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.
We don't need to be worried about it in Cape Cod Bay at this point. The Division of Marine Fisheries put out an advisory, and their advisory is really just indicating that oxygen has reached a value of about 4 milligrams/liter. And so we really start to worry about it when it gets to be 2 milligrams/liter or lower. So it's really just letting people know that things are getting low, but it's not anything that we need to necessarily worry about at this time because it hasn't gotten to the point where it's causing any significant negative influences on the organisms that live in those bottom waters.
GG Can you speak to like the history of this issue? Is this becoming more of an issue in Cape Cod Bay? Is this something we've been observing for a long time?
MS The sort of seasonal cycle is pretty normal. It's been getting warm in the summers and cooling off in the winters for as long as we can remember, and so this process where the water column becomes stratified is very normal. And that means that anything that's using up oxygen is going to drive the oxygen to lower levels during this period of time. What is sort of unusual is in 2019, we saw the oxygen get so low that it started to kill benthic organisms, and lobster fishers in the region started pulling up dead lobsters in their pots. And that's really when we became aware of this being sort of an unusual event. And so we went out in 2020 and we saw similarly low values. And so ever since those two summers, we've been sort of keeping a closer eye on this and we haven't seen oxygen levels get that low since.
GG Scientists don't know the cause of these low oxygen levels. Any theories? Any ideas?
MS We don't know exactly, but we have a pretty good idea that the lack of vertical mixing that supplies oxygen combined with the decomposition of organic matter is really what drives the oxygen down. In 2019 and 2020, we had very anomalous algae blooms. It was a new species of algae. These blooms were found pretty deep in the water column, but it was levels that were five times anything we'd ever seen. And so we really think those really high levels of algae biomass, when they died and sank to the bottom, are what depleted the oxygen in 2019 and 2020. That was a new species and we hypothesized that perhaps that new species was here to stay, but we really haven't seen blooms of this species since. And so that's the part we really don't understand. We know that the climate is changing. We know when the climate changes, new organisms can become established. But we really aren't at the point where we can predict when these new blooms are going to occur. And we didn't see a bloom any of the consecutive summers since 2020. And so we think the low oxygen is really directly related to these unusual algae blooms.
GG Is there any sort of common misconception about low oxygen zones that you would like to debunk?
MS In Cape Cod Bay, the seasonal cycle, that's a pretty natural cycle. The unusual part is when we get these species that haven't bloomed before. And so we don't really understand what's causing that, but we think that's directly related to the unusual low oxygen we've seen in Cape Cods Bay. I don't believe what's going on in Cape Cod Bay is due to human-introduced nutrients. The bottom waters of Cape Cod Bay is a giant reservoir of nutrients. It's really the result of a new species more than it is something that humans have done, I think, to change the system.
