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- Title
Regionally Variable Responses of Nitrogen Fixation to Iron and Phosphorus Enrichment in the Pacific Ocean.
- Authors
Tanita, Iwao; Shiozaki, Takuhei; Kodama, Taketoshi; Hashihama, Fuminori; Sato, Mitsuhide; Takahashi, Kazutaka; Furuya, Ken
- Abstract
Iron and phosphorus are major nutrients limiting marine nitrogen fixation. We directly assessed iron and phosphate enrichment using the surface water of the western and central North Pacific subtropical gyre (NPSG), central equatorial Pacific, and eastern South Pacific subtropical gyre (SPSG). Diazotrophic phylotypes showed regionally distinct distribution patterns. The abundances of nifH in UCYN‐A, Trichodesmium, Crocosphaera, and γ‐24774A11 and filaments of Richelia were significantly higher in the central and western NPSG than in the eastern SPSG. The surface dissolved iron concentration was <0.2 nmol l−1 in most of the regions; however, nitrogen fixation was enhanced by the addition of iron only in the western NPSG and equatorial upwelling. In the western NPSG, where surface phosphate was almost exhausted, and levels varied below 7 nmol l−1, diazotrophs also exhibited either phosphate limitation or colimitation by iron and phosphate. The present study demonstrated the limitation of nitrogen fixation by iron and phosphorus individually and colimitation by iron and phosphorus in the western NPSG for the first time, and the data showed that the nutrients limiting nitrogen fixation in the Pacific Ocean differed among the regions, indicating that the response of nitrogen fixation to environmental changes is not uniform. Plain Language Summary: While nitrogen is an essential element for biota, nitrogenous nutrients tend to be scarce in the ocean. Therefore, biological nitrogen fixation is important for providing nitrogen in a bioavailable form to fuel marine ecosystems. For nitrogen‐fixing organisms, the availability of iron and phosphorus are key factors, and iron is reported to be limiting in North Pacific subtropical waters (NPSG). However, although the surface phosphate concentration is extremely low in the western NPSG compared to the eastern NPSG, phosphorus limitation in the western NPSG has not been examined to date. We used enrichment experiments to investigate iron and phosphorus limitation of nitrogen fixation at a basin scale in the Pacific Ocean. Our results showed that nitrogen fixation was limited by iron and/or phosphate in the western NPSG and by iron in the central equatorial area but not by either iron or phosphate in the central NPSG and eastern South Pacific. Phosphorus limitations are considered to become severe when the upward phosphate supply is weakened by increased ocean stratification caused by global warming. Thus, the limiting nutrients for nitrogen fixation vary among regions, and the response of nitrogen fixation to environmental changes is not uniform in the ocean. Key Points: Limiting factors for nitrogen fixation as examined by enrichment experiments differed among the different regions in the Pacific OceanLimitation of N2 fixation by Fe and P and their colimitation were shown in the western North Pacific subtropical gyre for the first time
- Subjects
PACIFIC Ocean; NITROGEN fixation; IRON in water; PHOSPHORUS in water; SEAWATER
- Publication
Journal of Geophysical Research. Biogeosciences, 2021, Vol 126, Issue 9, p1
- ISSN
2169-8953
- Publication type
Article
- DOI
10.1029/2021JG006542