“We found unexpected blue carbon hotspots”

Oceans are one of the largest carbon stores on our planet. Around a quarter of the emissions we emit each year end up here. Seagrass meadows along the coasts play a role that has so far been underestimated: they absorb a lot of CO₂. But the ecosystem is sensitive to climate change and could lose a large part of its storage function if their health degrades. Angela Stevenson GEOMAR Helmholtz Center for Ocean Research Kiel researches seagrass meadows in the German Baltic Sea. Now she can show her first results.

Angela, how important are seagrass meadows for the ocean?

They hold the soil together: Their intricate roots and rhizomes prevent the soil from being stirred up by water movements creating the muddy, anoxic sediments that are key to inhibiting microbial activity that would re-mineralize the buried carbon and thus emit CO₂.

Do sediments below seagrass meadows store more carbon than sediments elsewhere?

Yes, our samples have shown that the sediments below seagrass meadows are two to sixty times richer in organic carbon than sediments without seagrass. We find more organic carbon in sediments of seagrass meadows sheltered from wave exposure. Based on previous measures outside of Germany, we can roughly say that one meter squared of seafloor below seagrass meadows stores around 27 to 52 grams organic carbon per year.

Storage and sequestration potential of seagrass in the German Baltic Sea

German Baltic Sea seagrass meadows cover an area of approximately 285 square kilometers, which burry 29 to 56 Kilotonnes of CO₂ per year.

Currently, the German Baltic Sea meadows store 3 to 12 Megatonnes organic carbon (integrated to 25 to 100 cm sediment depth). This amounts to 11.5 to 46 Megatonnes of CO₂.

Where does the carbon in the sea floor come from?

Much of the CO₂ is taken up by the seagrass plants during photosynthesis, some comes from the rich biodiversity that lives and dies in these systems. When they die, they sink to the ocean floor, where the CO₂ is stored as carbon. Yet another part comes from particles like plankton that are intercepted by the dense canopy of seagrass leaves. In some of the sediments we found well preserved wood pieces that suggest anoxic conditions persisted prior to seagrass establishing there suggesting that the carbon in these locations originated from pre-existing terrestrial sources.

What influences the CO₂ uptake by seagrass meadows?

The storage capability of seagrass meadows can generally be explained by factors such as wave exposure and seawater depth. Strong movements stir up the soil and re-suspend the buried carbon.

Water quality also has a major influence on CO2 uptake: murky waters allow less sunlight to penetrate to the seafloor where the plants grow. Without this light energy, the seagrass cannot photosynthesize efficiently, thus take up less CO2, ‘feed’ less efficiently, thereby compromising their health and ability to grow.

What is the current situation of the seagrass meadows in the German Baltic Sea?

Our preliminary results show a wide range of carbon stored in the German Baltic Sea coast, much greater than that previously reported for Zostera marina in the Baltic Sea outside of Germany. Interestingly, on our SCUBA diving research expeditions in seagrass meadows of some bays, we observed unexpectedly high organic carbon stocks below the seagrass meadows there.

The seagrass in some of these locations are exposed to high levels of wave activity, so we anticipated low carbon values, but instead we found carbon hot spots that were 50 times greater relative to the bare sediment baseline.

What could be the origin of these hotspots?

That’s what we want to find out. These unexpected organic carbon hotspots cannot be explained by wave exposure or sea water depth alone. As already mentioned, we also found well-preserved pieces of wood in some sediment samples. This is typical for peat-forming habitats. We therefore suspect that terrestrial influences also have an effect in the carbon stored in marine sediments. These could have developed in times before seagrass settled here, possibly even before sea levels rose after the Ice Age. But that's a hypothesis for now. We still have to date our samples to see when the material was deposited here.

What can we do to protect and enhance seagrass as Blue Carbon storage?

Together with national partners, GEOMAR is developing techniques to restore seagrass meadows in the BMBF funded project “SeaStore”. The data from our project in the Helmholtz Climate Initiative will be very instructive to identify sites where a high carbon storage can be expected. For example, we hope to transplant young shoots from healthy meadows to areas where the seagrass has vanished. We want to eventually scale this up across Germany to restore all habitable areas for seagrasses along the German Baltic Sea. At the coast of Schleswig-Holstein, for example, that would be an area of approximately 448 km².

Press release

Carbon-Hotspots in the Baltic Sea

Is there something citizens can do to help?

That is another idea we are working on. We want to involve the many amateur divers in Germany to monitor the health of the newly restored seagrass meadows and help with seagrass planting – a kind of underwater community gardening.