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August 22, 2019

Is the Reduction of Arctic Ocean Sea Ice Changing the Nitrogen Cycle?
—Potential Impacts on the Marine Ecosystem and Marine Resources—

1. Key Points

Ammonium, nitrate, and nitrite are major nutrients (salts) that are indispensable for biological production in the ocean, where ammonium converts to nitrite and nitrate through nitrification reactions.
In the Arctic Ocean, it was determined that nitrification is primarily restricted by light.
As a result of investigating light intensity in the Arctic Ocean, it was found that the marine regions where light levels could suppress nitrification have increased over the past two decades. For this reason, it was estimated that nitrification is declining throughout the Arctic.
Further reductions in sea ice could possibly suppress nitrification and increase the relative amount of ammonium nitrogen. Consequently, it is thought that this suppression may also impact the marine ecosystem.


Takuhei Shiozaki and collaborators from the Earth Surface System Research Center (ESS) of the Japan Agency for Marine–Earth Science and Technology (JAMSTEC) have found that the reduction of sea ice in the Arctic Ocean impacts the marine nitrogen cycle by suppressing nitrification. Nitrogen in the forms of ammonium, nitrate, and nitrite are some of the most important nutrients for marine biological production. In the ocean, nitrification occurs as ammonium is decomposed from organic matter and is converted into inorganic nitrogen, such as nitrite and nitrate, through microbial activity. Nitrification is central to the marine nitrogen cycle, and it plays an important role in determining the proportion of each type of inorganic nitrogen in the ocean. Furthermore, the concentration of ammonium, light levels, and pH are known to affect nitrification. In recent years, it has been indicated that the progressive melting of Arctic sea ice and the acidification of the Arctic Ocean could impact the rate of nitrification, but the actual conditions remained unclear.

Using its oceanographic research vessel, the MIRAI, the ESS has observed and recorded how nitrification responds to light levels and pH levels on the continental shelf in the Chukchi Sea and in the basin of the western Arctic Ocean (Fig. 1). As a result, it was discovered that although the nitrification rate has been reduced due to declining pH levels, this impact was less than that of light (i.e., light affected nitrification more than did pH). When the light intensity exceeded 0.11 mol photons m-2 d-1, nitrification was significantly suppressed (Fig. 2a). Moreover, when 0.11 mol photons m-2 d-1 was taken as the threshold for light intensity, the regions where light intensity exceeded this threshold were found at water depths of approximately ≤80 m on the continental shelf and in the basin of the Arctic Ocean (Fig. 3), and nitrification was found to be suppressed in these areas. In fact, when satellite data from the past 20 years were analyzed for the light conditions of these respective environments, it was found that the marine areas in which the light intensity exceeded the threshold at the seafloor or at a water depth of 50 m (due to reductions in sea ice) have expanded in the Arctic Ocean overall (Fig. 4). These findings led the project scientists to think that nitrification may be greatly suppressed by open-water conditions (i.e., those with reduced levels of sea ice). The deeper that light penetrates the ocean, the more inhibited nitrification becomes, resulting in the conversion of ammonium into nitrate becoming more difficult. It is this suppression that is thought to increase the relative amount of ammonium nitrogen in the ocean (Fig. 5). Ammonium nitrogen allows phytoplankton to undergo organic synthesis with less energy consumption, and the relative increase in the amount of ammonium nitrogen acts to the advantage of the lower trophic levels that supports the bottom of the Arctic Ocean food chain, and may have a large impact on the production of higher trophic levels.

Until now, anthropogenic changes in the nitrogen cycle have been due to the excessive nitrogen loads caused by the mass production of nitrogen fertilizers and the burning of fossil fuels, and it has been considered to be a problem limited to regions in the mid- to low latitudes, where population densities are highest. There are few major cities and farmlands in polar regions, and the direct nitrogen load on the ocean is minor compared to the regions at mid- to low latitudes. However, it was observed in this study that through the acceleration of ice melting in the Arctic Ocean, the light environment is changing, and consequently, the nitrogen cycle is also changing. This can be considered one of the overlooked impacts of human activities. Going forward, it will be necessary to clarify the impacts that the suppression of nitrification has on the nitrogen cycle and marine ecosystem in the Arctic Ocean.

The results of this study will be published on August 22 (JST) in Global Biogeochemical Cycles, an academic journal published by the American Geophysical Union. This research was supported by the Grant-in-Aid from the Japan Society for the Promotion of Science (Grants Numbers: JP26850115, JP15H05712, JP15H05822, and JP18H03361) and the Sasakawa Scientific Research Grant (Grant Number: 28-702). Furthermore, the voyage for this study was completed as part of an Arctic research promotion project, the Arctic Challenge for Sustainability (ArCS).

Title: Factors regulating nitrification in the Arctic Ocean: Potential impact of sea ice reduction and ocean acidification
Authors: Takuhei Shiozaki1, Minoru Ijichi2, Amane Fujiwara3, Akiko Makabe4, Shigeto Nishino3,Chisato Yoshikawa5 and Naomi Harada1
1 Earth Surface System Research Center, JAMSTEC
2 Atmosphere and Ocean Research Institute, The University of Tokyo (Current: Bioengineering Lab. Co., Ltd, Tokyo Metropolitan University, Kyoto Sangyo University)
3 Institute of Arctic Climate and Environment Research, JAMSTEC
4 Project Team for Development of New‐generation Research Protocol for Submarine Resources, JAMSTEC
5 Department of Biogeochemistry, JAMSTEC

Figure 1

Figure 1. Sampling stations in the Arctic Ocean from 2016 (red) and 2017 green). Gray bold and dashed lines denote the 100-m and 50-m isobaths, respectively.

Figure 2

Figure 2. (a) Results of the light control experiement and (b) the relationship between the nitrification rate and light intensity in the photic zone of the study area.

Figure 3

Figure 3. Verticle distribution of nitrification rates on the continental shelf and the off-shelf basin in the Arctic Ocean. Brown lines indicate depth of the seafloor, and green stippled lines represent the depth at which the light intensity becomes 0.11 mol photons m-2 d-1. At Sites 05 and 06, the light intensity reached more than 0.11 mol photons m-2 d-1 immediately above the seafloor.

Figure 4

Figure 4. (a) Total area of minimum sea ice cover and the marine regions where light intensities exceed 0.11 mol photons m-2 d-1 at water depths of ≤50 m above the seafloor, or at a depth of 50 m (light-rich province). (b) Spatial distribution of the light-rich provice according to annual frequency in (b) 1998–2006 and (c) 2007–2017. The gray area denotes the area that is never covered by sea ice throughout the year. The white line denotes the 50-m isobath, the black line denotes the sea ice extent recorded between 1998 and 2006, while the red line marks the period of 2007–2017.

Figure 5

Figure 5. Schematic of inorganic nitrogen cycling under sea ice-covered and open-water conditions (i.e., when sea ice is absent).


(For this study)
Naomi Harada, Director-General, Earth Surface System Research Center
(For press release)
Public Relations Section, Marine Science and Technology Strategy Department
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