Answer by Ryan Carlyle, BSChE, Subsea Hydraulics Engineer
It's been proposed, and I believe a test pilot is under way. The Swedish government is funding research into dead zone oxygenation for the Baltic Sea. Can oxygen pump breathe life into ocean 'dead zone?'
The idea of dead zones (eutrophication) is really quite interesting. You could just as easily call them "too much life" zones. By adding nutrients to the ocean, we cause blooms of plankton and bacteria, which consume most of the oxygen from the water. So there's often actually more biomass near the surface in "dead zones" than normal ocean. It's just low-value biomass like cyanobacteria, which humans don't use and few things eat.
Dead zones tend to disproportionately affect seafloor oxygen levels because:
- There is insufficient light for photosynthesis to produce much in-situ oxygen
- Dead organisms fall to the sea floor, where decay consumes oxygen
- Higher-density deep saline layers don't mix well with fresher nutrient-rich surface waters entering the ocean from rivers
Dead zones due to excess nutrient loading or poor water column mixing can be naturally-occurring phenomena, and taken in aggregate are a very tiny portion of ocean area. A dead zone is not inherently "bad" for the environment. It's something that happens in nature (to a lesser extent) even when humans aren't dumping enormous amounts of fertilizer and treated sewage into the ocean.
It's entirely possible to have a low-saline surface layer that is teeming with life, such as algae and phytoplankton blooms which feed zooplankton, fish, and so forth. Then that near-surface ecosystem deposits a "pellagic rain" of organic matter onto the sea floor, which de-oxygenates the deeper layers due to decomposition. This ecosystem stratification can be complex. For example, the latest Gulf of Mexico dead zone map highlights some very high-productivity commercial fishing regions around the Atchafalaya river delta alluvial fan:
Latest Bottom Dissolved Oxygen Maps and Data
Marine life does tend to avoid the hypoxic zones, but it's not a straightforward effect. Dead zones directly decrease Texas shrimp catches, but much less so for Louisiana shrimp catches. The difference is thought to be because the different shrimping seasons catch shrimp at different points in the lifecycle -- Louisiana juvenile shrimp move closer to land avoid the hypoxic zone, thus increasing the concentration in shallow trawling regions and therefore fishing yields. On net, dead zones are bad for commercial fishing, but in complex ways.
"Dead" zones are arguably better than open ocean, which is nearly lifeless due to lack of nutrients and thus has the opposite problem. If we could transport the excess nutrients away from the coast and to the open ocean, they would be good for the environment. The main issue with eutrophication is that human activity has expanded it beyond the natural level. It tends to encroach into productive fisheries and affect high-productivity shelf zones.
Deeper water tends to be less productive anyway, so reducing seafloor oxygen is kind of a mixed bag. Deposition of organic matter onto the seafloor in anoxic conditions is a major natural form of carbon sequestration -- it is the source of the majority of the world's oil deposits. So injecting oxygen to prevent dead zones could have unintended consequences by releasing additional carbon back into the environment.
So, it's a complex subject. We don't entirely know what will happen if we inject oxygen into dead zones. It could dramatically increase ecosystem productivity, or it could have far-reaching unintended consequences. I'm glad it's being studied on a small scale for now.
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