What’s causing a nitrogen build-up in the oceans?

The researchers used nutrient data from the past 20 years from different ocean depths for the study. “The ocean mixes at a slow pace and the water in the deeper ocean is older than that on the surface,” explains Gruber. The result is that samples taken from the deeper ocean allow conclusions to be drawn about the past chemical composition of the ocean surface – up to 40 years back.

Until now, researchers had assumed that the man-induced increase in nitrogen build-up in the oceans via the atmosphere was too small to be detected, mainly because large amounts of nitrate are present in the ocean, especially at depth.

A method developed by Gruber, which uses the ratio of nitrate to phosphate, enabled detection of the man-induced nitrogen build-up and the researchers could clearly identify the anthropogenic signal in the upper 500 metres of the ocean. The increase in the nitrate concentration, particularly in the vicinity of the surface, corresponds to almost a doubling of the local concentration. ETH doctoral student Simon Yang confirmed the detected nitrate increase by computer calculations using an Earth system model. The model calculations show a similar picture.

Fertilising the ocean

The researchers can only speculate about the consequences of such a build-up of reactive nitrogen in the ocean. “Since the biologically available nitrogen in the North Pacific is relatively scarce in comparison with other nutrients, the build-up of nitrate may fertilize the growth of algae,” says Gruber. It is also conceivable that certain types of bacterial species may become rarer as a result, namely those that focus on nitrogen fixation. These bacteria can transform molecular nitrogen into biologically available nitrogen compounds. But when such nitrogen compounds accumulate due to the addition from the atmosphere, these microorganisms lose their ecological niche and are outcompeted by other organisms.

The scientists plan to conduct a detailed investigation of the effects of nitrogen build-up in the ocean. This is particularly important as emissions are likely to continue increasing globally, causing a further increase in the atmospheric deposition of reactive nitrogen to the oceans.

Published in collaboration with ETH Zurich

Author: Fabio Bergamin studied Biotechnology in Basel and Strasbourg and received a PhD from the University of Bern in Immunology.

Image: A Green Sea turtle swims over a reef near the surf break known as ‘Pipeline’ on the North Shore of Oahu, Hawaii  March 20, 2013. REUTERS/Hugh Gentry.