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Human impacts

New Nitrogen in Town: Nitrogen Deposition on the Open Ocean

Source: Ren, H.; Chen, Y.-C.; Wang, X. T.; Wong, G. T. F.; Cohen, A. L.; DeCarlo, T. M.; Weigand, M. A.; Mii, H.-S.; Sigman, D. M. 21st-century rise in anthropogenic nitrogen deposition on a remote coral reef. Science 2017, 356 (6339), 749. DOI: 10.1126/science.aal3869

 

Nitrogen: what is it good for?

Nitrogen is one of three nutrients essential to the ocean ecosystem. Its importance is due to small photosynthetic organisms called phytoplankton. Phytoplankton take light, water, and carbon dioxide and convert them into sugars that they burn for energy. Slightly larger organisms eat phytoplankton for these sugars and in turn get eaten by even larger organisms. Phytoplankton support the entire oceanic food chain by being primary producers, or the first rung in the food chain ladder. In addition to water and carbon dioxide, phytoplankton rely on nutrients to carry out their cellular processes. The three main nutrients they need, referred to as macronutrients, are iron, phosphorus, and nitrogen. Because nitrogen availability and phytoplankton productivity are tightly linked, keeping track of how much nitrogen is in the water is important for understanding trends in ocean productivity.

 

Picky nitrogen eaters

Nitrogen can exist in several forms in the ocean, but only “bioavailable” nitrogen can be used by phytoplankton. The bonds in nitrogen gas are too strong for most organisms to break, but phytoplankton can readily make use of nitrogen in other compounds, such as nitrate (NO3-) and ammonium (NH4+). The three main sources of usable nitrogen in the upper ocean where phytoplankton reside are: 1. Upwelling of nutrient-rich water from below 2. Local nitrogen fixation (conversion by bacteria of unusable nitrogen into bioavailable nitrogen) 3. Nitrogen-containing particulates settling from the atmosphere into the ocean. The third source, referred to as “atmospheric deposition,” has become increasingly influential with the rise of fossil fuel burning. Models estimate atmospheric nitrogen being added to the ocean has doubled over the past 100 years. Though the settling of atmospheric nitrogen over land has been well documented, not as much research has been done on deposition over the ocean. Ren et al. sought to measure whether this increased nitrogen input from human activity has had any noticeable impact on the water.

Nitrogen is particularly pertinent to productivity in the open ocean, where water tends to be low in nutrients. Close to the coast, where nutrients are abundant, phytoplankton enjoy a surplus of nitrogen and other macronutrients. But in the open ocean, phytoplankton growth can be limited by nutrient availability. Thus additional nitrogen can increase phytoplankton growth, sometimes enough to cause surges called blooms. To mimic the open ocean setting, the researchers studied the Dongsha Atoll in the South China Sea (Figure 1). The Atoll is situated in deep enough water to resemble open ocean conditions, but is still in proximity to China, a major emitter of atmospheric nitrogen.

Figure 1. Map of the South China Sea, with the Dongsha Atoll (also known as the Pratas Reef) marked with a star. Adapted from Yeu Ninje via Wikimedia Commons.

 

To track the human-originated nitrogen in seawater, the researchers considered the relative isotopic weight of nitrogen in fossil fuels. Nitrogen atoms can have differing weights depending on the number of neutrons they contain (more neutrons = heavier atom). Nitrogen atoms of different weights are called isotopes. Because fossil fuels contain a higher proportion of the light nitrogen isotope than is found in nature, isotopes are useful for detecting the influence of fossil fuel combustion on natural systems. Nitrogen isotope composition is reported as a ratio of heavy to light isotopes in the notation δ15N. Fossil fuel combustion frees more light nitrogen isotopes into the atmosphere, lowering the atmospheric δ15N value. So when dust containing nitrogen from fossil fuels settles out of the atmosphere into the ocean, the ocean ends up with more of this light nitrogen isotope and a lower δ15N.

 

Nitrogen records in coral skeletons

Corals have been shown to serve as an archive for past water conditions. The atomic composition of their skeletons can reflect changes over time in the relative amounts of nitrogen’s heavy and light isotopes in the surrounding seawater. The researchers sampled coral skeletons from the Dongsha Atoll to produce a 45-year record of δ15N of the local water (Figure 2).

Figure 2. Bird’s eye view of the Dongsha Atoll. Source: NASA Johnson Space Center via Wikimedia Commons.

 

A 45-year decline in δ15N

From the coral skeleton samples, the researchers noted a significant decline in the δ15N over the past 45 years. Ren et al. examined each of the three nitrogen sources (vertical mixing, nitrogen fixation, and atmospheric deposition) to the Atoll to determine which source was responsible for the observed decline.

A decrease in vertical mixing could increase the relative importance of other sources of the lighter nitrogen isotope to the Atoll, lowering the water’s δ15N. But explaining the observed decline would require a 50% drop in vertical mixing, which has not been observed. Next, the decline in δ15N could have been caused by a boost in local nitrogen fixation. But again, there would need to have been a five-fold surge in nitrogen fixation over this period, which also has not been observed.

The only remaining explanation for the steady 45-year decline in δ15N is that the amount of human-emitted atmospheric nitrogen has risen. The northern South China Sea is situated such that it receives a lot of the emissions coming from China. The observed δ15N decline  suggests that the atmospheric inputs of nitrogen grew to be about 20% of the total nitrogen input to the Dongsha Atoll. The rapid decrease of δ15N in the 2000’s coincides with the rise in nitrogen emissions from fossil fuel burning, suggesting that fossil fuel combustion is the cause of the heightened nitrogen input to the South China Sea.

 

Conclusions

Human activity is known to have profoundly altered coastal and terrestrial nitrogen cycling, but whether or not it has affected the open ocean remained unclear. This paper suggests that fossil fuel burning has indeed caused detectable changes in the composition of the open ocean. If the rise in fossil fuel consumption continues, more and more nitrogen will be deposited in the open ocean. This growing source of bioavailable nitrogen has the potential to cause sweeping changes in open ocean productivity and to the greater oceanic ecosystem.

 

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