Influence of Human Nitrogen Emissions Found on Dongsha Coral Reefs: NTU Researcher

Supported by the Ministry of Science and Technology and National Taiwan University (NTU), Assistant Prof. Haojia Abby Ren (任昊佳) at the NTU Department of Geosciences led an international team to provide the first direct evidence for the influence of anthropogenic nitrogen (N) sources on the open ocean. This work was published on Science on May 18, 2017. Deputy Minister of Science and Technology Yu-Chin Hsu (許有進) and NTU Executive Vice President Ching-Ray Chang (張慶瑞) co-hosted a press conference on May 19 for the publication of the research. The study shows that nitrogen emitted through the use of fossil fuels can spread to the open ocean and be used immediately by ocean organisms. Human nitrogen emissions to the natural environment have frequently caused eutrophication of lakes, rivers, and coastal seas. This induces rapid reproduction of algae and other plankton, decreases the dissolved oxygen content of water bodies, and gradually leads water organisms to decline and even extinction. Once again, this finding warns human beings of their increasing impact on the natural environment. We have to remember that when fulfilling our desires, we are also putting pressure on nature and the organisms in it.

Nitrogen circulation in ocean and natural regulation of organisms

Marine phytoplankton, like all organisms, require nitrogen to live and grow. Although the majority of the air we breathe is N₂ , the nitrogen in the atmosphere is unavailable for use by most phytoplankton, except certain bacteria and cyanobacteria capable of breaking the strong N–N triple bond. In order for organisms to be able to use nitrogen, N₂ gas must first be converted to a more chemically available form such as ammonium, nitrate, or organic nitrogen. The inert nature of N₂ means that biologically available nitrogen is often in short supply in the ocean, limiting phytoplankton growth.

Human damage to the balance of nitrogen circulation can spread to open ocean

Within the last century, human activities have raised the speed of nitrogen fixation. In the early 20th century, Nobel Chemistry Prize winners Fritz Haber and Carl Bosch developed an industrial process to produce reactive nitrogen from atmospheric N₂ stores. The Haber-Bosch reaction soon became the means to produce nitrogen-rich fertilizers. Meanwhile, the adoption of internal combustion engines and other industrial burning processes led to widespread release of oxidized nitrogen (NOx) to the atmosphere. Through these activities, humans have more than doubled the amount of fixed nitrogen that is pumped into the biosphere every year. Most of these are deposited on land, but some have escaped and traveled far. Modeling studies suggest that the oceans far away from the continents are not immune from the impacts of humankind’s nitrogen fertilization experiment. However, little evidence exists to support this. The signal of anthropogenic nitrogen deposition is diluted by ocean mixing, and its impact may also be counteracted by the organisms capable of fixing atmospheric N₂ .

Dongsha coral records human influence on nitrogen circulation in open sea

Anthropogenic sources of nitrogen are often isotopically lighter than the nitrogen circulating through natural processes in ecosystems. Using ^15/14N stable isotope analysis, Dr. Ren and her team tracked the appearance of this isotopically light nitrogen in seasonally resolved coral from Dongsha atoll, a semi-closed circular coral reef atoll located 300 km away from the nearest continents in the northern South China Sea. The authors find that the light-nitrogen signal increased just before 2000, coincident with massive increases in fossil fuel combustion in Asia, but decades later than predicted by modeling work. The amplitude of change suggests that, by 2010, anthropogenic atmospheric N deposition represented about one fifth of the annual N input to the surface ocean in this region, which appears to be at the lower end of other estimates.

This discovery on the spreading extent of humankind’s unintentional nitrogen experiment should add urgency to recent concerns about the multidimensional planetary boundaries that humanity is pressuring. And it also highlights the urgency to monitor the ocean environment with significant spatial coverage and through time to better understand the human footprint on the open ocean. This study points to the tremendous potential value of a network of coral-bound N isotope records from ocean islands and offshore reefs deliver this.

*Click HERE to read the full paper “21st Century Rise in Anthropogenic Nitrogen Deposition on a Remote Coral Reef” on the May 2017 issue of Science.

*Read the original article at Dr. Ren’s Research Blog.

Research Contact:

Assistant Prof. Haojia Abby Ren
Department of Geosciences, National Taiwan University
TEL: (office) +886-2-3366-2908, +886-928-172-624
E-mail: abbyren@ntu.edu.tw