Marine Biosphere Through Mixing, by Swimming, May Have an Effect on Climate
A recent study shows how Marine life stirs ocean, by swimming and kinetic activities, possibly enough to affect climate. Interview with William Dewar, co-author of the report that will be published in the forthcoming issue of Journal of Marine Research.
Can you tell us what phytoplankton are and why is their photosynthesis is so remarkable?
Phytoplankton are the plants of the sea. They are the things which convert incoming radiant energy from the sun to more usable forms of energy for the biosphere. Basically, what they do is they make carbohydrates—that’s the chemical energy storage. The chemical energy gets stored in phytoplankton and ends up powering the biosphere both on the land and in the sea. Plants on land all do this, while everything else lives off of them and the same sort of thing happens in the ocean with the phytoplankton playing that role.
You measure the power stored by phytoplankton to be approximately 63 terawatts . How did you come to this conclusion?
It’s based on satellite observations. Satellites measure what is called ocean color, which is a measure of the phytoplankton content of near surface waters of the ocean, so you see variable distributions in concentrations of phytoplankton over the world ocean. Those are largely governed by regional mechanics and regional events which are places where deep water comes to the surface. Those tend to be rich areas of phytoplankton because the nutrient content of that water tends to be pretty high. In such places, phytoplankton bloom and one can see a lot of ocean color from satellite projections. A lot of people have spent time figuring out how to connect direct measurements of ocean color to activities of the phytoplankton and we’re principally interested in something called “net primary production.” When the energy comes in, the phytoplankton receive it and turn it in to carbohydrates, then they themselves use some of that energy to live—they have to live when the sun goes down and they have to live on cloudy days, and at those times, they consume some of their own stored energy. However, they don’t consume it all, they leave some behind. That’s what the biologists refer to routinely as net primary production. From the satellite projections, we’re able to get global estimates of net primary production that we can, in turn, relate to the stored energy that that net primary production represents. Mostly, because we have good knowledge of which energy is sequestered through photosynthetic process, net primary production is a fairly straightforward step to get to the energy. This is where we get this idea of 63 terawatts.
Is that a lot?
That ends up being quite surprisingly large, in fact, I was quite shocked when we got that number. To give you an idea of how one thinks of that relatively speaking, the amount of energy put into the ocean by means of tidal force is between 3 and 4 terawatts. The amount of energy directly applied to the ocean by the winds is something not very well known, but estimates range from 30 to 60 terawatts. So you can see that this chemical fixation rate at 63 terawatts is at least as big as anything else, and in some cases, far larger.
You also estimate that the marine biosphere invests one terawatt in mechanical energy . How is this mechanical energy being manifested?
Basically, this mechanical energy is being manifested by means of kinetic activities of the marine biosphere—the fish. They swim, primarily that’s what we’re talking about, the swimming motions of the living creatures in the ocean. The phytoplankton don’t directly contribute to this budget; what they do is to provide the energy which drives the rest of the marine biosphere. It starts by the feeding of the zooplankton on phytoplankton. It turns out interestingly enough, a significant fraction of the biomass in the zooplankton exhibits quite aggressive vertical migration on a daily basis. This is the vertical biome migration, that we talk about in the paper, which is interesting because these relatively small creatures move up and down in the water column, as far down as depths of one kilometer and they come back to the surface. What they’re doing is evading predation by going to deep waters when the sun comes up and when the sun goes down they come back to the surface. If you think about these many gigatons of zooplankton moving up and down in the water column on a daily basis, you can see that that would be responsible for a reasonable amount of stirring in the ocean.
What are the implications of this mixing in terms of climate control?
The climate that we live in can be thought of as a heat engine. There’s an excess of heat that comes into the equatorial zone and in deficit at the poles, which is consistent with our intuitive feeling: the equator’s hot, the poles are cold. The climate is the way in which the globe ends up evening out the imbalances where you have too much heat; the fluid media of the globe, which are the atmosphere and the ocean, move the heat to the poles. In the process, we develop the climate that we live in. In the ocean, the important component of that heat transport is the overturning cell; waters at the surface sink to depth because they become cold and heavy and they flow away from the poles filling up the ocean. Eventually they come to the surface near the equatorial zone and move back to the poles. When they come up near the equatorial zone, they get hot and as they move to the poles they get cold again. In the process, they lose a lot of heat, moving heat from the equatorial zone to the poles. That aspect of the ocean is the one that is strongly coupled to the climate. How that overturning circulation gets set into motion is central to this issue of the ocean’s participation in climate. In that sense, one of the ways in which people put forward to explain why the overturning circulation exists has to do with mixing. The way that occurs is that as the heat comes into the surface waters, it gets mixed downwards. There’s a tendency for that water to flow up, and as it does so, it gets filled in from the sides and you get this deep overturning cell that we’re talking about. If you understand how mixing takes place, then you have a way of talking about how the overturning circulation gets put into motion.
Traditionally, people have talked about the winds and the tides as driving the mixing. What we’ve suggested is that the marine biosphere through mixing, by swimming and kinetic activities, has as much of an effect as either one of those. That is a control of the climate; the impact of mixing and setting in the overturning circulation into motion.
You suggest that human and environmental decimation of whale and big fish populations may have had a measurable impact on the total biomixing in the world’s oceans…
This comes from our efforts to estimate what the total mixing of the energy provided by the biosphere is, and roughly speaking, we think half of it comes from the zooplankton and half of it comes from everything else. The number we have looked at in more detail is 400 gigawatts, 0.4 terawatts. Suppose that comes from the swimmers in the ocean, we can appeal to trophic systems theory to break that up into contributions from the very apex predators, from the next trophic level down and so on. Current estimates are that there have been fairly large impacts on the populations of the apex predators, as well as on trophic level 4 where we have bill fish, etc., so the current levels are smaller than they would be because of fishing activity. If we take relatively conservative estimates of what that impact is, we can get an estimate of mixing energies of the upper trophic systems of what has been lost in the ocean due to fishing. That number, interestingly enough, compares quite well to twice that of what people think the islands of Hawaii have on the mixing of the ocean. So it’s a surprisingly large number and one begins to think, perhaps there have been some changes in the mixing of the ocean as a result of the fishing activity we have engaged in, or more properly, in the changing population structure of the ocean overall.
William Dewar, thank you
 Dewar et al., forthcoming issue of the Journal of Marine Research
William Dewar is the Chair of the Department of Oceanography at Florida State University.
Interview by Thanh Tam Candice Vu.