The Climate in Emergency

A weekly blog on science, news, and ideas related to climate change

Dead Zones?

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In previous years I have written New Years’ retrospectives, recapping notable climate-related news stories from over the past twelve months.

This year, a retrospective of the past few weeks might be in order.

While I’ve been occupied writing holiday posts–for Yule, for Christmas, for New Years’ Day–and generally being distracted by family obligations, we’ve seen California’s worst wildfire ever (followed by a deadly mudslide just today, which is not unrelated), a rather startling case of Extreme Winter, and a new and really frightening report on marine dead zones. And there have been various political issues. Let’s pick one of these stories and catch ourselves up, shall we?

Please note that where I make statements of fact without linking to a source, it’s because I’m using a source I already linked to.

Dead Zone

The term, “dead zone” is, unfortunately, not a metaphor. These are areas, usually along the coast, but sometimes out at sea, where there is so little oxygen in the water that animals can’t live. It’s a horrifying idea. Imagine minding your own business, living as you usually do, and all of a sudden breathing does no good. Dead zones aren’t spontaneous. They are caused when flushes of nutrients (usually runoff from over-fertilized farm fields or lawns, or from sewage treatment plants) trigger massive algae blooms in the water. Although algae itself make oxygen, when the supply of fertilizer is exhausted, the algae die off and decompose and bacteria go through a population explosion. While not all bacteria breathe oxygen, these do, and there are so many of them that they use up the local supply, causing a dead zone.

In some circumstances, a dead zone can also be caused by algae directly, since algae, too, must breathe (I mean “breathe” loosely here, since all this happens under water)–it is a misconception that plant breathing is the reverse of animal breathing, that plants breathe in carbon dioxide and breathe out oxygen. Instead, plants breathe in oxygen just as we do, and for the same reason–to “burn” sugars for energy. The difference is that we get our sugars by eating, whereas plants make sugars by photosynthesis. Free oxygen is a byproduct of photosynthesis, and fortunately for us, plants make more of it than they need. But in warm, shallow water, a super-abundance of algae can sometimes run short of oxygen at night, when of course photosynthesis stops but breathing doesn’t. In Mobile Bay, in the summer, if the wind and tide are just right, this type of dead zone can move towards the shore, driving anything capable of fleeing before it. Long about dawn, anyone on the right stretch of shoreline can scoop up as much seafood as they want. Before the reason for this influx was discovered, it seemed like magic, an unearned gift from the sea. It’s called the jubilee.

Jubilees occur, less predictably, in other areas, too, such as the Chesapeake Bay, anywhere a dead zone can develop and then move towards shore. The size, shape, and duration of a dead zone depends on many factors, including, temperature, salinity, and wind direction. Dead zones are often low-down in the water column, leaving oxygenated water near the surface, which is why jubilees involve bottom-dwelling species, such as flounder or crab.

Dead zones occur in certain areas every summer, but their shape and size vary from year to year. Evidence of dead zones has been found in sediments going back at least to the late 1800’s, but the same study shows a worsening of the problem since 1950. It may be possible for a dead zone to form without human help, but humans unquestionably cause most of them.

In any case, the problem is less that individual animals die in the short-term, and more an issue of habitat loss. Because of dead zones, the places where marine life can exist are now smaller.

It’s worth noting that there are parts of the ocean where very little lives, and very little has ever lived because there is not much in the way of nutrients for various reasons. These are not dead zones. By definition, a dead zone is a place where life would occur if something had not used up so much of the oxygen.

Ok, Where Does Climate Change Come In?

Dead zones are mostly a story about pollution and land use–the factors that send excess nutrients downstream and into the sea. As such, the problem is sort of a cousin to climate change; the two have causes in common. But climate change also has a direct influence, most obviously because the warmer the water is, the less oxygen it can carry–and the less oxygen must be used up before a dead zone occurs. Also, warmer water raises the metabolisms of the animals that live in it, meaning that they need more oxygen, using the precious stuff up faster–and possibly also making dead zones occur at higher oxygen saturation levels.

Also, remember that salinity and wind direction are also factors in dead zones–and climate change can alter both.

The mechanisms here are a little complex, and I’m not going to describe all of them. Fresher water is lighter than saltier water, which means the two tend to resist mixing. River water flowing into the Chesapeake Bay, for example, or raining onto it, tends to float on top of saltwater flowing in  from the ocean. This resistance to mixing is not absolute–the surface waters of the Bay get brackish pretty quickly–but it is enough that the water on the bottom has trouble getting oxygen from the air. If the algae and sea grass in the water can’t produce enough of their own oxygen, a dead zone develops. The salty water is effectively under an air-tight lid, unless wind blows and stirs the layers.

Well, as sea level rises, more saltwater flows into the Bay. As the deeper waters get saltier, the resistance to mixing gets stronger, and dead zones get more likely.

In fact, although the dead zones of the Chesapeake Bay are now shrinking (thanks to concerted efforts in the Chesapeake watershed to limit nutrient run off), the amount of excess nutrient in the Bay water is shrinking faster. That is, the Bay has been dying more easily now than it used to, and the problem is getting worse. No one is exactly sure why, and various feedback loops and long-term ecological changes  (water dies easier if it’s been sick for a while?) could be in play, but sea level rise could be part of the answer, as could rising temperatures. Changes in wind direction may also play a role, as winds from the south have become less common since the early 1980’s, in favor of winds from the west. Since the Chesapeake is large, north to south, and skinny east to west, the change in wind direction has meant less wave action, and thus less mixing in Bay waters. I don’t know that the change in wind direction has anything to do with climate change–but I don’t know that it doesn’t, either.

As often happens, there are other factors that could be involved, some of which could actually mean climate change reduces the size of dead zones, long term. No one knows for sure.

But so far, as climate change progresses, dead zones have been getting worse. I suppose that could be a coincidence….

What’s the Story?

The reason I’m bringing all of this up now is that a study has just come out showing that although the Chesapeake dead zones are shrinking, dead zones elsewhere are getting much worse–and dead zones are even occurring and worsening in the open ocean, which is generally much more resilient.

Each area’s dead zone has its own history and its own context. How long has the zone been occurring, which industries cause it, who gets hurt by it, what is the relative political power of each, what details of local geography and ecology make the situation worse or better, what stresses other than low oxygen levels might be bothering marine life…. I’m reluctant to make generalized statements without first looking into the rabbit hole of information on each zone. Climate change may be a factor in some zones but not others.

But these zones are worth watching. Is there one near you? Does something you do, or don’t do, help cause a dead zone down stream? Are your state, local, and Federal representatives aware of the problem and concerned about it?

There are zones in the water that kill fish and many of them are growing.

 

 

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Author: Caroline Ailanthus

I am a creative science writer. That is, most of my writing is creative rather than technical, but my topic is usually science. I enjoy explaining things and exploring ideas. I have one published novel and another on the way. I have a master's degree in Conservation Biology and I work full-time as a writer.

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