A New England Pond

"Welcome to Black Pond. This small pond, nestled here in the Green Mountains of Vermont, is a good example of an ecosystem that is beloved by many people and one that has been very important in the history of ecology."

Let's take a closer look at the pond from both of these points of view. First, some science, what do we mean by an ecosystem?

Ecosystem is a concept in modem ecology. The terms ecosystem and ecology come from a Greek-word "eco" meaning home. Ecology studies living organisms in their relationships to one another and to their non-living environment. In other words, living things at home.

At the largest level home can mean the earth itself. In this case ecologists have a term to point to, all the living things on earth and to all their relationships to one and to their non-living environment. They call our living-earth-home the "biosphere."

Within the biosphere there are smaller units of study. Examples would be a tropical rainforest like one in Ecuador, a coral reef like this one in Puerto Rico, a volcanic island in the Pacific Ocean, a suburban backyard in Ohio, or a-New England pond in Vermont. Smaller units like these with natural boundaries are called ecosystems.

Ecology is a relatively new science. People have noticed and studied animals and plants, of course for many centuries. But the scientific study of the complex relationships between plants, animals and their non-living environment was only really begun in our own 20th century. One of the first modem scientific studies in ecology, as a matter of fact was done on a small pond in Florida back in 1957 by a pioneer ecologist named Howard Odum.

Many of the discoveries Odum made in Silver Springs, Florida could apply to our own pond in Vermont. Odum looked for instance at the details of how the Florida pond cycled energy and matter. Let's do the same with Black Pond.

"The sun is shining today at Black Pond. The sun, our sun, is the source of all the energy of all the plants and all the animals who live in and around the pond."

In the Black Fond community, as in almost all earth ecosystems, it is the job of the green plants to capture some of that sunlight energy. Through a chemical process called photosynthesis these green plants can use the energy of sunlight to build living tissues in their own leaves, stems, roots, flowers and seeds. As far as energy goes, these living plant tissues are like the storage gas tank in an automobile or the coal piles of an electric power plant. The plant tissues store sunlight in chemical molecules and then make it available in usable portions to other living things in the pond ecosystem.

In ecological terms, the green plants are the producers and the other living things that depend on the plants for their life energy are the consumers.

In Black Pond, as in most ponds and lakes, the most important producers are the ones you often don't notice--producers like this periphyton algae, for instance. It carpets rocks, submerged trees and pondweed stems. Other producers float in the water like these filamentous algae strands. And then there are free floating and for the most part invisible to the naked eye planktonic algae seen here under a microscope.

These algae may be small and inconspicuous but they are responsible for the lion's share of the photosynthetic production that supports all the rest of the pond's living population.

Besides the algae there are also large numbers of pondweed producers in Black Pond. There are the pond lilies, the Potamogeton, the rushes, bushy pondweeds and the horsetails (one of a group of plants with close ancestors that go back to the prehistoric ages when coal was formed).

Along the shores grow other producers--grasses, sedges and cattails.

The Odum's calculated that their pond in Silver Springs, Florida was able to capture about 5% of the sunlight energy that reached the pond, convert it and store it as chemical energy in plant tissues.

Once the sunlight is captured, the consumers compete for their share of this newly available chemically stored energy.

Again, the largest group of consumers is the least conspicuous, one-called animals for example, like these paramecium and the many celled but still microscopically small rotifers. These tiny consumers feed on the algae and other plant parts and in turn are themselves food for larger animals like hydra, fairy shrimp, freshwater sponges and water fleas.

In a typical food pyramid these smallest of animal consumers are then food for hundreds of species of larger mayflies, dragonflies, water striders, water scorpions, bugs, beetles, mosquito and black fly larvae, spiders, snails, clams and worms. Some of these small invertebrate animals (that is, animals without backbones) live their entire lives under water. Some like the water striders live most of their life on the water--that is, they are able to literally walk on water as the surface tension of the water keeps them dry.

Others insects like the mosquitoes, black flies, mayflies and dragon flies live under the water for part of their life--the egg and the immature larvae stages, and then come out into the air--in some cases for only one day--to mate and reproduce.

All of these small invertebrates are in their turn food for larger pond animals Like frogs, turtles, salamanders, snakes, fish and birds. These animals make their living near the top of the energy pyramid.

Mammals are also high in the food pyramid. They cannot live under water all the time since they need to breathe oxygen from the air. Many mammals, however, do make at least part of their living in the pond ecosystem. There are vegetarians like mice, beaver and muskrat and an occasional visiting deer or moose.

Carnivorous mammals like bats, shrews, mink, raccoon and otters also feed on pond insects, amphibians, reptiles and fish. And some mammals, like human beings, feed on both plants and animals.

Notice that each step up the energy (food) pyramid the available energy gets less. On the average you lose 90% of the remaining energy with each step up the pyramid. By the time you get to the highest level of meat eaters only a very small portion of the original sun energy is left and available for the life activities of the fish, heron, or human.

Each of the hundreds of plant and animals species in Black Pond occupy what ecologists call a niche. A niche means the particular role an organism plays in the ecosystem community. A rotifer, for instance, feeds on certain single celled animals and plants and is food for larger invertebrates; an insect larvae feeds on worm larvae and is food for a frog; a fish feeds on a frog and is food for a hawk. The combination of all these niches makes for a very complicated food web as in this diagram.

If one member of this food web increases or declines in numbers, or if a new organism comes into the web, it affects all the other members of the web. This happens often. The pond owners, for instance, report that twenty years ago bullfrogs used to be much more plentiful in the pond than they are today. At night in the spring and summer the air would be booming with their calls and the shallow areas of the pond would be bubbling with clumps of frog eggs and wriggling with hundreds of polliwogs. They are still there today but in much smaller numbers.

The fish population has also changed over the past fifty years. Formerly there were large numbers of perch, pumpkinseed sunfish, bullheads and rock bass. Today the most abundant fish in the pond is the large mouth bass. No doubt the populations of the smaller invertebrates have also changed over the past few decades though we have no records to show just how much.

Why has the population of frog and fish changed so much in such a comparatively short time? We can't be sure. There is some evidence today that frog populations in many northern ponds and lakes are down dramatically in recent years. The cause is unknown.

The change in fish species may be due to the change in character of the land surrounding the pond. A hundred years ago much of the valley and even the mountains surrounding Black Pond, like other parts of New England, were farm and grazing land for sheep and cows. Today, like much of New England, it is almost all forest.

Energy cycles through the pond food web in a one- way flow. In the plant and animal wastes and in the tissues of dead animals and plants there is still some stored energy left. This last bit of energy is used by another important member of the ecosystem, the decomposers.

The decomposers are mostly small and inconspicuous. They are the microscopic bacteria, the molds and other fungi that live on the pond bottom and cling to plant stems and underwater logs and dead leaves. Here they do their quiet work, breaking down the large organic molecules of living tissue into small. inorganic ones.

After they have finished their work the original sun energy has now all been used. Not lost, but dissipated, no longer available for life activities. Unable to be recycled.

This is the energy story, a one-way flow. In every ecosystem we also have a cycling of matter. This is not a one-way flow. It is a recycling one.

Air, water and earth provide the inorganic raw materials that living things use to carry on their life activities.

Air is a reservoir of gases that circulates above the pond. Small portions of this air dissolve in the pond. This dissolved air provides oxygen for all the living things in the pond and brings carbon dioxide and nitrogen for the use of the producer plants.

From the soil under the pond and the rocks and earth around the pond we get small quantities of minerals that contain nitrogen, phosphorus, chlorine, sodium, sulfur, and other elements that are needed to make up living tissues.

All of these--water, gases, minerals--are non-living matter. They are called the abiotic part of the ecosystem.

These abiotic atoms and molecules are taken up by the producer plants and - again powered by the energy of sunlight -- built into plant's living tissues. Leaves, stems, roots, flowers and seeds.

The consumers take in the larger organic molecules that the plant has made and convert them into their living tissues -hearts, muscles, bones, blood and nerves.

The decomposers break down these larger organic molecules, converting them back into the abiotic from which they all came in the first place.

Ecologists have been able to study in detail the way particular elements cycle in an ecosystem.

Take water first.

"It is raining today at Black Pond. as it often does in the spring, summer and fall. This rain, of course, is the source of all the water in the pond."

Besides the water that falls directly into the pond some water bubbles up from underneath the pond where there are springs. And another good portion of the water comes down the mountain in Tinker Brook and empties into the Pond at the southern end.

What comes in also goes out.

"The water from Black Pond flows out the end of the pond and becomes the Black River, which wanders around southern Vermont until it eventually flows into the Connecticut River, which flows into the Atlantic Ocean."

The clear cool water in the pond does not all flow through however. Some of it is taken up by the living creatures, plant and animal, that make up the pond's living community. The plants use the water, combining it with carbon dioxide and rearranging the carbon, oxygen and hydrogen atoms to make sugar in the process of photosynthesis.

Plants and animals both carry on respiration to power their life activities. In respiration--the reverse of photosynthesis--the sugar is broken back down into carbon dioxide and water. And now, matter wise, we are right back where we started. No gains. No losses. All atoms recycled.

As with the water cycle, a similar story shows how other life needed elements like nitrogen, phosphorus, sulfur, copper, iodine, sodium and chlorine cycle in and out of living things.

Nitrogen, for instance, has its abiotic form as nitrogen gas. Four fifths of the air above the pond is nitrogen in this form. Nitrogen-fixing plants and bacteria are able to take this nitrogen gas from the air and from its dissolved form in the water and fix it into a mineral form of ammonia and of nitrite and nitrate compounds.

These ammonia and nitrite and nitrate molecules can then be taken up by plants and animals and combined with sugars and starches to build plant and animal amino acids, enzymes, and proteins. These large molecules are in turn the basic building blocks for plant and animal tissues and these tissues together make up the structures of roots, stem and leaves--legs, hearts and brains.

When the plant and animal wastes or remains are worked over by the decomposing bacteria and fungi, the large protein, enzyme and amino acid molecules are broken back down into ammonia, nitrites and nitrates and then other decomposers--denitrifying bacteria--complete the cycle and turn the ammonia, nitrites and nitrates back into nitrogen gas, carbon dioxide and water.

Often this recycling process is slow and one of the intermediate molecules of decay processes can be methane gas, a compound of carbon and hydrogen, CH4, sometimes called natural gas. You can see this methane gas bubble up when you wiggle a canoe paddle into the muck at the bottom of Black Pond. If you get enough bubbles together you can light them on fire!

This is similar to the decay process that was responsible for the deposits of coal back in the Carboniferous Age of the earth some three hundred million years ago. When plants and animals in ancient wetlands died and were buried before the decay process could be completed, the partially decayed remains built up over the centuries. Carbon, nitrogen, hydrogen and oxygen atoms still held large amounts of sun captured energy in their half-decayed state. Today we mine that coal to help power our industrial society.

And so the cycle at Black Pond, as in all other ecosystems yesterday and today, continues day after day, year after year, century after century--abiotic to producer to consumer to decomposer to abiotic to producer to ...

Well, actually, in a pond ecosystem it doesn't continue unchanged century after century. All things change on this earth, and ponds and lakes change faster than most. In a process that ecologists call succession, a small pond like this will change dramatically over the centuries, sometimes over a few decades.

These plants that live close to the shore grow and die and grow and die and the remains of the plant parts fall to the pond bottom.

They do not decay as fast as they are deposited and soon the bottom of the pond is filling in and the plants are creeping out in the muck toward the deeper parts of the pond. In these deeper parts the floating plants and the pondweeds do the same thing, continually adding to the silt and muck at the bottom of the pond, continually making the pond shallower.

Eventually the pond gets so shallow that the grasses and sedges and cattails can grow everywhere and there is no open water at all. Now we have a marsh or a swamp or a bog. Eventually, a few decades or hundreds of years hence, we have a meadow or a forest with no standing water at all.

Look around in a state like Vermont or Maine or Wisconsin or Michigan and you can see countless examples of ponds and lakes that were formed a few thousands or even a few hundreds of years ago and that are now marshes, meadows or forests.

This process of succession is a natural one. It goes one whether human beings are around or not. Human beings, however, do have an influence on the pond ecosystem here and most everywhere on earth these days.

Farm and grazing land used to surround the pond. The two houses that front on the pond used to send their sewage from sink and toilet directly into the pond. The resulting erosion and pollution warmed and fertilized the pond's water and may have something to do with the changes of frog and fish populations over the past decades.

The owners of the pond built a dam at the southern end some ninety years ago to deepen the pond. They also did some dredging in past years to take muck from the bottom and deepen the pond.

And in recent years they have tried to harvest some of the floating underwater pondweeds to slow down and partially reverse the pond's natural filling in.

In addition to these forceful measures, the owners have also taken care to not dump human wastes into the pond now and to as much as possible prevent erosion from nearby shores and roads. All of these measures help delay the pond's natural succession and work to stabilize the present food webs and plant and animal population numbers.

Many other environmental influences on the pond are difficult to measure and to control. The air above the pond not only has oxygen, nitrogen, carbon dioxide and water, it also has small quantities of sulfur, lead and nitrogen compounds that were created in power plants and automobiles and snowmobiles and power lawnmowers and other industrial age appliances. Some of these compounds dissolve in rain water and make for acid rain. The acid rain in turn makes Black Pond more acidic.

Any change in the acidity of the water in the pond will have an effect on the living inhabitants of the pond. How much and how severe these changes will be is a subject of active research today.

So too with other changes, many of them impossible to predict or to control. If species from other parts of the world invade the pond, like the European Zebra Mussel or the Eurasian watermillfoil, they can cause great changes in the food webs and the elemental cycling.

These kind of changes have indeed gone on in all past ages and will go on in all future ages though with the speed of modern travel they are all accelerated today.

As you can see, Black Pond, like all ecosystems today, is a dynamic one. It is changing now and it will change in the future. But with all the changes it will also remain a complex community with a fascinating web of life.

Those who study pond ecosystems scientifically and those who explore this small pond in a more personal way have much in common. A famous American writer once lived for two years next to a small pond in New England. Henry David Thoreau put it simply in his classic book about Walden Pond, "We can never get enough of nature."