The water in the Kiel Fjord is still cold in April at a good seven degrees. The visibility of the divers at the Geomar Helmholtz Center for Ocean Research is quite good for Baltic Sea conditions at a good two meters. Scientist Angela Stevenson and her colleague Tadhg O Corcora sowed seeds they had harvested themselves for the first time this year – with success. “There’s grass,” Stevenson cheers as he emerges from the test fields. In February they had put out the first seeds at this point.
“Sea grasses are our coral reefs – they increase biodiversity and store carbon in the soil,” says biologist Thorsten Reusch. He heads the Marine Evolutionary Ecology in Kiel. The benefit for climate protection per square meter is similar to that of raised bogs. “One hectare of seagrass stores about two tons of carbon dioxide per year.” A total of around ten megatons of CO2 are stored in seagrass meadows in German waters.
Intensive farming threatens seagrass beds
According to Reusch, there are currently less than 300 square kilometers of seagrass meadows in the German Baltic Sea. About 60 percent of the meadow area from the beginning of the 19th century has been lost. The main cause is intensive agriculture: the entry of nitrogen compounds from fertilizers promotes the growth of plankton algae. They take away light and, in extreme cases, suffocate the seaweed.
The warming of the seas is also endangering the remaining areas. “The Baltic Sea is even warming up three times faster than the world ocean,” explains Reusch. “She broke the Paris climate target of limiting the rise in temperature to 1.5 degrees if possible.” His team found out that the threshold for local seagrass is 25 to 26 degrees: “If this temperature is exceeded for a longer period of time in the summer heat, the meadows die off,” says Reusch.
In summer, his team wants to look for plants in shallow water areas of the Bay of Kiel that have been exposed to higher temperatures. “We hope that the genetic variation in our populations already contains individuals that are prepared for future climate change. So we want to find the super seaweed that is already adapted to climate change.” However, it could also be the case that no plants survive the heat stress in lagoon-like areas for longer, for example between sandbanks. Reusch is optimistic – even with coral banks under heat stress, there are individual survivors, so-called super corals.
“Like picking flowers on land”
The people from Kiel have also been planting seaweed for two years. They have managed around 3000 square meters so far. “Within a year, we were able to determine densities at two locations at the end of summer that a natural meadow in the vicinity also had,” says Reusch. Last summer, the people of Kiel harvested the first 70,000 seeds from “their” meadows. “It’s like picking flowers on land,” says Stevenson. The Canadian came to northern Germany from British Columbia for work.
“Our goal for this summer is to harvest one million seeds,” says Reusch. The Kiel researchers have calculated that the number of meadows in German waters could be significantly increased by a permanent reduction in pollutant inputs. “With a reconstruction we can make a contribution to the German balance sheet in terms of CO2 emissions.”
