Lakes and ponds (also known as lentic systems) are a diverse set of inland freshwater habitats that exist across the globe and provide essential resources and habitats for both terrestrial and aquatic organisms. Although widely distributed and vital for many species, including humans, these habitats account for just over 3% of the Earth's surface (Downing et al. 2006). Importantly, these rare systems are increasingly threatened by the human demand for freshwater as well as human activity (Dudgeon et al. 2006). Recent evidence suggests that freshwater systems are more imperiled than marine and terrestrial ones (Sala et al. 2000, Dudgeon et al. 2006). Effective conservation and management of these systems relies on our ability to fulfill the growing human demand for freshwater while maintaining system integrity.The study of lentic systems is conducted within the field of limnology. Limnology is the study of the structural and functional interrelationships of organisms of inland waters as they are affected by their dynamic physical, chemical, and biotic environments (Wetzel 2001). Several excellent books published within the past decade provide in-depth coverage of lakes and ponds (Wetzel 2001, Brönmark & Hansson 2005, Moss 2010). Another excellent resource is the Encyclopedia of Inland Waters (2009). In this article, we present a brief overview of lentic systems and refer the reader to specific literature for more detailed information.

Physical and Chemical Structure

Lentic systems vary from ephemeral ponds that fill briefly during rain events and then dry out, to permanent water bodies such as lakes (Figure 1). The world's largest freshwater lake, based on area, is Lake Superior of North America (82,100 km2) while the deepest is Baikal Lake of Asia (max depth = 1741 m; mean depth 730 m). Lakes and ponds are formed by a diversity of processes. For example, the geologic uplift of mountains can lead to depressions that fill with water to create lakes. The subsidence of land along faults between tectonic plates has lead to the formation of deep rift lakes such as the Rift Valley Lakes of eastern Africa. Lakes can also form in the crater of an inactive volcano. The movement of glaciers has been an important force in creating lakes and ponds. For example, the Laurentian Great Lakes were created following the retreat of the Laurentide ice sheet during the last ice age 10,000 years ago. Rivers also play a role in creating lakes. For example, a wide meander in a river can be cut off from the main river to form an oxbow lake. Additionally, the flood plain of a river can inundate surrounding habitats creating small ponds as floodwaters recede.

Humans also create lentic habitats. Across the world, humans have constructed impoundments and reservoirs to provide water resources for humans and livestock, generate energy, or control flooding. While large water bodies have historically received most of the attention, ponds are the numerically dominant lentic habitat (Downing et al. 2006). Ponds are generally <2 hectares in size, shallow (<3 m), and dominated by aquatic plants. Ponds have a diversity of ecological and geological origins including man-made impoundments. Moreover, due to their small size, ponds often have characteristics that are distinct from larger water bodies, which we will point out below.

Once formed, lentic systems can undergo successional development in which the water body is gradually transformed and incorporated into the terrestrial landscape. Successional development is driven by sedimentation of inorganic and organic matter into the system. Sedimentation rates are influenced by the size and depth of the system, the surrounding terrestrial landscape, climate, productivity, and decomposition (Wetzel 2001). Shallow ponds provide an excellent example of successional development. As sedimentation occurs, the depth of the pond is reduced which allows vegetation to gradually encroach into the system. Over time, this leads to swamp conditions with standing water present among the vegetation. As sedimentation continues, marsh conditions are established with no standing water but water saturated sediments. The terminal stage of succession can include predominately dry land or bogs depending on such factors as the location of the water table and climatic conditions.

The structure and function of ponds and lakes are determined by factors such as turbulence, temperature, water clarity, habitat size, and water depth. Wind turbulence and temperature interact to influence stratification and water circulation within lakes. In the spring, wind turbulence circulates the water throughout a lake supplying oxygen to the entire water column. However, as the temperature increases during the summer and wind subsides, thermal stratification occurs, producing distinct layers in the water column; the upper warm-water epilimnion is separated from the lower cold-water hypolimnion by the thermocline. Oxygen concentration in the hypolimnion tends to decline compared to the epilimnion due to the lack of water circulation. Additionally, without mixing to replenish dissolved oxygen, respiration by benthic organisms within the hypolimnion can further reduce oxygen concentrations. As the temperature begins to cool in the autumn, whole lake circulation is restored and the oxygen concentration is equalized. During the winter, thermal stratification can occur again, but the pattern is reversed (inverse stratification); cold water rests over warm water because water density is highest at 4°C. In addition to increasing oxygen concentrations, water circulation plays an important role in circulating nutrients. Given that lentic systems are broadly distributed across different elevations and latitudes, there are a number of possible stratification patterns (Wetzel 2001). Many tropical lakes, for instance, do not experience seasonal temperatures below 4°C to initiate thermal stratification and remain well mixed. Similarly, ponds often show little thermal stratification during the summer due to their shallow depth, which facilitates wind-mediated water circulation.

Another critical factor in lakes and ponds is light transmission, which is required for photosynthesis in primary producers. Generally, the water column is divided into the photic and aphotic zones. In the photic zone, light penetration is >1% and plants undergo photosynthesis. In contrast, light penetration is <1% in the aphotic zone and respiration exceeds photosynthesis. Water depth as well as water clarity influence light transmission. For example, a shallow, clear lake or pond may have a minimal aphotic zone because light transmission is high, which can enhance primary production. However, a deep lake with low water clarity may have a large aphotic zone because light transmission is blocked by suspended particles in the water column. Importantly, lakes and ponds can alternate between clear and turbid states as populations of primary producers respond to changes in light levels and nutrient availability (Scheffer & van Nes 2007).

As in any ecosystem, nutrient inputs and cycling have important influences on the structure and function of lentic systems. Nutrients are transported into lentic systems via terrestrial run-off, ground water flow, atmospheric deposition (e.g., rain), rock weathering, and direct input from terrestrial systems (e.g., leaf litter). The three most important nutrients are nitrogen, carbon, and phosphorus, which are essential elements to all organisms. While phosphorus has historically been considered the most limiting in lentic systems due to low input rates and the high propensity to form complexes with iron, leading to mineralization (Schindler 1974), recent work suggests that nitrogen can also be limited in lentic systems (Elser et al. 2007). The supply of nutrients into lentic systems determines the amount of primary productivity that can be supported. Oligotrophic systems are characterized by low nutrient supply and low primary productivity where as eutrophic systems are characterized by high nutrient supply and high primary productivity. While natural systems can be eutrophic, human activity can increase the input of nitrogen and phosphorus into lentic systems leading to anthropogenic eutrophication. Industrial, agricultural, or municipal wastewater can augment nutrient inputs into lentic systems, which can shift the environmental equilibrium and lead to eutrophication. Anthropogenic eutrophication can dramatically alter the food web structure and reduce water quality. Once eutrophication occurs, it is difficult to reverse owing to internal feedbacks (e.g., hysteresis).

Biotic Structure: Lake Zones and Food Webs

Nearly 6% of described species (i.e., 100,000 species) are supported by freshwater systems (Dudgeon et al. 2006). Moreover, some of the world's oldest lakes (>1 million years old) contain a high diversity of endemic species that have evolved through the process of adaptive radiation (Cristescu et al. 2010). In lakes and ponds, much of the species diversity is concentrated in the littoral zone, near the shore, where algae and plants thrive in the abundant light needed for photosynthesis. Living within the plant matter is a cornucopia of animals including snails, amphibians, crustaceans, insects, and fish. Beyond the littoral zone is the limnetic zone. This is the zone of open water where light is still able to penetrate and support photosynthetic algae (i.e., photic zone). Consumers in this zone include zooplankton, which feed on the algae, some insects, and fish. Finally, the benthic zone is the bottom sediment (e.g., mud, sand, rock) of the habitat, which is dominated by invertebrate species.

Primary producers are broadly divided into three groups, periphyton, phytoplankton, and macrophytes. Periphyton grows on substrates such as mud, sand, rocks, and leaf litter where it appears as a green or brown film. This is an important resource for a diversity of herbivores including macroinvertebrates (e.g., snails) and amphibian larvae. Phytoplankton is algae suspended in the water column. Phytoplankton is divided into several groups (e.g., cyanobacteria, diatoms, chrysophytes, dinoflagellates, desmids) that tend to dominate at different levels of lake productivity. Herbivores such as zooplankton are common consumers of phytoplankton. Macrophytes are generally the most visible group of primary producers in lakes and ponds (Figure 2). Macrophyte species possess a diversity of growth forms. Emergent macrophytes undergo photosynthesis above the water and are typically found close to the shoreline. As depth increases, submerged macrophytes that undergo photosynthesis under the water, dominate. Macrophytes provide resources for many herbivores as well as important refuge from predators.

The consumer species found in lentic habitats include worms, snails, amphibians, crustaceans, insects, reptiles, fish, and birds. Herbivorous groups such as snails, amphibian larvae, and crustaceans play an important role in controlling primary productivity and algal blooms. Additionally, these groups are a critical resource for predators. By integrating primary producers, herbivores, and predators together, a food web is formed where species interact both directly and indirectly with each other. Lakes and ponds have proven to be excellent model systems for studying food web interactions. Food web theory is extremely important in ecology because it provides a conceptual understanding of how changes in the abundance of species influence other species within the community. An important characteristic of lentic habitats is their connection with terrestrial systems. Indeed, many species depend on both aquatic and terrestrial habitats due to their complex life cycles (e.g., amphibians, insects). As such, energy (e.g., nutrients, organisms) moves dynamically between the two systems (i.e., reciprocal subsidies; Nakano & Murakami 2001, Lamberti et al. 2010).

Patterns in Species Richness

Species-area relationships have a long history in ecological research and work on lentic systems has widely supported an increase in species richness with increasing habitat area (MacArthur & Wilson 1967, Browne 1981, Wellborn et al. 1996, Dodson et al. 2000). However, there are many other biotic and abiotic factors that change alongside increases in habitat area, such as permanence, water acidity, and predation pressure. All of these may play important roles in influencing species richness and community structure (Wellborn et al. 1996, Brönmark & Hansson 1998, Wetzel 2000, Williams 2001). Physical environmental factors such as hydroperiod and water chemistry can limit the breadth of species distributions due to physiological constraints (Williams 2001). For example, species that are tolerant of pond drying inhabit a diversity of habitat types (i.e., lakes and ephemeral ponds) whereas species that are intolerant of pond drying are restricted to lakes (Wellborn et al. 1996, Williams 2001). As a consequence of these physiological constraints, species richness tends to increase as hydroperiod increases. However, biotic interactions also can play an important role in structuring communities. Factors such as competition and predation can function to limit the abundance of a species (Kerfoot & Sih 1987, Brönmark & Hansson 2002). For example, many species tolerant of pond drying are frequently outcompeted in permanent waters. Hence their abundance is low or they mainly occur in marginal habitat of the shores, which largely functions as a temporary habitat. Additionally, the transition to more permanent water is often associated with an increase in the abundance of predators (Wellborn et al. 1996). For example, fish play an important role in reducing the abundance of species and influencing species composition and richness in permanent lakes. Thus, abiotic and biotic factors can interact to influence species richness and community structure in lentic systems.

Threats to Lentic Ecosystems

The biodiversity of freshwater habitats is increasingly threatened by human activities. Habitat loss, eutrophication, acidification, chemical contamination, global warming, and exotic species are just some of the factors that have directly or indirectly impacted lentic systems (Brönmark & Hansson 2002). Substantial progress has been made in identifying how some of these threats affect aquatic communities, and how to develop management strategies to reduce or eliminate their impacts. For example, the input of phosphorus, a limiting nutrient in lentic systems, was identified as a major contributor to lake eutrophication (Schindler 1974). Consequently, laws have been enacted to treat wastewater and reduce sewage input of phosphorus into lentic systems. Although some lentic systems have recovered, eutrophication remains a persistent problem due to agricultural intensification and other non-point pollution sources. Another challenging problem is the introduction of exotic species including plants, animals, and pathogens into lentic systems (Figure 3). In North America, for example, exotic wetland plants (e.g., purple loosestrife [Lythrum salicaria], Eurasian watermilfoil [Myriophyllum spicatum]) and animals (e.g., grass carp [Ctenopharyngodon idella], zebra mussel [Dreissena polymorpha]) can outcompete native species for nutrients or space. The introduction of the amphibian chytrid fungus (Batrachochytrium dendrobatidis) into Central America is implicated in the extinction of numerous amphibian species (Kilpatrick et al. 2010). While the potential negative consequences of exotic species introduction are well recognized, controlling their spread is difficult and, once established, eliminating them from communities has been difficult. Lastly, human-mediated habitat loss and alteration are increasing concerns driven by the expanding need for agricultural lands as well as development. These activities not only lead to the loss of habitat for species and vital water resources for a region, but also fragment and alter the landscape in which the remaining habitats are embedded. The combination of these threats and the scarcity of freshwater habitats make lentic systems one of the most imperiled on the planet (Sala et al. 2000, Dudgeon et al. 2006).

This project was supported by a grant from NSF (DEB-0553768) and a fellowship from the David and Lucile Packard Foundation.

Adaptive radiation: The evolution of ecological and phenotypic diversity within a rapidly multiplying lineage

Aphotic zone: Depth of the water where there is little to no sunlight for photosynthesis to occur

Benthic zone: The sediment layer of lentic systems

Epilimnion: The top layer of water in a thermally stratified water body

Eutrophic: A lentic system with high nutrient concentrations and high plant growth

Food web: Feeding relationships among organisms within an ecosystem or community

Hydroperiod: The length of time that a lentic system contains standing water

Hypolimnion: The bottom layer of water in a thermally stratified water body

Hysteresis: A system that exhibits path dependence such that the output depends in part on the internal state of system and not only on its input

Inverse stratification: Minor density gradient that develops between surface waters at 0°C (less dense) and lower waters at 4°C (more dense) leading to weak stratification

Laurentian Great Lakes: The five largest North American lakes formed by the retreat of the Laurentide ice sheet about 10,000 years ago

Limnetic zone: The open-water habitat of lentic systems beyond the littoral zone

Limnology: The study of the structural and functional interrelationships of organisms of inland waters as they are affected by their dynamic physical, chemical, and biotic environments

Littoral zone: The near-shore habitat of lentic systems

Macrophyte: An aquatic plant that grows in or near water and is either emergent, submergent, or floating

Oligotrophic: A lentic system with low nutrient concentrations and low plant growth

Oxbow lake: Water body formed when a wide meander from a river system is cut off

Periphyton: Complex mixture of algae, cyanobacteria, heterotrophic microbes, and detritus that are attached to submerged surfaces in aquatic environments

Photic zone: Depth of the water that is exposed to sufficient sunlight for photosynthesis to occur

Phytoplankton: Photosynthesizing microscopic organisms that are suspended in the photic zone of the water column

Reciprocal subsidies: Energy flux among terrestrial and aquatic habitats

Successional development: Temporal changes in the composition or structure of an ecological community

Thermal stratification: The temperature gradient observed at different depths in a water body due to the change in water's density with temperature

Thermocline: The distinct layer in a thermally stratified water body in which temperature changes more rapidly with depth than it does in the epilimnion or hypolimnion