Mathematician has been a member of the faculty since 1980 and department head since 2004.
Sallie Chisholm, the McAfee Professor of Engineering with appointments in the Departments of Civil and Environmental Engineering and of Biology, studies the ecology of aquatic environments. She is a former director of the MIT-Woods Hole Joint program in Oceanography. In 1994 she participated in IRONEX, an ecosystem experiment to fertilize a patch of the Pacific Ocean with iron, a nutrient for tiny floating aquatic plants (phytoplankton) that is relatively scarce. The iron-seeding resulted in a dramatic though temporary increase in the concentration of the phytoplankton, which ingest atmospheric CO2 and emit oxygen during photosynthesis.
Dr. Chisholm co-authored a paper in the July 21, 1995 special ecology issue of Science describing this and other ecosystem experiments, in which scientists study and sometimes manipulate segments of the natural environment over a period of time to gather information about environmental indicators that can be used in ecological understanding. She recently discussed this experimental approach with Alice Waugh of Tech Talk.
Waugh: What can you learn from iron fertilization experiments that you can't learn in the lab?
Chisholm: We've always been handicapped by our inability to do experiments outside of bottles in oceanography. The minute you put the community in enclosures (bags, bottles or whatever), you dramatically change it so you're not able to study exactly what is going on in the natural system. Unenclosed experiments give us a tool for studying the effects of enhanced productivity on the entire food web response, so you can go back and look at historic conditions in the ocean and try to understand how changes in iron deposition in certain regions would affect productivity. Also, iron fertilization changes the structure of the plankton community, and this can only be studied thoroughly in unenclosed experiments. The community goes from a system which is dominated by very tiny cells to a system dominated by larger phytoplankton cells. There appears to be one particular group of diatoms that really takes over, for example, but we don't know if that's a transient or a long-term response because the experiment wasn't long enough to see whether or not the zooplankton that eat this species would also increase.
Waugh: Since phytoplankton remove carbon dioxide from the atmosphere, does this experiment have the potential to slow down the greenhouse effect if repeated on a much larger scale?
Chisholm: IRONEX wasn't designed with this in mind. The hypothesis drew a lot of attention because of the idea that maybe we could fertilize large regions of the oceans and sequester atmospheric CO2. But the experiment was done simply to test the hypothesis of whether or not iron was limited in these areas. Assuming that iron also limits [phytoplankton in] this huge region all around the Antarctic where there are also excessive amounts of [the nutrients] nitrogen and phosphorus, people have calculated how much iron it would take to fertilize that entire area-"the Geritol solution" to global warming. When [IRONEX principal investigator] John Martin first came up with this hypothesis, he was kind of joking around, saying, "Give me a tanker of iron and I'll give you an ice age." The press got hold of this notion, and it gained widespread attention from scientists, engineers, utilities companies and politicians.
But there would be major problems in executing such a plan. Also, by definition, in order for it to succeed, you're changing the entire structure of the food web. So you'd have to be willing to say you're going to dramatically change the ecosystem and also be willing to say you have no idea what the long-term consequences are, but you're going to do it anyway. I think it would be very short-sighted. It's just another apparent quick fix-and I don't think it would even work-for a problem that's been a century in the making. It's just trying to take the carbon we've transferred from the earth to the atmosphere and bury it somewhere else, rather than trying to find alternative fuel sources. Even if it was totally successful and everything worked, most of the models say it would only delay the predicted greenhouse warming by about five years.
Thus, the major application of this work is adding to our understanding of the role of the oceans in the global carbon cycle. Answering basic questions is actually the motivation behind many ecosystem experiments. Some of them have obvious applied dimensions like the acid rain experiments, but many of them were simply done to understand how ecosystems respond to perturbations.
Waugh: So we're still in the discovery phase about how ecosystems work as opposed to figuring out how to effectively manipulate them?
Chisholm: Absolutely, and that's why these experiments are so exciting. I think only through experimentation at this scale can we test hypotheses about what is a perturbation and what is not. These are dynamic systems that are changing naturally. We can't separate human influence from "natural change" because we don't know the natural dynamics. The whole science of ecology is in its infancy. That's why to me, it's so preposterous to think we can predict what will happen when we intentionally modify ecosystems. The iron hypothesis is a good example. Until recently, we didn't know that the phytoplankton in major areas of the oceans are limited by iron availability. All of the models of how these systems are regulated were fundamentally flawed. That's the state of the art. We're really just beginning.
Consider, for example, that 10 years ago we got lucky and discovered a tiny phytoplankton species that turns out to be responsible for 50 percent of the total chlorophyll over large areas of the Pacific Ocean. We knew that chlorophyll was there, but we didn't know it was in this one group. Discoveries like this, which change our picture of how these food webs are structured, are happening all the time. To me it's incredibly humbling.
Waugh: How long have you and other scientists been involved with ecosystem experiments?
Chisholm: My first exposure was the Hubbard Brook Experimental Forest in New Hampshire in the 1960s. They did some very compelling experiments where they clear-cut watersheds that were sealed by bedrock so they could collect rain and runoff and measure nutrient budgets. They compared clearcut watersheds with control watersheds that hadn't been clearcut and others that had been strip-cut. Obviously it was a pretty dramatic experiment and it's been going on for 20 or 30 years. Another one is the Experimental Lakes Area in northern Ontario, where they devoted 22 lakes to experimentation, fertilizing whole lakes with different nutrients or acidifying them to test various hypotheses. Probably more has been learned about the behavior of lakes from that study than all other ones combined. There have been a lot of rewards.
Waugh: Do ecosystem experiments often yield unexpected results?
Chisholm: In the case of Hubbard Brook, there was a major loss of nitrates from the system in the clearcut forest that was much larger than was predicted. It turned out to result from a complex interaction between cations [positively charged ions] in the soil and the bacteria that are playing the big role in the nitrogen cycle. You wouldn't have even looked at the components of the system that turned out to be responsible if you didn't see this huge influx of nitrogen.
Waugh: Like the Biosphere II experiment?
Chisholm: Exactly. There, they saw a steady decline in oxygen in their atmosphere but no concomitant increase in CO2 which you'd expect from the balance between photosynthesis and respiration; if oxygen is going down, carbon dioxide should be going up. And it turned out the CO2 indeed increased, but it was going into the cement. The oxygen decreased because the soil they had put in Biosphere was so rich with organic matter, because they wanted their crops to grow, that it was just loaded with bacteria that were consuming the oxygen. The system was starting out with this disequilibrium and it was just finding its own equilibrium. I love the Biosphere II example because it's such a perfect example of human arrogance: thinking we can just set up an artificial ecosystem and have it go along in perfect long-term equilibrium.
Waugh: How about ecosystem experiments where the human disturbance is unintentional?
Chisholm: Some of the classic cases are the inadvertent introduction of species. They outcompete the indigenous species and reveal community structures that we couldn't see before. The biggest inadvertent experiment, of course, is the elevated production of greenhouse gases such as CO2. Another unplanned ecosystem experiment is Lake Washington, which has been studied for years by limnologists. It received the sewage from the city of Seattle for years until it got so polluted that they diverted the sewage into Puget Sound. They watched the lake get enriched with phosphorus and become eutrophic and then they diverted it and are now studying its recovery. There are all sorts of interesting changes to the structure of the planktonic food web in the lake during the recovery that never could have been predicted.
Waugh: But "recovery" doesn't mean a return to exactly the same state as it was in before people interfered, does it?
Chisholm: No; it recovered in the sense that it became less disgusting for human recreation, but the species were not the same types and proportions as before. In fact, there are some new zooplankton species that turned up for reasons that are not totally clear, but the analysis is focusing on enhanced fisheries success, because the lake had been seeded with salmon and they put rubble in the streambeds and the salmon eggs grew, so the salmon were eating the predators of these other things, and so on-a whole chain of events. So it isn't the prehuman lake, but there are still things we can do to reverse the [negative] trends.
Waugh: Given the often negative and unexpected results of human interference in ecosystems, is it ever desirable to deliberately manipulate an ecosystem to achieve a desired effect?
Chisholm: There are already a lot of biological control measures in use in agriculture and natural ecosystems. I think this approach is often advisable in lieu of pesticide use in agriculture, and sometimes advisable for controlling species invasions in natural ecosystems. From my perspective, it's a very important area to foster because it requires that you understand the interactions between the species. It's just an inevitable direction. Like it or not, we have to manage ecosystems, especially on land. Humans have usurped something like 30 percent of the primary productivity in the terrestrial ecosystems, meaning that 30 percent of all the photosynthesis on land is under the direct control of humans (cropland and forest) or it has been diverted somehow, such as changing a grassland to a desert by removing all the water. So that means that we as a single species have dominated 30 percent of the resources available to all species.
A critical dimension to ecosystem research is maintaining long-term records of things like species composition and biogeochemical cycles. This is something ecologists are working very hard to get the funding to support. This is one of the problems when you have funding cycles of four years and ecosystem cycles of 100 years or more. That's why the Hubbard Brook database, which now spans more than 30 years, has been incredibly valuable. They have acid rain measurements for decades. If you don't know what the long-term natural variation in an ecosystem is, how can you know if humans have influenced it?
Waugh: Do these complexities and timelines pose a special problem for this field?
Chisholm: Yes, it's very difficult. In some ways ecology is an intrinsically intractable science. There are so many variables. If you're asking questions about how an ecosystem works, you can't address that at the level of every single molecule. You have to ask questions that are at an appropriate level of organization, and those levels are intimidatingly complex. But that's what's so exciting about ecosystem experiments: their success has shown us that one can do experimental science at the level of ecosystems. It's something I stress with my undergraduate ecology course, that ecology uses the same tools of scientific inquiry as other sciences.
A version of this article appeared in MIT Tech Talk on October 25, 1995.