ecosystems by rain, the experiments of primary productivity were conducted in December 2017 onboard MIRAI in this region. Primary productivity largely fluctuated due to the changes of water mass structure and the wet deposition of aerosol via rain. This is suggesting that marine ecosystems are influenced directly by aerosol.

Primary productivity was largely enhanced at the salinity-stratified water compared with the vertically-mixed water as shown in the increasing of photosynthetic rate. This is suggested as the effects of supplying nutrients from aerosol and the improvement of light-availability by stratification. However, the water taken during rain declined primary productivity remarkably. It is suggested that the surface water was contaminated by something substances inhibit phytoplankton growth with high concentration through the wet deposition of aerosol.

(3)Impact of dust deposition on phytoplankton biomass in the Bay of Bengal

Enormous flux of freshwater from the Ganges-Brahmaputra River supplies nutrients for phytoplankton in the Bay of Bengal (BoB) coastal region, but inhibits nutrient supply from deep layer in the offshore region due to a barrier layer formation. Dust deposition (DD) may thus be a potential source of nutrients. Modeled DD and satellite data were analyzed to see possible impacts of DD and/or of nutrient-rich subsurface water on the BoB phytoplankton biomass (Chl-a).

Modeled (using SPRINTARS) DD into the BoB was low during the winter (January, a), and high during the summer (July, b). High DD in July extends southward as far as the tropical region of the Indian Ocean.

To assess contribution of nutrients from DD and deep ocean layer to Chl-a, a multiple linear regression [i.e., Chl-a as a function of DD and sea surface temperature (SST)] was conducted. Partial regression coefficients for DD (βDD) and SST (βSST) are shown in (c) and (d), respectively.

Inferred from significant βSST (blue area in d) nutrient flux from deep layer was likely important in the tropical region and western area of the BoB. The DD was likely important nutrient source for phytoplankton in the central area of the BoB (areas with significant βDD in c).