Kiel Plant Center

Angela Stevenson - Seagrass and Blue Carbon

Angela Stevenson is a postdoctoral researcher in Thorsten Reusch’s group at GEOMAR in Kiel. She is interested in understanding more about the organic carbon captured and stored by seagrass in the Baltic Sea. Here she explains more about her research into so-called Blue Carbon, and why the health of this overlooked plant is important for the health of local ecosystems.

Angela Stevenson in lab at Geomar
Angela Stevenson at GEOMAR. Copyright: KPC / Rosemary Wilson

What actually are seagrasses?
Seagrasses are marine plants - not seaweeds - that live in coastal waters around the world. As vascular plants they have roots, stems and leaves and even produce flowers and seeds. They need light to survive, so they are generally found at depths of about 1 to 8 metres and grow in patches we call meadows which act as nurseries and feeding areas for fish and invertebrates. These biodiversity hotspots are considered some of the most productive ecosystems on Earth! Their intricate root system also plays a role in coastal protection. In storm-swept coastal areas, for example, you might see less piling of the sand on the beaches and coastal erosion where you have seagrasses present. They also help to improve the local water quality via oxygenation and capturing pathogens and particles.

Seagrass meadow
Seagrass meadow. Copyright: Uli Kunz, GEOMAR 

Why are they important for carbon capture?
The seagrass has blade-like leaves that are very good at capturing particles and this includes carbon and atmospheric carbon dioxide. Their ability to capture and store, or sequester, huge amounts of carbon make them important carbon hotspots. This is because they capture organic particles from outside as well as within their environment. That means it’s not just their own carbon and the carbon of the animals that live and die there, but particles brought into the meadow by water currents are also trapped and settle in the sediment. The plants themselves also efficiently lower the resuspension of sediments by reducing the water flow across the sediment and hence protecting buried carbon. In fact, seagrasses, and other marine plant systems such as mangroves and salt marshes, store around 35 times more carbon per square metre than terrestrial systems such as rainforests! The capture and long-term storage of carbon by marine plant systems is known as ‘Blue Carbon’. Incredibly, despite their potential to store large amounts of carbon, seagrasses have received very little attention in terms of conservation until relatively recently.

What is their distribution here in north Germany?
Seagrasses occur in the entire Baltic and North Sea coastal area. In the Baltic Sea, which is where my work is focussed, we have a total area of approximately 285km2 of the seagrass species Zostera marina. The North Sea has currently about 177km2 of the seagrass species Zostera noltii. They are very sensitive to external influences. We don’t really know why some patches die off and others flourish. If the waters are very dirty, for example, they won’t be able to feed because light can’t penetrate very well through murky waters, but other influences on land such as coastal development or pesticide run-off may well affect them too. My colleagues have actually mapped the distribution of seagrass in the Baltic area over the last ten years, and although there have been some losses, thankfully it seems the total area covered by seagrass in German territorial waters has increased.

Angela Stevenson scuba diving
Angela will spend the summer diving off the Baltic coast collecting samples. Copyright: Angela Stevenson

What is your research focussed on?
We know that seagrasses are very good at storing carbon, but we don’t know how well they can do this for us in Germany. We have some estimates from Nordic countries bordering the Baltic Sea, but it is not clear how much carbon is actually stored by seagrasses here because, to date, there have been very few measurements made in Germany. Carbon levels will vary depending on a number of factors such as whether the meadow is in a very sea swept area or in a protected bay, the sediment grain size, the water quality and so on. We want to understand how much blue carbon is stored here and which patches are accumulating carbon the fastest to basically quantify the blue carbon potential for Germany. My job is to sample along the entire German Baltic coast and quantify the carbon stocks and fluxes in the area.


Two scientists take sediment samples on a muddy shore
Angela's research involves taking sediment cores underwater along the Baltic coast. Here she (right) takes on shore sediment cores during an earlier project with colleagues from the Group of Aquatic Macrophyte Ecology (GAME). Copyright: GAME (Group of Aquatic Macrophyte Ecology)

How will you do this?
I will spend this summer diving along the coast taking sediment cores. We will take several cores at different depths, of about 30 cm each. The aim is to map the range of carbon in each patch. Then we will analyse the cores in the lab. When you open up a core you can already see the different levels of carbon because the sediments differ in colour. We will test for isotopes of carbon, nitrogen, and sulphur to identify whether the stored carbon is of terrestrial or marine origin, and geochronology to see how quickly it has been accumulating this material over time. For this we use a lead isotope called 210Pb. We will also take a core of sediment from a patch of sand close by which we know has never had seagrass growing there – this is our control and it will tell us how much more carbon the seagrass is storing relative to this nearby patch of bare sediment. All this data will help us model how much carbon is being accumulated annually, and which patches are accumulating carbon the fastest. While I work below ground, my colleagues will study the above ground biodiversity in the meadows and how climatic changes affect them so we can get a better picture of the overall health of the seagrasses. It really is a team effort.

Sediment core in the lab
Studying the sediment cores in the lab. Different colours indicate different carbon content. Copyright: Angela Stevenson

What the goals of your research?
It’s clear the carbon has to stay in the ground! It is important to consider that seagrasses, and other blue carbon and terrestrial systems, have been capturing and storing carbon for centuries to millennia therefore it’s important to keep these plants healthy so the stored carbon stays below ground and won’t be released again. Healthy seagrass meadows obviously have many benefits for local ecosystems and the health of coastal areas too. Beyond that, once we have identified which patches are accumulating carbon the fastest we can advise where restoration efforts could be focussed to gain negative emissions. Ultimately our research and the data about how much carbon we’re talking about will clarify the Baltic seagrasses’ contributions to the national carbon budget as Germany sets out to achieve net-zero emissions by 2050, and advise politicians and NGOs on how to enhance carbon stocks.

Seagrass meadows act as nurseries and feeding areas for many species, such as this pipefish. Copyright: Uli Kunz, GEOMAR