Some occur seasonally. Some are permanent. Some are natural. Some are made worse by human activity.
Dead zones are areas within water bodies, usually in deep water near sediments, where there is insufficient oxygen to support life. The scientific name for this phenomenon is hypoxia, but “dead zone” better describes the consequences.
Inputs of nitrogen and phosphorus to surface waters fertilize and fuel algal blooms. When blooms fade, algae sink and are decomposed by bacteria that consume oxygen in the process.
If you spend a day at the beach or on a boat, you would probably have no idea that a dead zone lurked beneath the surface. The water looks the same. Fish can swim out of these areas (not always), but hypoxia weakens them and negatively affects their ability to reproduce. Organisms that don’t move (clams) or move slowly (shrimp) are more likely to be affected. This is bad news for fisheries and coastal livelihoods.
Temperatures also play a role in the formation of dead zones. Warm water holds less oxygen than cold water, so seasonal (i.e. summer) or climate-related increases in water temperatures make matters worse.
Currently, the largest dead zone in the world is in the Baltic Sea. The largest dead zone in the US is in the northern Gulf of Mexico, below the mouth of the Mississippi River. This year, the size of this dead zone is forecasted to be between the size of New Jersey (about 20,000 km2) and Vermont (about 25,000 km2). Seasonal dead zones also occur in Chesapeake Bay and Lake Erie.
So what can be done?
Reduce the loss of nitrogen and phosphorus from crop and livestock production and improve sewage treatment capabilities.
For the past 20 years, the USEPA has been working with states to identify opportunities to reduce nitrogen and phosphorus loads to the Gulf of Mexico from the Mississippi River. The drainage basin of the Mississippi River occupies nearly 40% of the area of the continental US and includes the corn belt. There are strong opinions about whether or not to force states to take action, so efforts are focused on encouraging farmers to voluntarily change their practices and helping cities improve sewage treatment. Unfortunately, nutrient loads to the Gulf show no sign of waning and recently appear to be increasing. Absent changes in political will, the dead zone in the Gulf of Mexico will continue for the foreseeable future.
A couple years ago, I had the opportunity to participate in an outreach program called Baltic Warriors. This was a role-playing game (LARP) where the dead zone in the Baltic Sea was “personified” as Viking warriors who were reanimated as zombies. The zombies would periodically escape the sea and stalk people living in the region, creating urgent societal problems. I even did a "zombie run" in Helsinki at the conclusion of the program (sadly I have no photos of this). While clearly fiction, I appreciate the effort to make environmental problems, like dead zones, more tangible.
The dead zone in the Baltic Sea reanimated this Viking warrior
I was on a panel discussion after the Baltic Warrior event in Stockholm, talking about what to do about dead zones