I have found that an understanding of natural aquatic ecosystems improves one’s success in keeping fishes and invertebrates in captivity. When we know how a habitat “should” function, we can better duplicate it in the aquarium or pond and make the necessary modifications where it is impossible to “copy nature”. Today I’d like to discuss the biology of natural freshwater environments, with the hope that you will find some information that will be of value in maintaining your own outdoor pond.
The study of freshwater habitats such as rivers, lakes and ponds is termed limnology. The science of limnology encompasses all of the factors that make up the ecology of a freshwater area, such as the interactions of plants and animals and the physical conditions that affect the habitat. Included among these are climate, geography and water and soil chemistry. The following is just a short overview of this fascinating yet complicated science.
A pond is generally defined as a shallow body of fresh water that allows for the growth of rooted plants across its width (the deeper water of lakes limits the penetration of sunlight, and therefore rooted plants grow only along the shoreline). Due to the shallow nature of pond water, temperatures across it are fairly stable, while the dissolved oxygen content may vary widely throughout the day. There is a blurred distinction between ponds, swamps, marshes and bogs -- while each of these habitats has specific characteristics (bogs, for example, contain acidic water and often support the growth of sphagnum moss), the various names are used interchangeably in many places.
Water Chemistry and Characteristics
Water has a number of characteristics that influence the types and numbers of animals that might live in it. A working knowledge of these factors will help you to understand your pond’s ecology, and should contribute to your enjoyment of it.
Oxygen and Carbon Dioxide
Both oxygen and carbon dioxide enter the pond from the atmosphere. Additional oxygen and carbon dioxide is released, respectively, by plants and animals during their respiratory processes. Pond water will generally contain a good deal less oxygen than does the air above it (the atmosphere is made up of 21% oxygen).
The water's oxygen content may be increased naturally by wind action and the presence of aquatic plants, or by locating the pond in a shaded area (cool water holds significantly more oxygen than warm water). Unless your pond is very lightly stocked with small fish and invertebrates, it will require supplementary aeration via a pump-driven filter or similar system.
The pond water’s oxygen content is generally highest during the day, when oxygen is being released as a byproduct of photosynthesis, and lowest at night, when plants cease photosynthesis, but animals continue to use the water's oxygen.
Temperature and Sunlight
Water quite effectively holds heat, and so the temperature in an aquatic environment is often fairly stable. However, in the shallow ponds typical of most backyards situations, the water temperature may closely mirror that of the air above it.
The depth to which sunlight can penetrate the water influences the ability of plants to grow therein. Silt or other materials dissolved in the water will limit the transmission of light, so that murky water will support less plant growth than will clear water.
The pH (acidity or alkalinity) of the water greatly affects species diversity. A pond’s pH is affected by a number of factors, including carbon dioxide content, bottom substrate, and the character of the water that is added to the pond. Be sure to research the pH requirements of any aquatic animals that you intend to introduce into your pond. Nitrate, phosphate and sulfate levels will also affect water quality and the types of animals that can live in the pond.
Although superficially uniform in appearance, even the smallest of backyard ponds will usually be organized into distinct habitats, each of which supports specific plants and animals.
Open water areas in larger ponds support drifting plants and actively swimming animals such as fishes and turtles. A strong attraction of water molecules for each other produces a quite resilient film at the pond’s surface. Certain invertebrates, such as fishing spiders and water striders, are able to utilize this film as a home or hunting ground.
Phytoplankton, which is composed largely of one-celled, floating plants known as algae, will be found throughout this area. Also dwelling here are enormous numbers of rotifers, diatoms, crustaceans and other invertebrates, known collectively as zooplankton.
The Pond Bottom
The pond bottom, especially in shallow areas, is home to a great variety of creatures, including the larvae of dragonflies, damselflies and other aquatic insects, crayfishes, snails, sponges, clams and catfishes. Depending upon the water's depth, rooted plants may also grow here.
The Littoral Zone
The habitat containing the greatest species diversity is known as the littoral zone. In a natural situation, this habitat is defined as the area extending from the water's edge to the point at which rooted plants cease to grow. Due to the shallowness of its water, the entire pond is generally one large littoral zone. The littoral zone is particularly important for smaller animals and for the young of larger species that inhabit the open water areas.
The littoral habitat is further subdivided into three zones, each defined by the types of plants that grow within. The area closest to the shore is known as the emergent plant zone, and is inhabited by plants that are rooted in the bottom and have stems and leaves that extend above the surface. Rushes, grasses and sedges are the dominant forms of plant life here, and they provide shelter to a wide array of birds, mammals, fishes, amphibians and invertebrates.
The floating-leaf plant zone begins where the emergent zone ends. The plants that thrive here, such as water lilies and duckweed, effectively block sunlight and thus the pond bottom may be bare of other plants.
Furthest from shore is the submerged plant zone, an area of underwater plants such as hornwort and watercress. Larger fishes such as pickerel use the plants here as cover from which to launch attacks on smaller fish, tadpoles, frogs and other prey.
Naturally-occurring ponds change over time, a process known as habitat succession. Aquatic plants become more abundant, and their dead leaves begin to fill up the pond, making it ever more shallow. Eventually, emergent vegetation extends across the pond and a thick, muddy bottom forms. Land plants begin to take root along the edges, and in time a grassy or forested area is established. In backyard ponds, you can largely control this process, although it is sometimes interesting to allow the pond to go unattended for several years, so as to observe these changes for yourself.
The International Society of Limnology posts interesting newsletters and field research reports at: http://www.limnology.org/