It’s one of the first warm spring days and the ice has recently left the small bay of Sandviken, located about 100 kilometres north of Stockholm. The water is mirror-like and a few swans complete the picturesque scene. But could this idyllic bay also be a climate villain, by emitting greenhouse gases that contribute to global warming? In search of answers to that question a group of researchers from Stockholm University will spend the day by the bay, taking samples of air, water and sediment.

“We will measure methane, carbon dioxide and nitrous oxide to see if there is a net uptake or release of greenhouse gases from the bay”, explains atmospheric researcher Julika Zinke, as she installs the measuring system, consisting of several boxes, hoses and cables, on a small motorboat at the pier. “In the long term this will tell us whether the bay acts as a carbon sink or a carbon source.”

Photo: Lisa Bergqvist

The WEGAS (Water Equilibration Gas Analyzer System) equipment allows continuous measurements of greenhouse gases, and a similar system has previously been installed on a houseboat at the Askö Laboratory and on board the research vessel Electra. This is the first time the measuring equipment is installed on a small motorboat, which is necessary to cover different parts of the shallow bay – apart from one deeper hole, Sandviken is no deeper than three metres.

“Until now, we have only tested the system as a whole in the shower at the office”, Julika Zinke says with a smile when all the parts are finally in place.

Phytoplankton bloom causes undersaturation

The boat leaves the pier and cruises back and forth across the bay. To get the full picture of the emissions and carbon uptake in the bay, it’s important to cover both nearshore and offshore parts, as well as parts with different amounts and types of vegetation. Professor Christoph Humborg, Scientific leader of the Baltic Sea Centre and the CoastClim research cooperation is monitoring the incoming data.

“What we have here is a typical late winter-early spring situation”, he says, adding: “We have a little bit of oversaturation of methane in the water compared to the air. So, there is more methane in the water, and that can be emitted to the air. But in case of carbon dioxide the water is undersaturated. This is because there is a first phytoplankton bloom going on, where the plankton use carbon dioxide for their photosynthesis.”

By further measurements during the year the researchers will assess whether Sandviken is a carbon sink (taking up greenhouse gases over the annual cycle) or a carbon source (emitting more gases over the year than is taken up). This is a key question for CoastClim, where researchers from different disciplines at the Universities of Stockholm and Helsinki are working together to explore the potential of coastal ecosystems to provide natural solutions to mitigate climate change.

Link between degradation and emissions

Sandviken and the other bays included in the ongoing sampling campaign are also being monitored for nutrient concentrations, Secchi depth, turbulence and several other parameters as part of the Thriving Bays restoration project. In the seven-year project, researchers are testing different techniques to restore heavily eutrophied bays to good ecological status.

“The measurements in these bays give us a unique opportunity to assess the extent to which greenhouse gas emissions are the result of degradation of a coastal area”, says Christoph Humborg. “In the longer term, it will also be possible to assess the climate benefits of restoration.”

Sediment samples add to the picture

A major component of coastal greenhouse gas emissions, expressed as carbon dioxide equivalents, is methane, previous research has shown. Methane is formed under certain circumstances when organic material is decomposed, mainly in anaerobic environments and in the presence of certain microorganisms. Learning more about these processes is another important aspect of understanding the climate impact of the coasts. 

On a subsequent trip by the motorboat, researchers Alexis Fonseca and Martijn Hermans take sediment cores from the bottom of the bay – a dirty job best done in galloon dungarees – to measure the methane content and identify the microorganisms present.

Back on land, biogeochemist Martijn Hermans takes one of the sediment cores and punctures it at different depths. He extracts some sediment from the various holes, using a syringe, then injects it into a container with a super-saturated and salty solution.

Researcher Martijn Hermans. Photo: Lisa Bergqvist

"The super-saline solution stops the microbial activity and, because it's also super-saturated, it makes the dissolved methane in the sediment go into the gas phase," explains Martijn Hermans. "But it's very important that the containers are kept upside down so that the methane can't escape.”

The subsequent analysis provides the researchers a profile of the methane concentration in the sediments at different depths.

"Based on this, you can predict, for example, how much methane will be released into the water", says Martijn Hermans.

The role of microorganisms

Alexis Fonseca slices other of the sediment cores into pieces corresponding to the depths at which the methane concentration is to be measured.

"I'm a child again," he jokes, stirring the boxes containing the different slices of sediment, which get darker the deeper he goes into the core.

However, the samples are used for more than amusement. Alexis Fonsecas research is focused on identifying microorganisms – mainly bacteria and archaea – in the sediment. 

Using DNA and RNA analysis methods called genomics, metagenomics and transcriptomics, he is able to identify species, their functions and their metabolism.

Researcher Alexis Fonseca. Photo: Lisa Bergqvist

“The microorganisms are important, because they are responsible for the biogeochemistry”, he explains. “Some of the bacteria and archaea are producing methane, while others are consuming it. My expectation is to find producing organisms in the bottom layers of the sediments, and consumers higher up.”

After stirring, portions of the samples are transferred to tubes and placed in liquid nitrogen at -196 degrees Celsius during transport to the laboratory, where analysis can begin.

“This time, we are also testing a new approach – if there are viruses present that are infecting the bacteria and archaea, and how that impacts their abilities to produce and consume methane”, Alexis Fonseca reveals. “All these processes are important to understand the greenhouse gas fluxes from a bay like this.”

Text: Lisa Bergqvist