Taiwan – Prof Abby Hao-Jia Ren of the Department of Geosciences led an international team seeking to obtain the first direct evidence for the influence of anthropogenic nitrogen (N) sources on the open ocean. The team’s findings were published in a study titled “21st-Century Rise in Anthropogenic Nitrogen Deposition on a Remote Coral Reef” in the journal Science in May.
NTU Executive Vice President Ching-Ray Chang joined Deputy Minister of Science and Technology Yu-Chin Hsu in co-hosting a press conference on 19 May to mark the article’s publication.
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.
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. In order for these 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.
During the last hundred years, human activities have raised the speed of nitrogen fixation primarily through the use of nitrogen-rich fertilisers and combustion of fossil fuels, which have more than doubled the amount of fixed nitrogen that is pumped into the biosphere every year. Modelling studies suggest that the oceans far away from the major continents are not immune from the impact of humankind’s nitrogen fertilisation experiment; however, little evidence exists to support this.
Anthropogenic sources of nitrogen are often isotopically lighter than the nitrogen circulating through natural processes in ecosystems. Using 15/14N stable isotope analysis, Prof 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 kilometres away from the nearest continents in the northern South China Sea.
The authors found that the light-nitrogen signal increased just before the year 2000, coincident with massive increase in fossil fuel combustion in Asia, but decades later than predicted by modelling 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 highlights the urgent need 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.