The Pond Ecosystems;
The freshwater aquatic environment differs from the terrestrial environment in many significant ways. Water acts as a solvent for many of the chemical elements necessary for sustaining life. These include nutrients, minerals, and many gasses (oxygen, nitrogen and carbon dioxide being the most important to your pond). Aquatic organisms live in an environment that surrounds them with the basic elements of life. All that is needed is an energy source (sunlight), to begin the nutrient cycle in this ecosystem. The absorption of sunlight in a pond depends on water clarity, which in turn is affected by dissolved material like minerals, suspended organic and inorganic material, such as algae and dirt. This suspended material in the water is called turbidity. Other factors to light absorption include artificial inhibitors like shading and dyes.
Understanding the current condition of your pond is another important factor for anticipating what type of maintenance problems you will be facing. Generally, ponds are classified into one of three types: oligotrophic (new), mesotrophic (middle aged) or eutrophic (old).
Oligotrophic (new) ponds are either freshly built or have aged slowly due good design and proper maintenance procedures. Small ponds can be drained and cleaned every few years this restores them to an oligotrophic condition.
Mesotrophic (middle aged) ponds have an intermediate level of nutrients and plants. They experience moderate algae blooms on an intermittent basis.
Eutrophic (old) ponds generally have high nutrient levels, large amounts of sludge, turbid or cloudy water, and large algae and aquatic plant populations.
Remember, these classifications are a measure of the condition of your pond, not the age in years or seasons. Proper maintenance can mitigate or reverse this aging process. A maintenance program using a microbial treatment is an effective way to restore your pond to a balanced and controlled condition.
The first signs of low oxygen levels are odors and then fish kills. Typically DO is measured in parts per million (ppm) with 0 ppm representing a complete lack of oxygen and 15 ppm representing the saturation point of water. Normal oxygen content in a healthy pond will run between 5 ppm to 10 ppm. Fish and other organisms utilize oxygen to accommodate metabolism, they then excrete carbon dioxide, for use by plants and algae to regenerate oxygen in the pond.
This oxygen cycle, and the oxygen balance are affected by what is known as the biological oxygen demand or BOD in the pond. This involves the decay process of plants and animals and consumes substantial amounts of oxygen in the ecosystem. It is interesting to note that an ecosystem in a good healthy balance will not have wide swings in the oxygen levels but will stabilize according to the many conditions present. It is worth repeating that one method of assuring good dissolved oxygen levels is through
mechanical aeration
of the water. Many aeration devices are available for this purpose.
An additional nutrient source from landscaping is from runoff from the area surrounding a pond. Some reports state that up to 4% of the fertilizer applied to the area adjacent to a pond may runoff into the pond. To make the matter worse, it is accepted practice to extend the fertilizer area out into the adjacent pond to assure complete coverage on the terrestrial areas. This is sabotage to your lake maintenance plan. Careful applications of slow release fertilizers are the best method to use when fertilizing around an aquatic system. One solution to this problem is to fertilize the last 15 to 20 feet out from your lake with hand spreaders. Be very careful to not over fertilize, monitor and control run-off into your lakes, try switching to organic fertilizers in the area around your pond; they release more slowly into the water system after rain or irrigation. In some instances, like catfish ponds, fertilizer will be added to ponds to grow Planktonic algae; it's free fish food and blocks sunlight, preventing weed growths. This has nothing to do with your beautiful reflection pond getting loaded with harmful nutrients by negligent landscaping. Fertilizers and landscape materials will not have any beneficial effect on your pond and need to be controlled in every way possible.
An additional source of nutrient is waterfowl. As mentioned above, it takes only 0.1 ppm of phosphate or 5 -7 ppm of nitrate to reduce your water quality and thus the enjoyment of your pond. Research shows that it takes 4.2 ducks per surface acre per year to produce this much nitrates and only ½ duck surface acre per year to produce the minimum phosphate. This means that if your one-acre pond has as few as five resident ducks, they could be producing enough nitrates to overload the system, and up to ten times the amount of phosphate needed to foul the pond.
The sediment or sludge at the bottom of the pond can accumulate at a rate of one to five inches per year or more. This rate is higher in warm, nutrient-rich waters. From a practical perspective, an irrigation pond with one acre of surface area can lose water storage capacity by 80,000 gallons per year by the loss of volume caused from sediment build up. Naturally-occurring bacteria are present in all ponds, but generally are not present in large enough numbers to combat the sedimentation and extra nutrients available. Supplemental microbes will reduce this sedimentation by digesting the organic components which can exceed 90% of the biomass. The remaining inorganic materials, like sand or clay, will remain on the bottom as a component of your healthy ecosystem. This remaining bottom sediment, called the benthic layer, will provide an anaerobic zone, which is important to the denitrification process.
The various forms and stages of phosphorus in the aquatic system is quite complex. Phosphorus exists in water in either a particulate phase or a dissolved phase. Particulate matter includes living and dead plankton, precipitates of phosphorus, phosphorus adsorbed to particulates, and amorphous phosphorus. The dissolved phase includes inorganic phosphorus and organic phosphorus. Phosphorus in natural waters is usually found in the form of phosphates (PO
Ultimately, the pond keeper needs only be concerned about keeping the phosphorus in the system in a form that cannot be utilized by plants for weed and algae growth.
Alum or aluminum sulfate is one of the most widely used products to treat artificial ponds for clarity and phosphate control. The aluminum ions bond with phosphate molecules in the water. The resulting aluminum phosphate is insoluble in water. This precipitate settles to the bottom of the pond and the phosphate is no longer available as a nutrient
Another important chemical cycle is the Nitrogen/ Ammonia chain. Ammonia is the second most important gas in your pond after Oxygen. Excessive levels of ammonia can be brought on by landscape intrusions, excessive feeding of fish, excrement from fish and waterfowl and other decomposing plant and animal material. The presence of ammonia in the pond is normal and ammonia can exist in two forms: un-ionized ammonia (NH3) and ammonium ion (NH
Another important factor in the pond ecosystem is temperature. Most aquatic organisms are cold-blooded ("poikilothermic" ) meaning they are unable to internally regulate their core body temperature. Therefore, temperature exerts a major influence on the biological activity and growth of aquatic organisms. In general, higher water temperatures produce greater biological activity. Fish, insects, zooplankton, phytoplankton, and other aquatic species all have preferred temperature ranges. As temperatures get too far above or below this preferred range, the number of individuals of the species decreases until finally there are few, or none.
Most algae in ponds has a fairly narrow temperature range when compared to other organisms, but the range is broad enough that algae is a problem in many areas from April to October , with a longer season in warmer climates. Algae growth as, well as many other organisms, will become dormant at water temperatures below 50
Water Chemistry, Nutrient Levels and Pond Dynamics.
A healthy system will have an abundance of life. All the nutrition needed will be available without becoming excessive, water chemistry will be stable and outside influences will be minimal.
The most important factors influencing water quality are the amount of sunlight, oxygen levels, nutrients levels and pH of the water.
Sunlight is the primary energy source for a pond. It drives photosynthesis in plants and affects water temperature. Shallow bodies of water, less than six feet deep, are generally more difficult to manage than deeper ponds because full light penetration and warmer water can cause more frequent and severe algae blooms. Low levels of sunlight can adversely affect the oxygen levels, as happens during a sustained cloudy period. Algae in the system drop in the production of photosynthetic oxygen and begin to consume oxygen thus depleting oxygen levels.
Dissolved Oxygen
or (DO) - Oxygen is the most important gas in the aquatic environment and adequate DO levels are critical in maintaining a stable ecosystem. Cold water holds more DO than warm water. In warm water zones the DO levels are lower, which has an adverse effect on most organisms. Low DO levels can lead to reduced microbial growth and minimal aerobic digestion of nutrients. Oxygen enters ponds by several sources such as photosynthesis and wind or wave action. Aquatic plants and algae produce oxygen-using photosynthesis whenever light is present. When there is no light, the plants and algae use oxygen, this is the reason fish kills can occur at night in ponds with heavy planktonic blooms. Wind and wave action allows oxygen to mix with surface waters, and allow oxygen to diffuse into the water. The transfer of oxygen and other gasses into and out of the system is facilitated by bubbles in the water, which provide a medium for gas exchange. Oxygen levels can also be increased mechanically through aeration. This is often the primary method of oxygenation in a pond and is discussed in detail in the section on
aeration and water circulation
. DO levels below three to four parts per million will cause stress situations in the pond.
The most substantial sources of nutrients in medium sized ponds are dead vegetation, landscape debris, runoff from the area surrounding the pond, and waterfowl. As aquatic vegetation grows and dyes, it sinks to the pond bottom, adding to the nutrient level for future aquatic growth. This is called nutrient cycling. Additionally, landscape activity can be a substantial factor in the amount of decaying vegetation. From grass clippings, to trimmings from tree and bushes, to falling leaves, pine needles and pinecones, the landscaping around your lake is a major consideration for your lake maintenance plans and expectations. Lack of control over these factors can be a primary reason your lake slips inexorably into eutrophic condition.
This is discussed in the chapter on
As one example of this, there has been great deal of enthusiasm for adding barley straw to a pond as a natural algae control. This is actually a complex method of binding phosphates. The bacteria in the pond break down the barley straw and during this process hydrogen peroxide is produced. The hydrogen peroxide in the water is a natural algaecide and an oxidizing agent and which helps bind up the phosphates in the water.
Organic phosphate is phosphate that is bound to plant or animal tissue. Organic phosphates are formed primarily by biological processes. One primary source of phosphates in a pond is found in waterfowl excrement. Organic phosphates may occur as a result of the breakdown of organic pesticides which contain phosphates. They may exist in solution, as loose fragments, or in the bodies of aquatic organisms.
Orthophosphate is sometimes referred to as "reactive phosphorus." Orthophosphate is the most stable kind of phosphate, and is the form used by plants. Orthophosphate is produced by natural processes of metabolism.
Polyphosphates (also known as metaphosphates or condensed phosphates) are strong complexing agents for some metal ions. In water, polyphosphates are unstable and will eventually convert to orthophosphate, particularly in lower oxygen environments. Inorganic phosphate is phosphate has been bound up and is not associated with organic material. Types of inorganic phosphate include orthophosphate and polyphosphates.
Some pond keepers remove phosphorus from their water through the addition of alum (aluminum sulfate or potassium aluminum sulfate). The alum causes phosphates to form a fluffy precipitate (floc), aluminum phosphate, which has a lower solubility than the aluminum sulfate and will sink to the lake bottom. Within hours of applying the alum, the upper layer of water will become clear and relatively free of phosphates, but the precipitate or floc can leave a milky appearance in shallow water particularly if the Alum treatment is done on a windy day with strong wave action. .
pH is the measure of the water's relative acidity, based on a logarithmic scale 0 to 14. On this scale 7 is neutral, less than 7 is acidic, and above 7 is alkaline. This value represents the concentrations of hydrogen ions present in the water. It is important to note that knowing the pH doesn't give a complete picture. A pH test of distilled water can show almost any value since just a tiny amount of residual impurities, either acid or base, can have a major effect on the ratio of the two.
The Alkalinity of the water is related to the actual number of base components and can be thought of as the "intensity" of the pH. If the alkalinity is low, it indicates that even a small amount of acid can cause a large change in pH. Alkalinity is related to the amount of dissolved Calcium, Magnesium, and other compounds in the water and as such, alkalinity tends to be higher in "harder" water and is slowly increased by evaporation which concentrates the source compounds. Alkalinity is naturally decreased over time through bacterial action which produces acidic compounds that combine with and reduce the alkalinity components.
Alkalinity, or hardness, is most often measured in ppm and shown as calcium carbonate (CaCO
3)
equivalents. Hard waters have the capability to buffer fish from heavy metals that can be toxic, like copper in many common algaecides. This buffering can also reduce the effectiveness of these same algaecides. Waters rich with minerals are termed hard, and will be alkaline, with a pH above 7. Waters without minerals, soft water, will be acidic, with a pH below 7. Most freshwater life including bacteria prefers a pH range between 6.5 and 9.0. Each organism has varying tolerances. Higher alkalinities are associated with increased productivity along with increased growth of algae and aquatic plants. Many algaecides lose effectiveness in alkaline conditions, making algae control more difficult.
Temperature is also important because of its influence on water chemistry. The rate of chemical reactions generally increases at higher temperature, which in turn affects biological activity. An important example of the effects of temperature on water chemistry is its impact on oxygen. Warm water holds less oxygen than cool water, so it may be saturated with oxygen but still not contain enough for survival of aquatic life. Some compounds are also more toxic to aquatic life at higher temperatures.
Temperatures in ponds will continuously change, but well circulated ponds will have more stability than poorly circulated ones. The most obvious reason for temperature change in lakes is the change in seasonal air temperature. Daily variation also may occur, especially in the surface layers, which are warm during the day and cool at night. In deeper lakes (typically greater than 15 feet for small lakes and 30 feet for larger ones) during summer, the water separates into layers of distinctly different density caused by differences in temperature. This does not occur in smaller ponds.
