Nissum Fjord helps researchers understand the CO₂ cycle of coastal areas

Author: Maria Knadel

On World Environment Day and in connection with EGN Week, Geopark Vestjylland highlights new research from Nissum Fjord, showing how a local coastal landscape can contribute to our understanding of global climate processes.

Last year, I followed the young researchers during their fieldwork in Thorsminde, where they worked with measuring equipment, water samples and data logging by the fjord. At the time, the project was still ongoing. Now, the results have been published in the scientific article Quantifying tidally-driven CO₂ air-sea exchange in a Danish coastal lagoon, written by Janine Steffens, Linnea Henriksson and Isaac R. Santos. The article is based on high-resolution measurements from Thorsminde and Nissum Fjord in 2025.

The fjord’s “breathing” is controlled by the movement of the water

The study shows that CO₂ dynamics in Nissum Fjord are largely controlled by the movement of water through the sluice at Thorsminde. When seawater flows into the fjord, the CO₂ content in the water increases. When the fjord drains out towards the sea, CO₂ levels decrease. In this way, the sluice acts as a rhythm in the fjord’s “breathing”, where the direction of the water influences whether the fjord absorbs or releases CO₂.

During the fieldwork, the researchers measured dissolved CO₂, oxygen and radon every 10 minutes over four days. Radon is used as a tracer because it can show where groundwater flows into the coastal marine environment. The measurements gave the researchers a detailed picture of the interaction between water exchange, groundwater, biological activity and sluice management.

The results indicate that Nissum Fjord acted as a net sink for atmospheric CO₂ during the measurement period. In other words, the fjord absorbed more CO₂ than it released on the days when the researchers carried out their measurements. At the same time, the study shows that CO₂ levels change rapidly, and that seawater, freshwater, groundwater and biological production all play a role.

Plants, algae and groundwater affect the CO₂ balance

An important part of the explanation lies in the fjord’s biological activity. When algae and aquatic plants grow, they use CO₂ and release oxygen, just like plants on land. The researchers found signs that this production may help keep CO₂ levels low in the fjord.

At the same time, nutrients from streams and groundwater may stimulate biological production. Groundwater can therefore affect the CO₂ balance both by adding substances to the fjord and by supporting the processes that remove CO₂ from the water.

A local landscape with global significance

World Environment Day and EGN Week are opportunities to focus on the connection between nature, climate and people. The study from Nissum Fjord shows how geology, water chemistry, biology and human regulation together influence the fjord’s CO₂ cycle.

The shape of the fjord, the groundwater flow and the sluice at Thorsminde create a natural laboratory where researchers can study processes with significance far beyond West Jutland. For Geopark Vestjylland, the research is a strong example of how concrete places such as Nissum Fjord can contribute to international climate research and provide new knowledge about global climate and environmental challenges.

Read the full scientific article here