Fertilisers increase greenhouse gas emissions in the Baltic Sea
Fertilisers from agriculture that reaches the ocean cause a large increase of the greenhouse gas nitrous oxide in the atmosphere. In certain coastal areas of the Baltic Sea, emissions reached levels among the highest ever recorded in coastal seas worldwide.
“These findings suggest that polluted coastal waters may contribute more to climate change than previously understood,” says Henry Lok Shan Cheung, doctoral student at the Department of Marine Sciences.
Fertilisers are essential for modern agriculture and food production, but a large fraction of these nutrients – especially nitrogen – leak into the groundwater and eventually reaches rivers and the ocean. This causes eutrophication, which leads to harmful algal blooms, oxygen depletion and fish deaths.
However, the excess nitrogen not only leads to eutrophication, but also contributes to global warming. This is because nitrogen can be converted into nitrous oxide, which is a particularly potent greenhouse gas: 1 gram of nitrous oxide warms the atmosphere almost 300 times more than 1 gram of carbon dioxide.
Nitrous oxide – a potent greenhouse gas
“Greenhouse gas emissions are often associated with carbon dioxide from fossil fuels, or methane from livestock. However, eutrophication in the ocean can lead to emissions of nitrous oxide – one of the most potent greenhouse gases. That is why it is important that we study this process more closely,” says Henry Lok Shan Cheung, a PhD student at the Department of Marine Sciences.
In his thesis, Henry Lok Shan Cheung has investigated an area stretching from the deep Nordic fjords to the shallow Baltic Sea. In the Baltic Sea, he has discovered that eutrophication caused by fertilisers significantly stimulates nitrous oxide production in sediments that subsequentially release to the atmosphere.
In certain areas of the Baltic Sea, nitrous oxide emissions reached levels among the highest ever recorded in coastal seas worldwide.
“My research findings therefore suggest that polluted coastal waters may contribute more to climate change than previously understood,” says Henry Lok Shan Cheung.
Fjords behave differently
Nitrogen is converted into nitrous oxide through a microbial process known as denitrification. Scientists have long believed that this was the dominant way in which the ocean breaks down excess nutrients.
In Nordic fjords, Henry Lok Shan Cheung’s research findings point to another possible process that may play a significantly greater role. These findings have been published in the journal Nature Communications.
“In Nordic fjords, nitrogen is buried in the sediment on the ocean floor, rather than being converted into nitrous oxide. The fjords appear to act as highly effective natural sinks that provide a nature-based solution to combat nutrient pollution and nitrous oxide emission in coastal waters,” says Henry Lok Shan Cheung.
“We don’t know yet if further disturbances to the ocean by human activities may interfere with this mechanism, so this is something we want to investigate further”, says Henry Lok Shan Cheung.
Relevant to coastal areas worldwide
As eutrophication affects coastal regions all over the world, the findings are relevant far beyond Sweden and the Nordic countries.
“Overall, these results improve our understanding of how coastal seas respond to human activities and environmental change. In the long term, this knowledge can support better strategies to reduce nutrient pollution, protect marine ecosystems, and improve climate assessments,” says Henry Lok Shan Cheung.
Writer: Annika Wall
Link to thesis: ”Nitrogen cycling in a changing ocean: overlooked pathways, key ecosystems and large-scale implications”
Link to paper in Nature Communications: "Long-term nitrogen burial exceeds denitrification in global fjords"
Contact: Henry Lok Shan Cheung, doctoral student at the Department of Marine Sciences. Email: lok.shan.cheung@gu.se
