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Ponds and Pond Ecology - Part Two

Nature's lessons for pondkeepers.

By Stephen M. Meyer

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In part one of this series examined the origins of natural ponds and the process of "pond aging." We will pick up where we left off by continuing our discussion of pond aging.

For our purposes ponds pass through three basic life stages. Ponds that are oligotrophic are in the early stages of their life. These young ponds are generally very low in nutrients and are sparsely populated by plants and animals. Because life-supporting nutrients are in short supply, productivity — the rate at which new life is created — is also very low.

Ponds that are eutrophic are at the other end of the age scale. These are old ponds that have abundant concentrations of nutrients and are teeming with life. Eutrophic ponds are very productive, often creating well over 1 gram of carbon per square meter of pond surface per day. In fact, eutrophic ponds and their surrounding wetlands can have productivity rates that approach those of tropical rain forests.

Ponds that are neither very young nor very old are mesotrophic. That is, these ponds are in middle age.

Where do garden ponds fit into this age scheme? When compared in terms of principal nutrients (nitrogen and phosphorus), transparency (light transmission) and algal and bacterial populations, garden ponds tend to have characteristic profiles quite similar to eutrophic natural ponds. In other words, our backyard ponds are biologically and chemically in old age even though they may be quite young chronologically.

Eutrophic ponds have several times the nitrogen concentrations of younger, more pristine ponds. Phosphorus levels in the former may be 1000 times higher, or more, than those of the latter. These higher levels of nutrients mean higher levels of biological activity — especially phytoplankton (suspended algae) and, subsequently, zooplankton (microscopic animals, many of which eat phytoplankton). As a result of this proliferation of life-forms, the transparency of the water (often referred to as turbidity) in eutrophic ponds is usually only a fraction of that of oligotrophic waters, but may be more comparable to mesotrophic ponds. In addition, eutrophic waters may support 10 to 100 times the bacterial concentrations of younger ponds.

We can use the eutrophic pond as a model to explore one of the most vexing problems facing garden pondkeepers: planktonic algae blooms. It is the explosive growth of this algae that turns pond water pea-soup green and makes it difficult to view the fish.

The Ecology of Algae
Algal productivity in any pond is determined by the relative availability of four basic factors: 1) the intensity and duration of sunlight striking the pond and the concentrations of 2) carbon dioxide, 3)nitrogen and 4) phosphorus in the water. In almost all natural and artificial ponds, sunlight and carbon dioxide are in such overabundance that the degree of algal growth is almost unlimited. Algae are fairly efficient users of light — even providing deep shade for ponds usually fails to cut into the algae population. Similarly, there is no practical way to reduce dissolved carbon dioxide levels to the point where algal productivity will be affected.

In eutrophic waters, the critical limiting factors that set the bounds of algae growth are dissolved nitrogen and phosphorus. By definition, both are relatively abundant in eutrophic ponds and, therefore, massive algal blooms are a common occurrence. Research shows that algae (and other plants) use these nutrients at an average ratio of 7 parts nitrogen to 1 part phosphorus. Therefore, if the ratio of nitrogen to phosphorus in a given pond is less than 7:1, the limiting factor is nitrogen. If more nitrogen were added, algae productivity would increase until a 7:1 ratio was achieved. Reducing nitrogen levels would cut algal productivity. Conversely, reducing phosphorus concentrations would have no observable effect on algal growth until the nitrogen:phosphorus ratio reached 7:1.

On the other hand, if the nitrogen to phosphorus ratio is greater than 7:1, the limiting factor is phosphorus. Adding more nitrogen would not affect algae growth, but adding more phosphorus would. Cutting phosphorus would proportionately reduce algal productivity. In almost every case that I can recall, the nitrogen to phosphorus ratio in garden ponds exceeds 7:1. In other words, there is more nitrogen in the water than the algae can use given the concentration of phosphorus. Phosphorus, then, is the controlling factor in most garden ponds and, because of this, reducing phosphorus will cut algal productivity.

The first option for reducing phosphorus is to limit the amount entering the pond. Unfortunately, the domestic water supplies in most areas of this country are being contaminated with ever increasing amounts of phosphorus (and nitrates, as well). Thus, a considerable amount of the phosphorus in your garden pond may come straight from the tap. Water changes and "topping off" for evaporation merely add more of this nutrient to the water, which is why large water changes often make algae blooms worse, not better.

There is little you can do about phosphorus in your tap water in the short run. Although of no immediate practical value, convincing merchants to offer and consumers to purchase non-phosphate products may reduce source concentrations over the long term. Similarly, in agricultural regions, shifting to less intensive use of chemical fertilizers can reduce groundwater contamination. Both garden and natural aquatic habitats will benefit greatly, but the results will take some time before they become apparent.

A second major source of phosphorus in garden ponds is fish food. On average, every gram of fish food contains about 32 milligrams of phosphorus. Only about 20 percent of that is locked up in fish growth, which means that much of the rest ends up in the water and is available to algae. If you maintain a high fish load in your pond or if you overfeed, you are — for all practical purposes — in the algae farming business. Reducing the fish load and taking precautions against overfeeding can significantly cut the planktonic algae problem.

Consider, for example, a 2000-gallon pond with perhaps 50 12-inch fish — a fish load that, unfortunately, is actually on the low side of many koi ponds. With standard feeding practices, these fish would consume about 600 grams of pellets each day, which would contain roughly 19,200 milligrams of phosphorus. If only half of this amount becomes available for algae use, 1.2 milligrams per liter (mg/l) of phosphorus are being added to the water every day! You can see that this is why a koi pond ends up mimicking a eutrophic pond in a fraction of the time it takes nature.

Cutting the fish load back to about five 12-inch fish would drop phosphorus levels almost to mesotrophic levels. Also, phytoplankton levels would drop proportionately.

In an undisturbed (i.e., by human activity) natural pond, a careful balance is maintained between algae, zooplankton and other forms of life, both smaller and larger. Bacteria and higher plants compete with algae for important nutrients — especially nitrogen and phosphorus.

Interestingly, the form of nitrogen most readily used by algae and other plants is actually ammonia, not nitrate. In fact, plants end up having to convert nitrate to ammonia when only the former is available to them. Thus, a well-established and properly maintained biological filter can make life tougher for planktonic algae by providing a good home to nitrifying bacteria that convert ammonia to nitrate. (Incidentally, this helps to explain partially the apparent paradox of why biological filters often help clear ponds of planktonic algae, even though most people associate its nitrate product with plant foods and fertilizers.)

"Proper" maintenance is the key element here. As it turns out, a heavily "clogged" biological filter can help to reduce phosphate levels. Pond enthusiasts have long made the distinction between what they believed to be the "good" bacteria in their biological filters — the nitrifying bacteria — and the "bad" bacteria — the heterotrophs. This distinction is wrong.

As a biological filter begins to clog with organic matter, heterotrophic bacteria populations take up residence. Pond ecology studies have shown that these bacteria are far more efficient consumers of phosphorus than the much larger and less productive algae. On average, eight heterotrophic bacteria generations are produced for each generation of planktonic algae that is produced. Once heterotroph populations in the biofilter reach critical mass, they can cause the planktonic algae population to crash by consuming so much phosphorus that little is left for the algae.

This explains why garden ponds frequently become crystal clear as the biological filter becomes heavily clogged. And it explains why these ponds often become green right after the biological filter is cleaned. It is not because nitrifying bacteria were washed away, but because the heterotrophs were dislodged.

This suggests that pond biological filters should be made much larger — perhaps two or three times — than the size required merely for nitrification purposes. The filters should be big enough to allow for substantial organic "clogging" while still permitting good flow rates for biological filtration. Cleaning should be limited to a single portion of the filter at any one time.

Of course, aquatic plants have always been used by nature and pondkeepers to compete with algae for nutrients. Emergent and submergent plants are effective in soaking up nitrogen and phosphorus from pond waters. The key is to have enough plants. Plants represent almost 90 percent of the life in natural ponds (about a third of which is algae of all types). In contrast, the proportion of the pond biomass accounted for by plants in garden ponds may be 10 percent or less. In many koi ponds, plants (almost all of which are algae) may comprise no more than 1 percent of the living organisms — the relative proportion of fish may be more than 97 percent.

A garden pond planted to minimize algae blooms should attempt to follow nature's guidelines — that is, 90 percent of the macro-biomass should be in plants and the remaining 10 percent in fish. This means that for each 12-inch koi (about 360 grams) there should be 3.2 kilograms of emergent and submergent plants. Herbivores — plant eaters — represent three times the pond biomass as the carnivores. Much of this herbivore population is comprised of microscopic animals: the zooplankton. Zooplankton, such as Daphnia, graze on algae much as cows and sheep graze a meadow. This grazing can have a surprisingly significant effect on algae populations. For example, one study of a Scottish lake found that grazing Daphnia cut phytoplankton biomass by 80 percent during the spring.

The frequent and often needless use of parasiticides (which are actually just insecticides) and other pond "medications" can seriously impair zooplankton populations. The duration and intensity of algal blooms can be reduced by allowing healthy zooplankton populations to exist in your garden pond. Consider that every time you dump a parasiticide in your pond because you notice a fish scratching against the bottom, you are making life easier for planktonic algae.

There is also an ironic twist in the great quest to keep a pond algae-free. This is the fact that planktonic algae is such an excellent consumer of ammonia — so much so that it can reduce pond concentrations to immeasurable levels. This leads to an important possibility.

Many pondkeepers live in colder climates and are forced to shut down pond biological filters during the winter. And, even in warmer climates where the filters are kept running, the nitrifying bacteria in these filters scale back their activities considerably. In either instance, it takes about two months before the biological filters are operating fully after the weather begins to warm. This raises the intriguing possibility that the algae bloom so loathed by pondkeepers each spring is saving the fish from ammonia poisoning — especially in an overcrowded pond!

Natural Disease Prevention
Natural pond models are not only useful because of the similarities that can be observed between natural and artificial ponds. They also highlight important differences that help us to understand garden pond ecology.

For example, on average, it can take a natural pond roughly 100 years to age from an oligotrophic (nutrient-poor) state to a eutrophic (nutrient-rich) state. Garden ponds, in contrast, usually become eutrophic in just one or two seasons! This is due to the fact that almost all garden ponds use 100-percent water recirculation and are burdened with unnaturally high fish loads from day one. The rapid acceleration of aging in garden ponds distinctly warps their ecology. There is no chance for a biological balance to develop.

When you consider the implications of plant versus fish biomass, it's difficult to imagine how a biological balance could be established in most garden ponds even in 100 years. While eutrophic ponds and garden ponds may be loaded with comparable levels of dissolved and suspended organic material, the vast proportion of dissolved and particulate organic matter in natural ponds originates from plants. Conversely, the great bulk of dissolved organic carbon (and a considerable amount of the particulate organic carbon, as well) in garden ponds is animal (fish)-derived.

Among other things, this difference has a strong influence on the bacteria that populate garden ponds. In natural ponds, cellulose, starch and other "plant material-oriented" bacteria dominate the water. In garden ponds, however, the bacterial population is decisively tipped in the direction of proteolytic bacteria — that is, bacteria that live off animal substances. The effect of this most definitely includes supporting massive populations of disease-causing bacteria.

Thus, the apparent equality of bacterial counts between eutrophic natural ponds and garden ponds is deceiving. From a fish's perspective it would much prefer to be in the natural pond where most of the bacteria are interested in plant substances. In the garden pond, most of the bacteria are looking for the fish.

As if this were not bad enough, in most instances the total biological load in garden ponds is several times that of natural ponds — even hyper-eutrophic ponds. The combined animal and plant biomass in garden ponds is often wildly out of balance with what nature intended. Given the preponderance of fish biomass in garden ponds, this works to the advantage of disease-causing organisms and fish parasites. The greater the fish load in your pond, the better the environment for fish pathogens.

In the case of the ich parasite and anchor worm, for example, their intermediate life stages have only a limited amount of time — often just a few hours or days — to find a host to parasitize. If they fail, they die. The probability of their finding a host is directly proportional to the density of animals in the pond. Thus, reducing the fish density by half also reduces the probability of finding a host by half. Keep this in mind next time you go shopping for more fish.

In fact, nature uses these relationships to keep fish populations in check. The "natural" or appropriate carrying capacity of a pond can be thought of as the population size that can be supported without causing catastrophic collapses to occur because of "natural" disasters. Such disasters include starvation due to exhaustion of food supplies, large increases in parasite and predator populations that result from an increase in the host/prey population, and severe degradation of the habitat (e.g., destruction of spawning areas, depletion of oxygen, etc.).

This is illustrated by comparing a stable fish population density that hovers around the carrying capacity in a natural pond with the all-too-frequent occurrence in garden ponds in which the owner stocks far too many animals in the pond and continues to mindlessly replace dead fish after each die-off. Major disease and parasite infestations are nature's way of saying that the carrying capacity of a pond has been significantly exceeded. The fact that these kinds of disease and parasite disasters do not befall natural fish populations unless some major environmental catastrophe occurs holds a lesson for pondkeepers. (Conversely, nature limits the density of fish pathogens by limiting the density of fish.)

The lesson should be obvious: Devastating epizootics (bacterial and parasite plagues) can be avoided if you keep the fish load in your garden pond low. And how low is low, you ask?

Fish Loads In Natural Ponds
Coldwater aquatic systems have lower natural fish loads than do warm water or tropical systems. This is related to the quantities of food available, not the purity of the water. Eutrophic ponds, for example, may hold five times or more the fish load of pristine mountain streams. In fact, the fish load generally increases when moving downstream for the same reason that the number of fish increases from younger to older ponds: food availability goes up.

Here again, the primary determinant of food availability is that most basic of plants: algae. Imagine that you wanted to raise a fingerling to the size of 1 kilogram. Studies have shown that the efficiency of biological energy conversion in aquatic habitats is about 10 percent This means that to add 1 kilogram to its mass an insectivorous fish will have to eat about 10 kilograms of insects and other macro-invertebrates. To raise 10 kilograms of insects requires about 100 kilograms of zooplankton for food. That 100 kilograms of zooplankton will consume about 1000 kilograms of algae. Thus, in this simplified example, for every kilogram of fish, a natural pond must produce at least 1000 kilograms of algae to support the food chain.

But once again, even those waters found in nature with the most abundant food supplies do not support fish loads anything like what one encounters on a typical garden pond tour. Garden pondkeepers frequently attempt to stuff into their artificial ponds three to 10 times the fish biomass of the most productive natural ponds. Aquaculture facilities get away with this because 1) their fish are in residence for relatively short durations (frequently just several months), 2) rearing ponds are drained and sterilized on a regular basis, and 3) regular fish deaths are an acceptable business loss. Don't bother to try this at home — it won't work for long.

Using eutrophic tropical inland waters as a model suggests that not more than 360 grams of fish be stocked per square meter of pond surface area. This is approximately one 12-inch fish per 10 square feet of pond surface area, which I have noted as a maximum limit numerous times over the years. This recommendation assumes that there is a minimum pond depth of 2 feet, adequate biological filtration and sufficient aeration. Actually, a more appropriate model for ponds in the northern half of the U.S. and into Canada would be about one 10-inch fish per 10 square feet of pond surface.

TABLE I
Koi Equivalence
Standard Length
(inches)
Equivalent Number of 12-inch Koi
Number of Fish Equal to One 12-inch Koi
6
0.1
10
8
0.3
3
10
0.5
2
12
1.0
1
14 - 16
2.0
18 - 22
5.0
24 - 28
12.0
30
20.0

You will find that Table I provides useful equivalence information that will let you assess the fish load in your pond in terms of 12-inch fish or equivalents of 12-inch fish. Fish length is measured in terms of standard length — that is, excluding the tail.

As you can see, a 6-inch fish is equivalent to 10 percent of a 12-inch fish, not one-half. In other words, one 12-inch fish is the equivalent bioload of 10 6-inch fish. This is because fish mass does not increase in proportion to the length, but rather in proportion to slightly more than the cube of the length. Thus, a 2-inch koi has roughly eight times the mass of a 1-inch koi.

To use the table, note the size of each fish and the percentage of a 12-inch fish that it represents. For example, if you have two 6-inch koi (two X 0.1 = 0.2), three 8-inch koi (three X 0.3 = 0.9) and one 20-inch koi (one X 5.0 = 5.0), this is equivalent (0.2 + 0.9 + 5.0 = 6.1) to approximately six 12-inch koi (six X 1.0 = 6) or 12 10-inch koi (12 X 0.5 = 6). If the pond is 60 square feet in area, using the tropical waters model, you would be right at the limit (one 12-inch koi per 10 square feet). If you chose to follow the temperate waters model (one 10-inch fish), you would need to sell or give away six fish to avoid overstocking the pond. Those readers whose fish are plagued each year by disease and parasites will be amazed at the impact a reduced fish load can have on the health of the pond and the fish. We will examine the role of plants in ponds in part three.

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