Takuhei Shiozaki

Large‐scale impact of the island mass effect through nitrogen fixation in the western South Pacific Ocean

We describe a new mechanism for the island mass effect fueled by nitrogen fixation. The nitrogen fixation activities and δ15N of suspended particles in the surface water in the South Pacific were examined. Active nitrogen fixation and abundant Trichodesmium spp. were observed near islands in the western subtropical region, which was attributable to the material supplied by land runoff. High primary production was extensively centered around the islands and was characterized by low δ15N of suspended particles and a reduction in phosphate concentrations at the surface compared with the subtropical gyre and eastern equatorial upwelling. This suggested that Trichodesmium spp. were advected to areas remote from these islands, and consequently, the elevated primary production fueled by nitrogen fixation extended over a large area around them. Because the proposed island mass effect is triggered by a terrigenous nutrient supply, this ecosystem is potentially vulnerable to human activity on small islands.

Heterotrophic bacteria as major nitrogen fixers in the euphotic zone of the Indian Ocean

Diazotrophy in the Indian Ocean is poorly understood compared to that in the Atlantic and Pacific Oceans. We first examined the basin-scale community structure of diazotrophs and their nitrogen fixation activity within the euphotic zone during the northeast monsoon period along about 69°E from 17°N to 20°S in the oligotrophic Indian Ocean, where a shallow nitracline (49–59 m) prevailed widely and the sea surface temperature (SST) was above 25°C. Phosphate was detectable at the surface throughout the study area. The dissolved iron concentration and the ratio of iron to nitrate + nitrite at the surface were significantly higher in the Arabian Sea than in the equatorial and southern Indian Ocean. Nitrogen fixation in the Arabian Sea (24.6–47.1 μmolN m−2 d−1) was also significantly greater than that in the equatorial and southern Indian Ocean (6.27–16.6 μmolN m−2 d−1), indicating that iron could control diazotrophy in the Indian Ocean. Phylogenetic analysis of nifH showed that most diazotrophs belonged to the Proteobacteria and that cyanobacterial diazotrophs were absent in the study area except in the Arabian Sea. Furthermore, nitrogen fixation was not associated with light intensity throughout the study area. These results are consistent with nitrogen fixation in the Indian Ocean, being largely performed by heterotrophic bacteria and not by cyanobacteria. The low cyanobacterial diazotrophy was attributed to the shallow nitracline, which is rarely observed in the Pacific and Atlantic oligotrophic oceans. Because the shallower nitracline favored enhanced upward nitrate flux, the competitive advantage of cyanobacterial diazotrophs over nondiazotrophic phytoplankton was not as significant as it is in other oligotrophic oceans.

Nitrification and its influence on biogeochemical cycles from the equatorial Pacific to the Arctic Ocean

We examined nitrification in the euphotic zone, its impact on the nitrogen cycles, and the controlling factors along a 7500 km transect from the equatorial Pacific Ocean to the Arctic Ocean. Ammonia oxidation occurred in the euphotic zone at most of the stations. The gene and transcript abundances for ammonia oxidation indicated that the shallow clade archaea were the major ammonia oxidizers throughout the study regions. Ammonia oxidation accounted for up to 87.4% (average 55.6%) of the rate of nitrate assimilation in the subtropical oligotrophic region. However, in the shallow Bering and Chukchi sea shelves (bottom ⩽67 m), the percentage was small (0–4.74%) because ammonia oxidation and the abundance of ammonia oxidizers were low, the light environment being one possible explanation for the low activity. With the exception of the shallow bottom stations, depth-integrated ammonia oxidation was positively correlated with depth-integrated primary production. Ammonia oxidation was low in the high-nutrient low-chlorophyll subarctic region and high in the Bering Sea Green Belt, and primary production in both was influenced by micronutrient supply. An ammonium kinetics experiment demonstrated that ammonia oxidation did not increase significantly with the addition of 31–1560 nm ammonium at most stations except in the Bering Sea Green Belt. Thus, the relationship between ammonia oxidation and primary production does not simply indicate that ammonia oxidation increased with ammonium supply through decomposition of organic matter produced by primary production but that ammonia oxidation might also be controlled by micronutrient availability as with primary production.

Nitrogen isotope ratios of nitrate and N* anomalies in the subtropical South Pacific

Nitrogen isotopic ratios of nitrate (δ15N– NO3−) were analyzed above 1000 m water depth along 17°S in the subtropical South Pacific during the revisit WOCE P21 cruise in 2009. The δ15N– NO3− and N* values were as high as 17‰ and as low as −18 μmol N L−1, respectively, at depths around 250 m east of 115°W, but were as low as 5‰ and as high as +1 μmol N L−1, respectively, in subsurface waters west of 170°W. The relationships among NO3− concentrations, N* values, δ15N– NO3− values, and oxygen and nitrite concentrations suggest that a few samples east of 90°W were from suboxic and nitrite-accumulated conditions and were possibly affected by in situ water column denitrification. Most of the high-δ15N– NO3− and negative-N* waters were probably generated by mixing between Subantarctic Mode Water from the Southern Ocean and Oxygen Deficit Zone Water from the eastern tropical South Pacific, with remineralization of organic matter occurring during transportation. Moreover, the relationship between δ15N– NO3− and N* values, as well as Trichodesmium abundances and size-specific nitrogen fixation rates at the surface, suggest that the low-δ15N– NO3− and positive-N* subsurface waters between 160°E and 170°W were generated by the input of remineralized particles created by in situ nitrogen fixation, mainly by Trichodesmium spp. Therefore, the δ15N values of sediments in this region are expected to reveal past changes in nitrogen fixation or denitrification rates in the subtropical South Pacific.

Basin scale variability of active diazotrophs and nitrogen fixation in the North Pacific, from the tropics to the subarctic Bering Sea

Nitrogen-fixing micro-organisms (diazotrophs) provide biologically available nitrogen to plankton communities and thereby greatly influence the productivity in many marine regions. Various cyanobacterial groups have traditionally been considered the major oceanic diazotrophs, but later non-cyanobacterial and presumably heterotrophic diazotrophs were also found to be widespread and potentially important in nitrogen fixation. However, the distribution and activity of different diazotroph groups is still poorly constrained for most oceanic ecosystems. Here, we examined diazotroph community structure and activity along a 7,500-km south-north transect between the central equatorial Pacific and the Bering Sea. Nitrogen fixation contributed up to 84% of new production in the upper waters of the subtropical gyre, where the diazotroph community included the gammaproteobacterium γ-24774A11 and highly active cyanobacterial phylotypes (>50% of total nifH transcript abundance). Nitrogen fixation was sometimes detectable down to 150 m depth and extended horizontally to the edge of the gyre at around 35°N. Nitrogen fixation was even detected far north on the Bering Sea shelf. In the Alaskan Coastal Waters on the Bering Sea shelf, low nitrate together with high dissolved iron concentrations seemed to foster diazotroph growth, including a prominent role of UCYN-A2, which was abundant near the surface (1.2 × 105 nifH gene copies L-1). Our study provides evidence for nitrogen fixation in the Bering Sea and suggests a clear contrast in the composition of diazotrophs between the tropical/subtropical gyre and the separate waters in the cold northern regions of the North Pacific.

Wind-induced upwelling in the western equatorial Pacific Ocean observed by multi-satellite sensors

Abstract R/V MIRAI (MR01-K05 Leg3) of Japan Marine Science and Technology Center (JAMSTEC) was stationed at the point of 2° North and 138° East in the western equatorial Pacific Ocean from November 9 to December 9, 2001. During this air–sea interaction research cruise SeaWiFS and NOAA/AVHRR local area coverage (LAC) scenes were received by the station onboard R/V MIRAI and the products derived from these satellites are verified against oceanographic observations including the parameter of sea surface temperature, salinity, chlorophyll-a and current velocity profile to the depth of 300 m. Level 3 wind vector products derived from QuickSCAT onboard SeaWinds are also collected and validated against in situ wind vectors. The diurnal change of amplitude of sea surface temperature decreases from 1.5° to 0° after the week-long prevailing northwest monsoon wind with the maximum gust more than 20 m/s while the observed surface salinity and chlorophyll-a concentration increase from 34.1 to 34.37 and from 0.05 to 0.14 mg/m3, respectively. The increase of chlorophyll-a and the decrease of sea surface temperature in this region are shown in the multi-date SeaWiFS chlorophyll-a concentration products and multi-channel sea surface temperature (MCSST) products from NOAA/AVHRR. Wind vector patterns before and after the gust more than 20 m/s are also observed by QuickSCAT. During the period of the week-long northwest monsoon wind the current velocity of upper 70 m reaches about 70 cm/s in the southeastward direction while the current velocity at the depth from 80 to 120 m indicates 50 cm/s in the northwestward direction. The current of upper 70 m corresponds to the northwest monsoon current (NMC) and the intrusion of NMC enhanced by the strong northwest monsoon winds (westerly wind bursts) causes a reversal in the sub-surface current (New Guinea coastal undercurrent, NGCUC) which creates a temporal upwelling in this region.

Distribution of major diazotrophs in the surface water of the Kuroshio from northeastern Taiwan to south of mainland Japan

 RESEARCH AND DEVELOPMENT CENTER FOR GLOBAL CHANGE, JAPAN AGENCY FOR MARINE-EARTH SCIENCE AND TECHNOLOGY, -, NATSUSHIMA-CHO, YOKOSUKA-CITY, KANAGAWA -, JAPAN, ORGANIZATION FOR MARINE SCIENCE AND TECHNOLOGY, NAGASAKI UNIVERSITY, BUNKYO-MACHI -, NAGASAKI CITY, NAGASAKI -, JAPAN AND DEPARTMENT OF MARINE SCIENCE, GRADUATE SCHOOL OF FISHERIES AND ENVIRONMENTAL SCIENCES, NAGASAKI UNIVERSITY, BUNKYO-MACHI -, NAGASAKI CITY, NAGASAKI -, JAPAN