Kelp, I understand, is a huge ~~carbon sink~~ oxygen producer and it grows incredibly fast. Overfishing leads to urchins eating them up though and they begin to starve. It becomes a self reinforcing cycle because predators won't eat the starved ones, which become zombie-like and survive to keep eating the kelp.
I'm not sure I'm remembering all the details right off the dome, but I know there was one group trying to make robots that destroy the zombie urchins so kelp forests can regrow.