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The Complexity of Freshwater Ecosystems
From: Columbia University
| By:
Dickson Despommier |
EDITOR'S INTRODUCTION |
As a species that aggressively alters the environment to suit our needs and wants, we are baffled by the simple elegance with which other species mold themselves to their conditions. Nature's adaptability and interdependence has led Dickson Despommier, professor of microbiology and environmental health sciences at Columbia University, to spend much of his professional and personal time meandering along America's freshwater rivers. Despommier has found that freshwater ecosystems rival those of rain forests and coral reefs in the number of species and their remarkable adaptation to changing conditions for life. |
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| The south branch of the Raritan River in Ken Lockwood Gorge in central New Jersey. | |
 he most startling thing I have ever learned in my life probably came from the person who discovered that there is microbial life in the bedrock. That is an enormously exciting concept for me to contemplate, because I am old enough to have grown up in an era when we did not even know what the genetic material was, let alone that a lot of it stems from these organisms' evolution. But for someone interested in symbiotic relationships and parasitism, to realize that there is life in bedrock, or even life coming out of the thermal vents at the bottom of the ocean, is as thrilling as it gets. |
On a similar note, one of the most impressive facts emerging from the human genome study is that every living thing's DNA is related to everybody else's. There are remnants and bits and pieces and unused sequences that connect virtually every life-form on earth with each other. For this reason, I believe that all ecosystems are relevant, necessary and interconnected. |
Niche living
G. Evelyn Hutchinson conceived of the modern notion of the niche concept and first presented it after a dinner. (Can you imagine sitting for two hours after a dinner, even if it was a good dinner, and listening to a mathematical treatise on the theoretical aspects of niche formation?) Nonetheless, I'm sure he made it quite interesting for those who managed to stay awake! Imagine for the moment that you are in a single bubble, and that bubble represents the entire niche in which you live. You can then seek out your level of comfort by finding in that bubble the concentration of constituents that corresponds to the optimal conditions for your life. For example, the levels of sodium chloride in bodies of water differ according to temperature, climate and currents. Fish are free to swim and are found mostly at their level of comfort within those conditions. |
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| A caught and released brown trout from the Willowemoc River in Livingston Manor, New York. | |
When a fish has to contend with two things that determine its comfort zone, like temperature and pressure, you can then imagine that the fish lives at the flat interface of two physical niche bubbles. As you keep adding more and more bubbles, you create an interface space occupied by all the niche bubbles. As each niche bubble is added to this environmental complexity, it is a mere snapshot of one moment in the time of any organism's life--characterizing its zone of comfort, or its essential niche, with regard to physical, chemical and biological requirements. Over time, the essential niche changes. That is how I try to imagine how an organism behaves in its essential niches. |
The niche that I will be talking about today--the physical niche, not the essential niche--is the freshwater ecosystem. You find them everywhere we have mountains. You can find cold freshwater environments in the central portions of Africa, all the way down the Andes, throughout the Rockies and throughout the eastern United States, too. Wherever you have mountains you have the potential for cold freshwater ecosystems that function independent of one another. And, of course, they all continue to evolve by the process of erosion. Introducing species of fish, particularly trout, into them has altered them dramatically with respect to energy flow and species dynamics. |
Freshwater rivers
There are basically two different kinds of river systems in the US. Freestone rivers are the ones that we are mostly familiar with--the bubbling water over rocks, imagined as pristine and high quality. Limestone rivers exhibit a totally different set of physical niche parameters. They change less over the course of a year than freestone rivers. Thus, temperature, turbidity and flow rates in limestone and freestone streams select different life-forms for each of these situations. |
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| Annual average water temperature in freestone and limestone streams. | |
At the origin of a limestone river, the temperature of the water is fairly constant, and the oxygen content is almost zero. It accumulates various chemical components including oxygen as it flows, modifying the environment along the river, as well as the water in it. Calcium is mobilized due to the basic pH of the water, allowing more to be dissolved than in freestone-river situations. Calcium is thus available for the life-forms that live there. The scud, a macroinvertebrate crustacean, is the dominant form of life in most limestone streams throughout the world. This is because it uses calcium to build its outer shell. |
This system can produce prodigious amounts of organisms, often of large size. The limestone river generates energy all year long, because stream plants are able to sustain themselves due to the slow currents. Energy flow most closely resembles that of a food chain: sunlight, plants, scuds and predator species. These are quite intimate and beautiful places, and, as illustrated, they are highly productive. This is the environment where Isaac Walton perfected his angling techniques in the south of England in the 1600s. Today, you can still catch salmonids in his favorite stream, the river Test. |
In contrast, the boisterous freestone rivers begin as steep gradients of water by flowing out of the melting ice of glaciers and mountain snowfields or from small spring-fed rivulets, often creating extensive dendritic systems. This pattern also exists throughout other parts of nature as well--tree roots, tree branches, river systems and our nervous system, for example. This probably has something to do with the ease with which these systems form, and the simplicity of the patterns that are reinforced by the processes that support them. Wherever you have mountains and water, this is the pattern you will observe, even on the Martian surface. |
Instability brings diversity
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| Dendritic branching of a typical river in the midwestern US. Note the extensive agricultural encroachment. These conditions favor periodic flooding. | |
Cold freshwater ecosystems rival the rain forest and the coral reef in the number and variety of species existing there. I know that sounds like an unsupportable statement, but the studies on species diversity bear this out again and again. However, unlike freestone rivers, rain forests and coral reefs are characterized by very stable environments that contain numerous possibilities for organisms to fill their physical niches. The longer geologic time that stable conditions prevail, the more diverse the life-forms found there. There is nothing stable about cold freshwater ecosystems. For that reason they select life-forms that can take advantage of the many instabilities (changes in turbidity, temperature, pH and oxygen concentration, to name a few) and have actually evolved in a seemingly disastrous situation to a highly productive and diverse assemblage of plants and animals. |
The distribution of life-forms in each depends upon a given dendritic river system. Where the river gradient is steep, the flow rates are fastest, and thus nutrient loading is low. There are very few species found at the headwaters of most steeply graded rivers. And as one moves into slower regions of the river, the biotic diversity index increases, because the river will then support plants and will hold nutrients in the water column longer in order to supply food for the organisms occupying this region of the river. As anybody who has walked along these river systems knows, the lower down the gradient you go, the warmer and slower the water gets. For example, if you walk along the Delaware River starting at Downsville, the water is about 48 degrees all year long. Downstream around Port Jervis and Easton, or even as far south as Philadelphia, the water temperature can rise into the 50s, 60s and sometimes even exceeds 75° F. The diversity of life that has been selected for in each of these gradient levels is quite different. In summary, the gradient of the river determines the level of its nutrients, its flow rate and even the temperature, all of which select for life-forms which are best suited for each zone of the river. |
The leaf cycle
What makes the whole ecological picture of the freestone river even more complex is the change of seasons. In freestone rivers, each organism has a life span that allows it to leave the river, so the energy flow is away from the water and onto the surrounding banks. Then there is a huge influx of energy back to the river in autumn, when leaves fall into the river. |
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| Beginning detritovore activity (creating carbon dioxide bubbles) on leaf surfaces in the early fall. | |
From the time at which leaves fall into the water and start to rot until the time they are entirely consumed by the macroinvertebrates, more than 200 days will have elapsed. The leaves themselves are colonized sequentially by a number of organisms, all of which take advantage of the previous ones to gain a foothold. There is a succession to life on the surface of a leaf when it falls into a freshwater ecosystem and is left to rot. |
If we place a leaf into an acid-impacted aquatic environment, the leaf will remain intact, without the benefit of microbial processing. This experiment was actually carried out at a biological station situated on the border of the Upper Peninsula of Michigan and Wisconsin. University of Notre Dame limnologists used two lakes, Peter and Paul, connected by a narrow isthmus. They poisoned out Peter with an organic acid and left Paul untouched as the "normal" ecosystem component. Then they stocked both lakes with rainbow trout and studied the effects of their efforts over the next few years. To this day, there are few trout that live in Peter, but the control lake continues to function normally. The leaf-rot rate in Peter is almost zero. |
This simple experiment tells us a lot about how things function or do not function after a human impact. Different types of leaves rot at different rates in both river and lake environments, so there is a time-release-like effect that extends this food source for all the disparate forms of life found there. As many as 1,500 different species of Trichoptera (caddis flies), 600 different species of Ephemeroptera (mayflies) and about 100 or so species of Plecoptera (stone flies) take advantage of this food source in streams throughout North America, plus a host of other invertebrates, including beetles. If an entire tree falls into the river, it will take a long time to rot away; provided that not too many at one time do so, this is a welcome event for the river, evening out the energy released into it. |
Delicate balance
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| Hendrickson's Pool on the Beaver Kill River in Roscoe, New York. | |
Most ecosystems, terrestrial or aquatic, have four trophic (nutrition) levels, more or less, in balance. If you take one of these away--for instance, the top carnivore level (trophic level 4)--others will start to affect the rest of the system, and it will come out of balance. Mary Power at UC Berkeley placed cages over a small portion of the American River and excluded the steelhead trout from eating smaller fish. The unchecked population of small fish then ate all the algae-eating invertebrates inside the caged area, and shortly thereafter the algae in it overgrew its expected limits and turned that protected plot of stream bottom into a "forest." Sportfishing has learned this lesson the hard way, by trial and much error. Fortunately, today more and more of those who sportfish are releasing their catch back for them to swim yet another day. |
Most encouragingly, a significant number of NGO-funded ecological studies are now ongoing throughout the United States, most of which are aimed at showing how cold-water ecosystems function in our behalf both ecologically and recreationally. It is entirely possible, therefore, that we humans may eventually come to appreciate the ecosystem services that these aquatic environments provide to us, and act as one to protect them from further harm. Then perhaps the world will revert to a more balanced place for all of its life-forms, with us still in it to appreciate our humble place in the global scheme of things. |
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