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The Human Impact on Freshwater Ecosystems
From: Columbia University
| By:
Dickson Despommier |
EDITOR'S INTRODUCTION |
The changes we make to the world around us, no matter how noble they are in benefiting humans, are not benign to other species. Dams create reservoirs that bring water to our communities, but they also drown creatures that were there before us and enable other species that cause human disease to thrive. Dickson Despommier, professor of microbiology and environmental sciences at Columbia University, studies the amazingly diverse species in freshwater ecosystems and the effects our choices have on the balance of life there. Despommier explains how humans cause imbalance in freshwater ecosystems, and how we can use our knowledge of environments, species and diseases to return the systems to equilibrium and control epidemics of waterborne diseases. |
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| A dam on the Green River in Utah. | |
 e all grew up with the periodic table of the elements. We all, so to speak, eat off the periodic table. G. Evelyn Hutchinson said that the difference between human beings and all other animals and plants on the planet is that for the other plants and animals the first 45 or so elements are considered crucial for life. Almost all plants and animals use the first 45 elements, but humans use all 92. We even make a few of them. The problems associated with using the rest of those elements could be classified as environmental health sciences: what do you do when you concentrate something like uranium into a very small space? Those elements all have public health effects. Hutchinson, even in his early days, realized the danger of working with all 92 elements. |
Today, ecosystem functions and ecosystem services are part of our language, and they are intermingled with issues of public health. Unequal relationships are often classified as parasitic, where the species taking advantage of another species becomes a "culprit." You know as well as I do that life is not like that--there is not polar bad and good in biological relationships, where every species aims for survival. But there are public health implications of the success of these species, and of our controlling their environments. |
Dammed mosquitoes
Within the contiguous United States there are more than 70,000 dams. There are more than 800,000 dams throughout the world. That has huge, amazing consequences for our lives on this planet. The mosquito is the parasite that everybody is worried about during travel. At the departments of epidemiology, we get your calls: "Is there still malaria in Indonesia?" Well, there is more "high speed" lead poisoning over there right now than there is malaria, but there will always be malaria. In the twentieth century, as many people have died from malaria as now live in the United States. The spread of malaria is enhanced through building dams and reservoirs behind the rivers, which create standing bodies of water that allow the mosquitoes to survive and flourish. No matter where you go there are mosquitoes, and there are more than 300 million people infected at any one time by that one parasite. |
Dams on the Tennessee River were established not only for hydroelectric power or to establish reservoirs but also to help eliminate malaria. They set up and coordinated about 10 dams so that they could control the mosquito population by opening and closing the dams. As the mosquito populations rose in a lake, they would lower the reservoir, and that would raise the reservoir below it. By lowering the first reservoir, all the larvae would be stranded on the shore and would die. Then that reservoir would be raised the next week so the one below it would be lowered, and so on, all the way down. By doing that, they actually controlled malaria without any drugs, without any insecticides, with nothing except these dams, which, of course, wrecked the entire river system. |
The other thing that eliminated malaria in this country was farming. Pigs do not like swamps, so pig farmers filled them in. The same thing worked in Ostia, in Rome, when the Romans filled in the surrounding swamps and established it as a major port city on the Mediterranean. Environmental intervention has been the best way to eliminate malaria from a given region. |
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| A typical limestone stream. Note the abundant macrophytes. | |
Insecticides or antimalarial drugs have never succeeded. Those work temporarily, but eventually the insects become resistant, so the best way to eliminate it is through environmental intervention. If you could in a given moment cure everybody of their infection and at the same time do some environmental intervention, there would not be any more in the next season. It does not carry over from year to year, like encephalitis viruses do. That is the difference. |
To propagate malaria, a mosquito has to bite an infected person, incubate the infection for a week and a half and then bite another person. Most mosquitoes are not infected, and mosquitoes usually do not bite twice. The odds of that happening are so remote that it is a miracle it is ever transmitted in the first place. In endemic areas with 100 percent infection, you might find only 1 percent of the mosquitoes infected, and 1 percent of those mosquitoes will bite twice. That is one in 100,000 or 200,000 mosquitoes. You have to get bitten 100,000 times to catch malaria. Unfortunately, you can actually get bitten 100,000 times in a week in those areas! Most people there do not even notice the bites. In the US, we have the mosquitoes, so we have the potential for the infection, but we do not have the parasite any longer. But every now and then someone carrying malaria steps off an airplane and gets bitten by a US mosquito. It has the potential to come back. |
Around 1990, malaria was transmitted at 5,000 feet up Mt. Kenya, but at 7,000 feet the mosquitoes were not there, because the temperature was wrong. Ten years later, malaria is transmitted at 7,000 feet, because the global temperature has changed. There is some evidence that changing patterns have encouraged the spread of some other pathogens, too, such as cholera. |
Parasites we haven't met yet
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| "Young of the Year" brook trout from the south branch of the Raritan River in central New Jersey. | |
Malaria is a pathogen we know well. But think about those numbers in light of the pathogens we have not even heard about yet, which may be coming from the hundreds of thousands of species in freshwater ecosystems, like pfisteria, a protozoan living in the estuaries of the Atlantic coastal plain. No one had even heard of pfisteria until the coastlines of Maryland, Virginia and North Carolina became overloaded with nutrients from warm temperatures, lots of rain and washed-out pig-waste lagoons. Other organisms disappeared because their niche was eliminated by these changes. But pfisteria had nothing to do but wait for the moment the conditions became right, and it is not going to go away. We have had a similar situation in New York with the West Nile virus, and it is not the last time that is going to happen, either. Witness the events of this last summer on Staten Island. |
West Nile virus is transmitted in mosquitoes both horizontally, from mosquito to person, and vertically, meaning it is carried from the parent insect to their eggs. There is no horizontal transmission during the wintertime, but the virus returns the moment the eggs hatch in the spring. Another parasite of note is elephantiasis, caused by a nematode. Around the world 250 million people have it, and it is a mosquito-borne disease as well. Then there are blackflies, which also breed in freshwater ecosystems, particularly in moving freshwater ecosystems. In tropical environments in Africa and in Central America, blackflies carry onchocersiasis, a blinding infection in humans. |
If you build a dam in the tropics and it creates a reservoir behind it which can house many more snails and many more mosquitoes, you also create a fishery below it which can now accommodate many more blackflies, because the blackflies need fast running water. By building the dam, you have done two disservices at the same time. Blackflies carry a disease called river blindness. It is called river blindness because the people who lived next to rivers were most susceptible to infection. The parasite produces larvae that cause blindness, because blackflies cannot travel very far. Hundreds of thousands to millions of people were infected, but through the miracle of science a drug was created that has eliminated river blindness. It is gone, so people can now move back to these areas. Unfortunately, they can now catch malaria and schistosomiasis. |
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| Stream erosion along the bank of a river whose trees have been removed. | |
Schistosomiasis is another helminth infection that is encouraged by dam building. People acquire the parasite in tropical Africa and South America when wading in water. All you have to do is step into the water to catch this infection. A little bit is not so bad for you, but a lot is terrible. Little kids who acquire the parasite at an early age suffer from bloating (ascites fluid) due to the parasite's eggs blocking the circulation to the liver. |
Logging, mining, land development, dams and reservoirs always contribute to the degradation of cold-water systems. Now, why do you think we have 70,000 dams in this country? We have 70,000 because we need them for irrigation and for drinking water. Those dams create 70,000 different public health settings. To protect the rivers that feed freshwater is essential to public health. When you take a canoe trip along the Delaware River from Downsville to Philadelphia, you think, "This is the cleanest river I have ever seen," and you are tempted to drink from it. Your immune system could probably withstand whatever you would ingest. But take your canoe below Philadelphia and see what happens. Nobody would drink that water, but the fish still have to. |
Human impact on river systems
The fragmentation of our ecosystems has caused huge effects on these riparian environments. The depletion of the ozone hole is probably due at least partly to the bromium released from all of the burning of wood throughout the world. When trees are removed, there is no way to hold rivers back, and the soil erodes. When Gene Likens was a faculty member at Dartmouth in 1967, he was concerned about the clear-cutting going on in the West. He said the only way to actually study an ecological process is to perturb it and hope to see quick returns, because most ecological processes take a long time. He wanted to watch it all happen at once. A paper company in New Hampshire gave him a watershed, and he chopped down virtually everything larger than two centimeters in diameter and left another watershed untouched. The third summer after he cleared the land, there was virtually no nutrient difference between the controlled watershed and the one he was working on. And then he studied the regrowth and succession. |
In the first two years, everything from the land--all the calcium, nitrogen, phosphorus--flowed back into the river. Over the next two or three years the primary production went up--all of the shade-intolerant shrubs and bushes grew and trapped the nutrients in the soil. Everything went back to looking normal, but it wasn't normal. His forest, part of the Hubbard Brook Ecology Study, grew back without interruption from 1967 to 1999, when it stopped growing. He does not know why. |
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| Fall input of leaves into a cold-water running river. | |
River systems have been abused and used for as long as they have existed. They have been used to develop more agricultural settings, so runoff now has an even greater impact than before, degrading the biodiversity of the system. These aquatic ecosystems are the most robust and richest that we know of, and every time we deforest we help eliminate them. |
A lot of chemicals are still on the loose. In North Carolina they are finding unlabeled drums of stuff. In order to get rid of chemicals like DDT that aren't used anymore, farmers have to call the EPA and pay them to take the drums away. A lot of farms are already running on marginal profit, so instead they hide them behind barns and the like. Floods float them out of their hiding places into the watersheds downstream. Under normal circumstances, nitrogen is a limiting factor in most terrestrial ecosystems. But growing crops takes more and more nitrogen out of the soil. A lot of agricultural industries are strategically placed along the coastline, and their effluent has a huge impact on the functioning of estuaries, while those farther inland impact the river systems that feed into them. |
In the eastern US, there is very little primary productivity in the Hudson River, but there is a tremendous amount of bacterial production. This drives the ecosystem. Down the coast, however, in the Delaware Bay and the Chesapeake Bay, primary productivity takes over, and the bacterial species' productivity diminishes greatly. Striped bass used to winter in the Chesapeake but now winter primarily in the Hudson River. This is probably because of the huge amounts of nitrogen being put into the Chesapeake Bay through encroachment, mainly from agricultural runoff. |
Golf courses are considered watersheds. I don't think so. If you looked at what the groundskeepers use to keep the grass green, you would be appalled. What happens when it rains? You do not get pennies from heaven. The reservoir fills up with agricultural runoff from the golf courses. The water company says, "Whatever you put in there, we can take out." That may be true, but do they? When I was growing up in Dumont, New Jersey, I used to catch trout in my local streams. You can still catch fish there now, too, but you would not want to eat them. We continue to spoil our local "pond," but the reasons for this irresponsible behavior escape me. |
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| "You can never wade into the same river twice."--Ovid. | |
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Getting out of it
There is obviously hope. I am not going to end on a rotten note. I think that if we are smart enough to get ourselves into this, we are smart enough to get ourselves out of it. The eighteenth century was the beginning of the industrial revolution, the nineteenth century was the honing of the industrial revolution, the twentieth and twenty-first centuries are the eras of pollution. The twenty-second and twenty-third centuries will be the eras of clean air, clean water, clean land, the "thank God we have learned how to do this" eras. It has actually been done on a small scale already. It has cost great amounts of money. We have also rehabilitated a lot of rivers by simply planting trees and shrubs along their banks. |
I lead a group involved in river repair and plantings each spring. Two years after planting by a little river in Roscoe, New York, you cannot even find the water because it is now overgrown with willows. Now we put trout in the river and they survive. That ecosystem is restored--it is healthy again. It will happen in many other places when we realize their true value to our own lives. Trust me, it will happen. |
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