Half a dozen divers are navigating the cold waters of the Baltic Sea off the coast of Dahme, a small beach town in northern Germany. Keeping the roots intact, conservationists carefully lift shoots from a donor meadow of seagrass and place them in a bag.

They only take a few shoots from each spot, which will be planted where the conservationists hope to regrow another meadow. They've already picked out a prime area for the young plants, which need sandy sediment, shelter from waves and currents and sufficient light and oxygen to thrive.

The scientists see the work as critical. Myriam Spicka, who heads the NGO Seagrass Conservation, says she has "seen the changes" in the more than 30 years she's been diving.…