River's respiration affects life, water quality
Jeff Richey measures the way the Amazon River “breathes.”
Jeffery Richey, Professor of Oceanography at the University of Washington’s School of Oceanography, is studying river respiration in the Amazon River basin. In August, 2005, Richey talked with Earth & Sky’s Eleanor Imster about the breathing Amazon.
Imster: In the Amazon rain forest, the trees and plants breathe in carbon dioxide, and sometime later, the river breaths CO2 back out. Can you tell me what’s happening?
Richey: Carbon is the central element in all this. It can be carbon dioxide in the air. It can be organic carbon, in your bodies, or in the tree trunks, or in the fish. So, it’s just transforming what the carbon is associated with. But they’re all carbon.
Imster: When it’s a gas in the air, it’s carbon dioxide, and that’s the greenhouse gas that’s warming our climate…
Richey: That’s correct. And the fact that the carbon dioxide in the atmosphere is increasing is why we’re interested. The trees take up carbon dioxide through photosynthesis. But, another point that’s important, only a small part of the carbon that’s taken up by the trees actually even gets into the river. Lots of it is respired back, goes right back into the atmosphere, or stays on land as litter, dead leaves and dead roots and all that kind of stuff.
Imster: How does it go back to the atmosphere right away?
Richey: At night, the trees respire. That’s why, over the course of a day, a mature forest doesn’t grow. The maple tree in your back yard isn’t growing very much any more, right? Or we as adults aren’t growing in more either. We eat during the day, and that gives us our basic fuel that we use for metabolism, the process of getting energy from that fuel. Then we breathe out the CO2 just doing that, just maintaining equilibrium.
Or think of your checkbook. Unfortunately, our bank accounts don’t grow as fast as we’d like. No matter how much we put into it, it goes out as fast. What the trees are doing is acting like a checkbook for the carbon dioxide in the atmosphere. The CO2 goes in, it contributes, and then a lot of it, but not all of it, goes right back to the atmosphere.
Imster: I thought that the trees breathed in carbon dioxide, and breathed out oxygen.
Richey: During the daytime, the carbon dioxide is taken up. And then, you’re right, oxygen goes back out. But there’s also CO2 that’s respired back out.
Imster: What percent of the CO2 that the trees take in is respired back out?
Richey: Probably 90 percent of it, over the course of the day, depending on how fast things are growing.
Imster: So, of the carbon that’s retained by the forest, some ends up on the forest floor in soil, debris, bark and leaf litter. Some of that gets washed into the river, and is eaten by river life – microorganisms, insects, animals and fish. Then it’s breathed back out into the atmosphere as carbon dioxide. I guess what I’m asking is, when you say the river is breathing out carbon dioxide, it’s because the animals that are in the water are breathing out carbon dioxide?
Richey: Yes. There are animals or plants somewhere along the way. That’s why we say it’s breathing. Part of the issue here is that there’s so much more carbon dioxide in the river than in the atmosphere that the carbon dioxide will go from the river to the atmosphere. Think of a bottle of Coca Cola. Shake it up and take top off, what happens?
Imster: Gas bubbles out.
Richey: Because the concentration of CO2 in the Coke is much higher than in the atmosphere, so it goes from the Coca Cola to the atmosphere. The rivers are nowhere near as concentrated in CO2 as Coca Cola is. But there’s still a lot more in the water than in the air, so it will go from the water to the air.
Imster: I understand that the ocean grabs CO2 out of the air. A lot of carbon dioxide gets removed from the atmosphere thanks to the ocean. Why doesn’t the Amazon River do that? It’s a huge river basin.
Richey: The reason is that ocean has, overall, less CO2 than the modern atmosphere. Gas will go from someplace where there’s more of it to someplace where there’s less of it. For example, if you had that bottle of Coca Cola with no CO2 in it and you opened it, CO2 from the air would go into the Coke.
The ocean’s main organic matter is completely internal, from algae and phytoplankton and that sort of thing. These organisms die and take and take the carbon out and bury it at the bottom of the ocean. Basically, the upper part of the ocean, the living part, is losing carbon that way. Whereas in the river basin, you’ve got all these vast amounts of trees and leaves and animals and worms and everything else. A river receives all this organic carbon subsidy or fuel from the land. So the source to the river is vastly greater than it is for the ocean.
Imster: So, there’s more life per square inch in the river than in the ocean?
Richey: Parts of the ocean are really rich, and this is where the great fisheries are and the big algae blooms and all that. But an awful lot of the ocean is pretty much a desert, there isn’t very much going on.
Whereas in the river, there’s so much fuel coming in that the animal population has lots to chew on, and so it’s working hard and turning out organic carbon into its bodies and in the process, releasing the carbon dioxide when it respires. There’s a tremendous amount of organic matter. Things are going wild – all that tropical life.
Imster: So in a warmer place, the forest and the river breathe faster?
Richey: Yes.
Imster: As our climate warms, will everything start to breathe a little bit faster?
Richey: Yes. That’s definitely one of the issues with global warming. Things will start to happen faster. But it’s not a direct relationship. You can’t say, “the temperature’s going to go up one degree and we’re going to increase metabolism by one percent.”
Imster: Let’s say X amount of carbon dioxide is taken in by the forest, and Y amount of carbon dioxide is released by the forest and the river. Are X and Y equal amounts?
Richey: Those are the exact kind of things we’re trying to discover.
When the forest takes up carbon dioxide, it releases oxygen. You sometimes hear that called “the lungs of the Earth.” So whenever CO2 is taken up, oxygen is released, and vice versa.
On a typical day in the rainforest, let’s say ten units of CO2 are taken up, during the daytime. But at nighttime, oxygen is consumed by the forest and the animals and CO2 is released. So, at nighttime, maybe nine and a half units go back to the atmosphere. If say, only seven units went back, the forest would grow pretty rapidly. In a mature forest, that just doesn’t happen.
Now to make these kinds of measurements turns out to be incredibly tricky, and that’s where a lot of uncertainly has come from. Out of those ten units taken up, there are still a few tenths of the units that are left over. The trees will grow a bit, dead leaves will fall, roots will respire into the soil. Animals come along and eat that. That stuff doesn’t go to the atmosphere. Our findings show that a certain amount of that goes into the water and ultimately gets respired back to the atmosphere someplace else than where it happened to start.
Imster: How do you make those measurements? How do you find out how much carbon dioxide is going into an area and how much comes back out?
Richey: There is a very large science program going on now called L.B.A., the Large–scale Biosphere Atmosphere Study of the Amazon. It’s a joint project of N.A.S.A., the Brazilian government, and the European Union. Basically, what this whole program is set up to do is to measure exactly your question. For example, there are towers that are put up with sensors that measure how much carbon dioxide there is over the course of 24 hours.
It’s a very complicated process to measure these things. The problem is that the difference between that daytime and nighttime is really pretty small. And what we’re looking for is that small part. Because that’s the part that says whether the forest is growing, how much impact deforestation and climate change will have.
Imster: In your research, you’re able to look at carbon samples from the river, and determine not just how old the carbon is, but what kind of plant it comes from. How do you know if the carbon came from grass or big trees?
Richey: One of the more detailed chemical techniques that we use are things called tracers. Imagine a string of a thousand white ping–pong balls streaming by in front of you. You want to know how fast those are going, but it’s just sort of blurry, you can’t see. But if every tenth one was bright red, you could see that one go by, and if every twentieth one was green you could see that one go by. And if you knew what the ratio of the red ones and the green ones was to the total, which is easy enough to figure out, then all you have to do is count the red ones, or the green ones and that will tell you where everything is coming from.
And we look for the equivalent of that in chemistry. We used two tracers. Carbon–14 is one for age. The other one, Carbon–13 is very sensitive to whether the plant material was a big old tree or whether it was a grass. Because it turns out that the tree and the grass, as you can imagine, have different photosynthesis pathways. And so this tracer, called Carbon–13 is very different from one to the other. If we know how much Carbon–13 is in the system, we can calculate how much of it might have come from grasses versus trees. It’s a neat little technique.
Imster: And where the river carbon is younger, does that mean that the respiration is happening faster?
Richey: It doesn’t necessarily mean that it’s happening faster. It’s that the organic matter that it comes from is much more recent and is being metabolized faster. If it’s more metabolically active, that shows several things. It shows that this material is still fresh, and part of the current carbon cycle activities.
It also means that that water system is responding to the land. This whole study on this came from a paper in Nature in 2002, which called attention to this whole outgassing the first place. One of the basic reasons why this all really interesting, is that this shows that the land and the water are much more tightly tied together than people thought.
Part of that’s just disciplines. People that study trees, you know they’re off under the trees and try to get as far away from streams as possible. And water people tend not to go up on land. It’s a lot more fun out in a boat someplace. The two communities had never really talked together much about this. And so, the core finding is how tightly coupled that land and the water are. It also means that if you start changing the land, the whole water system is going to change as well.
Imster: You mean that the land use, the kinds of plants growing, makes a difference in how much carbon dioxide is being inhaled?
Richey: Very much so.
Imster: So, the problem is finding ways to use the land that will minimize the carbon dioxide we put into the air. But people need to economically survive at the same time.
Richey: You just hit an important point in this whole thing. We can’t just shut everything down just to keep the CO2 in the atmosphere from going up. People have to make a living. This brings up the huge balancing act in all this, between the remedial actions you can take, versus people’s ability to make a living?
Imster: “Save the Amazon rainforest” is one of the rallying cries for conservationists. How much does the Amazon rainforest help in regulating the amount of carbon dioxide in the atmosphere?
Richey: A mature rainforest takes a whole bunch of carbon dioxide out of the air during the day, but it puts back a whole bunch at night. So it’s basically pretty close to being in equilibrium with the atmosphere. So it’s not really doing much, it’s that maple tree in the back yard sort of thing. It’s sustaining vast amounts of life and all sorts of other things, but in terms of the atmosphere, it’s pretty much neutral.
The big part of the rainforest is that they’re being cut down, and there’s so much carbon in them, then that CO2 goes back to the atmosphere.
Imster: Because those big trees are holding so much?
Richey: Exactly.
Imster: I read a quote of yours, “River breath is much deeper and faster than anyone realized, I understand about the faster – that carbon dioxide is going in and out – but what about the deeper?
Richey: There’s more of it. I mean, no one had ever thought about this before, that river systems could be doing this sort of thing. Not only is it going faster, but the amount of carbon that’s coming out of the rivers, in other words, the breathe, is just bigger, stronger, than people thought.
Imster: Are you able to measure whether the amount of carbon dioxide that the Amazon River is exhaling is increasing or decreasing over time?
Richey: In principle, we’d be able to do that. In practice, even figuring out how this works at all is right at the limit of our abilities – and funding. That said, we are focusing on some of the areas that have the greatest land use change and trying to see what the differences are between those areas and areas that are still intact. And we definitely see influence of the change in vegetation types, as you move from big trees to soybeans or squash or corn or whatever gets grown.
Imster: One thing that these findings are telling us is that how you use land, what you use it for, makes a big difference, and those differences may make the river respire more quickly, but so what?
Richey: Fair question. There are several “so whats” in all that, outside of whatever basic science interest there is. First of all, how that land and water are tied together is a critical element in these things. It’s not just the carbon, but it’s also the water and energy balance.
For example, our group is making computer models of both the Amazon and Mekong River in Southeast Asia. We’re asking what effect would it have if you changed the temperature, or changed the land cover, if you moved from forest to rice fields, or slash and burn agriculture? What effect is there going to be on the overall water cycle?
At the most simple level, the life of a river depends on two things. One is, how much water there is, and when, and its temperature. And then how much food is there in the water for the plants and the fish that you care about it eat.
One of the big issues in some of the big rivers in Southeast Asia is if dams go on upstream, is that going to change the food supply for the hugely economically important fisheries downstream? The whole CO2 thing is not just related to the sort of “gas” part of the story, but it’s the whole metabolism. Think of it as it gives you the pulse of the system. So, if the CO2 level’s changing in the water, that can be a sign that the overall metabolism is changing. So it signals something. It’s a harbinger, canary in the mine if you would.
Imster: And so what if the metabolism changes?
Richey: If the metabolism changes, you say, OK then, which metabolism are we caring about? What people care about will be fish, right? Because that’s their food source. And they also want to know about the water for the water quality aspect. All this discussion of CO2 and metabolism and all is sort of science–speak for water quality. If you’re changing the water quality, which directly affects people’s ability to drink it, then people will care. The ability to drink water, the ability to fish from water, and the ability to use water for irrigation are the basic people issues in all that.
Imster: And so if we have some idea about what will change things how, we might be able to make better decisions?
Richey: And this is where the words like “sustainability” come in. How can you optimize the use of land and water so you’re mitigating some of the climate change and pollution impacts, without taking away people’s ability to make a living from the land?
