Ponds

Ponds are small lentic or still bodies of freshwater. There is much disagreement about how to define a pond, as well as the proper method of distinguishing ponds from lakes. Some defining methods involve size requirements. For example, some areas of the United States define a pond as having surface areas of less than 10 acres. Other methods specify a maximum depth, stating that they must be as shallow as 12 to 15 feet (3.66 to 4.57 meters), or shallow enough that light can penetrate to the bottom uniformly across the water body so that plants may root all the way through the pond. This shallow depth also means that ponds do not have temperature layers or stratification caused by heat convection as lakes do; however, they may have thermally driven microclimates. Another criterion is that ponds must be a quiet body of water, lacking wave action on the shoreline, though they may have wind-driven currents. Regardless of these criteria, many ponds have been given the misnomer of lake and vice versa. Ponds can form in any depression in the ground that collects and retains enough groundwater or precipitation to pool. These depressions can result from a wide range of natural geological and ecological events. For example, kettle ponds were formed when glaciers retreating across North America and Europe ground out deep potholes in the landscape, which are now filled from underground aquifers. Many pond-forming activities are now constrained by human activities, such as filling or irrigation.

Vernal Ponds

Some ponds are temporary, only holding water for part of the year, and are known as ephemeral or vernal pools. Most vernal ponds fill with winter rains and snowmelt, meaning that they are at their peak level in the spring, hence the term vernal, meaning spring. Because these ponds tend to dry up annually, they are generally fishless, which allows for the safe development of amphibians and insect larvae that would normally be preyed upon. However, some species of fish, called killifish, have adapted to this environment by laying eggs that can survive partial dehydration. There are several species of organisms that rely upon these pools and are considered indicator species by scientists trying to identify pond types. Aquatic invertebrates and amphibians can serve as indicators of pond health and type because they have specific habitat requirements and different species respond differentially to changes in the biological, chemical, and physical makeup of a pond. Vernal pools are important breeding sites for frogs and toads, such as wood frogs (Rana sylvatica) and spadefoot toads (Scaphiosus), and mole salamanders like spotted salamanders (Ambystoma maculatum) and marbled salamanders (Ambystoma opacum). They are also habitats for daphnia and fairy shrimp, small invertebrates that survive the periodic dry periods by laying resting eggs in the dried mud of the pond bottom, which remain dormant until the pond refills with water in the spring. Some eggs can remain dormant for decades, ensuring the continuation of the species life cycle, even through years of drought.

Pond Seasonality

Most ponds retain water year round, and in temperate climates freeze partially or completely during the winter season. Aquatic organisms tend to tolerate a narrow temperature range, and have a variety of methods for dealing with seasonal changes. Cold-blooded animals, like reptiles and amphibians, can change their metabolism and reduce their activity levels to preserve energy stores. Turtles and frogs will often burrow in the mud of the pond or surrounding area and hibernate through the winter. Some frog species, such as the wood frog, can survive being partially frozen because of the presence of cryoprotectant chemicals that act like antifreeze in their blood and body tissues. Many insects also burrow and hibernate or lay winter eggs that will hatch in the spring. Fish and snails move to deeper waters and remain less active beneath the ice. Some aquatic plants form winter buds that drop to the bottom of the pond and germinate in the spring. Zooplankton, like rotifers or cladocerans, produce winter eggs, whereas protists and annelids protect themselves by forming cysts around their bodies.

Nutrient Cycling in a Closed Ecosystem

Many ponds have no surface outflow and are spring-fed, making them self-contained ecosystems. All ecosystems are made up of interactions among the biotic, or living, plants, animals, and microorganisms relating among themselves and to the abiotic, or nonliving, physical and chemical factors of the environment. In ponds, the abiotic factors include: water, carbon dioxide, oxygen, phosphoric salts, amino acids, and nitrogen. Like most closed ecosystems, ponds function through the interlocked relationship of two food chains cycling these nutrients through their environment. The first food chain consists of light-dependent primary producers or autotrophic organisms, and the organisms that eat them, called heterotrophs. Autotrophic organisms produce complex organic compounds from simple inorganic molecules. Though a few autotrophs can do this by using chemicals like hydrogen sulfide, most ecosystems, including ponds, derive their energy from the sun and are dominated by photosynthetic autotrophs. Photosynthesis involves the reduction of carbon dioxide molecules through the addition of oxygen, which creates chemical energy and sugars that are used by organisms to create biomass or growth.

The primary producers in ponds are phytoplankton, periphytic algae, submerged plants, floating plants, emergent plants, and shore plants. Phytoplankton, whose name means “wandering plants,” are microscopic algae that float in open water, giving it a green appearance. Periphytic algae are microscopic algae that attach to substrates, making pond rocks a slimy, greenish brown. This algae helps to produce oxygen at the bottom of ponds, which is important for decomposers found in the depths. Submerged plants grow with their entire structure completely under the water. Floating plants are rooted at the bottom of ponds, but extend to float on the surface. Emergent plants are rooted in shallow water and have their stems and leaves above the water most of the time, whereas shore plants are completely above the water most of the time.

The energy stored in the algae and plant biomass is moved up the food chain by grazing organisms. These organisms are called consumers, or heterotrophs, which include animals, bacteria, and fungus. Ponds are filled with tiny zooplankton, microscopic animals that eat phytoplankton or other small zooplankton. They include single-celled animals like the foraminifera, tiny crustaceans such as copepods, or immature stages of larger animals. Algae and zooplankton are eaten by macroinvertebrates, larger animals that have no backbone, including thousands of species of pond dwelling insects.

Plants, zooplankton, and microinvertebrates are then eaten by larger vertebrates, animals having a backbone, which include fish, frogs, salamanders, turtles, snakes, water fowl, wading birds, and semi-aquatic mammals like muskrats, beaver, and otters, as well as raccoons. Consumers that eat only plants, herbivores, are considered primary consumers. Consumers that eat primary consumers are called secondary consumers, and can be strict carnivores, eating only animals; or omnivores, which eat both animals and plants. Tertiary consumers are considered apex or top predators, and can feed on both primary and secondary consumers. An example of a tertiary predator in a pond is the otter, which may eat both herbivores, such as some fish, as well as carnivores like frogs.

When the nutrients in a pond cycle up through the food chain as organisms are eaten by larger and larger animals, they are lost to the surrounding environment only when animals leave the pond through external predation or migration. The rest of the nutrients are cycled back down to the abiotic pool of nutrients by a second food chain. The second food chain is made up of non-light dependent organisms that feed on and break down the detritus from the first food chain. These detritivores are bacteria, fungi, and other organisms that feed on the waste products of the producers and consumers. In turn, the detritivores break the organic waste back into their inorganic components, which the primary producers, plants and algae, can then again use in the process of photosynthesis. In this way, the nutrients are cycled continuously through the pond ecosystem.

Pond Habitats

Ponds are made up of four distinct habitat zones: the shoreline, the surface film, the open water, and the bottom water. The surface film and the open water make up the pelagic or upper habitats of the pond. The surface film refers to the surface of the pond on which insects, such as water striders, and free-floating organisms range. It also includes the water just below the surface, as well as the underside of floating plants.

The open water is home to large, free-swimming organisms like fish, and drifters like plankton. The bottom water and shoreline make up the benthic or lower level habitats in the pond. The bottom water habitat varies depending on the pond depth. Quiet, standing water ponds often have muddy and silted bottoms into which crayfish, mayfly nymphs, and other macro- and microorganisms burrow. The shoreline is considered the area of the pond where water meets land, and is dominated by rooted plants and air-breathing organisms that vary depending on the substrate type. This is the richest part of the pond in terms of biodiversity because of the dense plant life, which is used for foraging and breeding by many species.

Though not strictly considered part of a pond, the area of land around a pond is essential to the health of the pond and the organisms within it. It is called a buffer because it often serves to protect or buffer the pond from exposure to harmful external forces such as pollution. Wide vegetated buffers around ponds provide essential nesting sites, winter habitat, and escape cover for wildlife. Species of amphibians like the spotted salamander are especially dependent on large and healthy buffer zones around ponds because, though they are dependent on the ponds to lay their eggs and for their larvae to develop, the adults only remain at the pond long enough to breed. Afterward, they return to the surrounding terrestrial habitats to live and feed for the rest of the year and over winter. Reptiles like snakes and turtles may depend on the pond for food and cover, but they require the surrounding area to lay their eggs buried on dry land.

Without a healthy buffer zone of at least 50 feet, many species cannot survive, despite the presence of a pond. A vegetated buffer also controls soil erosion and increases the amount of water filtration through the soil. It helps to improve water quality, filtering out runoff and pollutants like fertilizers. In addition, buffers can isolate ponds from intensive wading from livestock, such as cattle, which can disturb pond vegetation, increase water turbidity (the amount of suspended particles in the water), accelerate bank erosion, and increase nitrogen levels in the water.

Pond Eutrophication

Life in ponds depends on a pool of nutrients that includes nitrogen and phosphorous. The amounts of these nutrients present in a pond can limit the amount and type of organisms that can survive in a pond. The rate of release of these compounds into the pond from the surrounding environment can vary depending on the temperature cycle and seasonality of the pond, as well as the amount of sunlight it receives and the regional climate. Sometimes, ponds receive excess amounts of nitrogen and phosphorous through both natural and artificial means. Natural cases of increased nutrient intake occur when nutrients accumulate through naturally occurring processes, such as depositional environments wherein sediments are laid down or where ephemeral or seasonal nutrient flows occur. Artificial nutrient enrichment occurs from human activities such as untreated sewage, fertilizer runoff, or allowing livestock direct access to ponds and rivers, into which they expel fecal matter.

Spikes in nitrogen and phosphorous levels can lead to eutrophication, which is the increase of plant biomass in a body of water. This process usually favors simple phytoplankton or algae over more complex plants, and often leads to algal blooms. The excess nutrients allow these photosynthetic species to convert even more sunlight into biomass, meaning that they can increase their populations rapidly. Algal blooms can coat water surfaces or make water cloudy, often turning ponds a shade of green, yellow, brown, or red depending on the species of algae. This overgrowth limits the amount of sunlight that can reach bottom-dwelling organisms, and can cause drastic changes in the amount of dissolved oxygen in the pond.

When photosynthesis occurs, oxygen is released from the breakdown of carbon dioxide and water. In a healthy pond, more oxygen is made during the day than can be consumed by respiring organisms and builds up in the pond. This oxygen reserve is then used by the pond organisms at night, without which they would otherwise suffocate. During an algal bloom, large excesses of oxygen are made during the day, but the unnaturally large populations of phytoplankton continue to undergo cellular respiration at night and can deplete oxygen stores within a pond. This oxygen depletion leads to hypoxic or oxygen-poor conditions in the pond, which can cause large die-offs of fish and other animal populations, especially immobile bottom dwellers. Through such die-offs, eutrophication can cause large changes in the species composition and richness of a pond. Eutrophication in a pond can also threaten species through toxic effects. When large amounts of certain species of algae die and are eaten, they can release both neurotoxins (affecting the brain) and hepatotoxins (affecting the liver). These toxins can cause the death of many animals, including species of zooplankton. Livestock can also become sick from ingesting toxic algal blooms, and the toxins can even pose a threat to human health.

Laws have been successful in reducing some instances of eutrophication by regulating the discharge and treatment of sewage, thereby reducing a source of nutrient inflow to groundwater sources. However, there is need for regulation dealing with the use of agricultural fertilizer and animal waste. Fertilizer and pesticides that are sprayed on crops often end up washed by rains into nearby water bodies, especially when applied liberally. Reducing the amount, frequency, and timing of application of agricultural chemicals, particularly those containing phosphorus, could help to reduce the occurrence of eutrophication. Vegetated buffers can again serve the health of a pond by helping to filter out chemicals that would otherwise wash into ponds, as well as keeping livestock from directly accessing water bodies.

Pond Pollution

Other types of pollution can also be harmful to ponds, changing the chemical makeup of the pond and affecting organisms. Foamy areas, fluorescence, or distinctive odors can signal pollution in a pond, but can also be misleading. Foam can be produced in ponds by harmless inorganic salts or some species of phytoplankton. Fluorescence and distinct odors can be caused by certain protozoan species. Carbon dioxide levels in water can be effected by pollution and are important to determining the pH of a pond, which can determine which types of life can survive in that environment. Agricultural pesticides and synthetic estrogens have been found contaminating ponds in agricultural and urbanized environments where frogs and fish are experiencing external and reproductive deformities, including the feminization of males. Higher levels of the isotope nitrogen-15 have been found among animals that live in ponds close to septic systems and thus human wastewater, which may contain hormones and other pharmaceuticals. Ongoing research and policy development are needed to determine the effects of pollution on pond ecosystems and the best ways to protect water bodies like ponds from contamination.

Climate Disruption to Ponds

As the global climate changes, disruptions to ponds are affecting which organisms will survive and where. Rising surface-water temperatures increase the difference in temperatures down the water column. This affects where algae live and may allow toxic cyanobacteria to thrive. Further, changing water levels due to flooding or rising groundwater also influence which aquatic plants grow and the size of their population.

Ponds also relate directly to some greenhouse gases responsible for climate change. They have historically been carbon sinks; however, British researchers found that after seven years of warming, ponds could only absorb about half the amount of carbon dioxide as they had previously. Higher levels of carbon dioxide can acidify the water. At the same time, greater anaerobic decay of dead organisms in ponds—particularly reedlike matter—produces methane gas, which captures as much as eighty times the heat as carbon dioxide does in the atmosphere and may accelerate climate change.

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