September 12, 2025 — The state Division of Marine Fisheries issued an advisory this week warning lobster fishers of low oxygen conditions in parts of Cape Cod Bay.
CAI’s Gilda Geist caught up with Malcolm Scully, a senior scientist at the Woods Hole Oceanographic Institution, to learn more about what low oxygen zones are and why they happen.
Gilda Geist What is a low oxygen zone?
Malcolm Scully A low oxygen zone is something that forms in waters around the world, really. In a lot of places, it’s a seasonal phenomenon. What happens is, as we move from the winter or spring months into summer, the surface waters begin to get very, very warm and the bottom waters remain cool. That is because the surface and the bottom waters are not mixing. Typically, organic material sinks to the bottom where bacteria use it, and that process uses up oxygen. As we move into the summer months, when there’s very little vertical mixing between the surface and the bottom, the oxygen levels will slowly go down. And by this time of year, we often see some of the lowest oxygen values of the season and it’ll stick around until we get a really strong wind mixing event, as the storms from the fall and the winter begin to roll through. And that will mix the oxygen from the surface down into those bottom waters and relieve this low oxygen condition.