Lake ecosystem
A lake ecosystem or lacustrine ecosystem includes
Lentic systems are diverse, ranging from a small, temporary rainwater pool a few inches deep to
Two important subclasses of lakes are ponds, which typically are small lakes that intergrade with wetlands, and water reservoirs. Over long periods of time, lakes, or bays within them, may gradually become enriched by nutrients and slowly fill in with organic sediments, a process called succession. When humans use the watershed, the volumes of sediment entering the lake can accelerate this process. The addition of sediments and nutrients to a lake is known as eutrophication.[4]
Zones
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Lake zones |
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Lake stratification |
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Lake types |
See also |
Lake ecosystems can be divided into zones. One common system divides lakes into three zones. The first, the littoral zone, is the shallow zone near the shore.[5] This is where rooted wetland plants occur. The offshore is divided into two further zones, an open water zone and a deep water zone. In the open water zone (or photic zone) sunlight supports photosynthetic algae and the species that feed upon them. In the deep water zone, sunlight is not available and the food web is based on detritus entering from the littoral and photic zones. Some systems use other names. The off shore areas may be called the pelagic zone, the photic zone may be called the limnetic zone and the aphotic zone may be called the profundal zone. Inland from the littoral zone, one can also frequently identify a riparian zone which has plants still affected by the presence of the lake—this can include effects from windfalls, spring flooding, and winter ice damage. The production of the lake as a whole is the result of production from plants growing in the littoral zone, combined with production from plankton growing in the open water.
Wetlands can be part of the lentic system, as they form naturally along most lake shores, the width of the wetland and littoral zone being dependent upon the slope of the shoreline and the amount of natural change in water levels, within and among years. Often dead trees accumulate in this zone, either from windfalls on the shore or logs transported to the site during floods. This woody debris provides important habitat for fish and nesting birds, as well as protecting shorelines from erosion.
Abiotic components
Light
Light provides the solar energy required to drive the process of
Temperature
Temperature is an important abiotic factor in lentic ecosystems because most of the biota are poikilothermic, where internal body temperatures are defined by the surrounding system. Water can be heated or cooled through radiation at the surface and conduction to or from the air and surrounding substrate.[6] Shallow ponds often have a continuous temperature gradient from warmer waters at the surface to cooler waters at the bottom. In addition, temperature fluctuations can vary greatly in these systems, both diurnally and seasonally.[1]
Temperature regimes are very different in large lakes. In temperate regions, for example, as air temperatures increase, the icy layer formed on the surface of the lake breaks up, leaving the water at approximately 4 °C. This is the temperature at which water has the highest density. As the season progresses, the warmer air temperatures heat the surface waters, making them less dense. The deeper waters remain cool and dense due to reduced light penetration. As the summer begins, two distinct layers become established, with such a large temperature difference between them that they remain stratified. The lowest zone in the lake is the coldest and is called the
Wind
In exposed systems, wind can create turbulent, spiral-formed surface currents called Langmuir circulations. Exactly how these currents become established is still not well understood, but it is evident that it involves some interaction between horizontal surface currents and surface gravity waves. The visible result of these rotations, which can be seen in any lake, are the surface foamlines that run parallel to the wind direction. Positively buoyant particles and small organisms concentrate in the foamline at the surface and negatively buoyant objects are found in the upwelling current between the two rotations. Objects with neutral buoyancy tend to be evenly distributed in the water column.[2][3] This turbulence circulates nutrients in the water column, making it crucial for many pelagic species, however its effect on benthic and profundal organisms is minimal to non-existent, respectively.[3] The degree of nutrient circulation is system specific, as it depends upon such factors as wind strength and duration, as well as lake or pool depth and productivity.
Chemistry
Oxygen is essential for organismal respiration. The amount of oxygen present in standing waters depends upon: 1) the area of transparent water exposed to the air, 2) the circulation of water within the system and 3) the amount of oxygen generated and used by organisms present.[1] In shallow, plant-rich pools there may be great fluctuations of oxygen, with extremely high concentrations occurring during the day due to photosynthesis and very low values at night when respiration is the dominant process of primary producers. Thermal stratification in larger systems can also affect the amount of oxygen present in different zones. The epilimnion is oxygen rich because it circulates quickly, gaining oxygen via contact with the air. The hypolimnion, however, circulates very slowly and has no atmospheric contact. Additionally, fewer green plants exist in the hypolimnion, so there is less oxygen released from photosynthesis. In spring and fall when the epilimnion and hypolimnion mix, oxygen becomes more evenly distributed in the system. Low oxygen levels are characteristic of the profundal zone due to the accumulation of decaying vegetation and animal matter that “rains” down from the pelagic and benthic zones and the inability to support primary producers.[1]
Biotic components
Bacteria
Bacteria are present in all regions of lentic waters. Free-living forms are associated with decomposing organic material,
Primary producers
Algae, including both phytoplankton and periphyton, are the principle photosynthesizers in ponds and lakes.[8] Phytoplankton are found drifting in the water column of the pelagic zone. Many species have a higher density than water, which should cause them to sink inadvertently down into the benthos. To combat this, phytoplankton have developed density-changing mechanisms, by forming vacuoles and gas vesicles, or by changing their shapes to induce drag, thus slowing their descent.[9] A very sophisticated adaptation utilized by a small number of species is a tail-like flagellum that can adjust vertical position, and allow movement in any direction.[2] Phytoplankton can also maintain their presence in the water column by being circulated in Langmuir rotations.[3] Periphytic algae, on the other hand, are attached to a substrate. In lakes and ponds, they can cover all benthic surfaces. Both types of plankton are important as food sources and as oxygen providers.[2]
Invertebrates
Zooplankton are tiny animals suspended in the water column. Like phytoplankton, these species have developed mechanisms that keep them from sinking to deeper waters, including drag-inducing body forms, and the active flicking of appendages (such as antennae or spines).[1] Remaining in the water column may have its advantages in terms of feeding, but this zone's lack of refugia leaves zooplankton vulnerable to predation. In response, some species, especially Daphnia sp., make daily vertical migrations in the water column by passively sinking to the darker lower depths during the day, and actively moving towards the surface during the night. Also, because conditions in a lentic system can be quite variable across seasons, zooplankton have the ability to switch from laying regular eggs to resting eggs when there is a lack of food, temperatures fall below 2 °C, or if predator abundance is high. These resting eggs have a diapause, or dormancy period, that should allow the zooplankton to encounter conditions that are more favorable to survival when they finally hatch.[11] The invertebrates that inhabit the benthic zone are numerically dominated by small species, and are species-rich compared to the zooplankton of the open water. They include: Crustaceans (e.g. crabs, crayfish, and shrimp), molluscs (e.g. clams and snails), and numerous types of insects.[2] These organisms are mostly found in the areas of macrophyte growth, where the richest resources, highly-oxygenated water, and warmest portion of the ecosystem are found. The structurally diverse macrophyte beds are important sites for the accumulation of organic matter, and provide an ideal area for colonization. The sediments and plants also offer a great deal of protection from predatory fishes.[3]
Very few invertebrates are able to inhabit the cold, dark, and oxygen-poor profundal zone. Those that can are often red in color, due to the presence of large amounts of hemoglobin, which greatly increases the amount of oxygen carried to cells.[1] Because the concentration of oxygen within this zone is low, most species construct tunnels or burrows in which they can hide, and utilize the minimum amount of movements necessary to circulate water through, drawing oxygen to them without expending too much energy.[1]
Fish and other vertebrates
Fish have a range of physiological tolerances that are dependent upon which species they belong to. They have different lethal temperatures, dissolved oxygen requirements, and spawning needs that are based on their activity levels and behaviors. Because fish are highly mobile, they are able to deal with unsuitable abiotic factors in one zone by simply moving to another. A detrital feeder in the profundal zone, for example, that finds the oxygen concentration has dropped too low may feed closer to the benthic zone. A fish might also alter its residence during different parts of its life history: hatching in a sediment nest, then moving to the weedy benthic zone to develop in a protected environment with food resources, and finally into the pelagic zone as an adult.
Other vertebrate taxa inhabit lentic systems as well. These include
Trophic relationships
Primary producers
Lentic systems gain most of their energy from photosynthesis performed by aquatic plants and algae.
Bacteria
The vast majority of bacteria in lakes and ponds obtain their energy by decomposing vegetation and animal matter. In the pelagic zone, dead fish and the occasional
The decomposition of organic materials can continue in the benthic and profundal zones if the matter falls through the water column before being completely digested by the pelagic bacteria. Bacteria are found in the greatest abundance here in sediments, where they are typically 2-1000 times more prevalent than in the water column.[11]
Benthic invertebrates
Benthic invertebrates, due to their high level of species richness, have many methods of prey capture.
Fish
Fish size, mobility, and sensory capabilities allow them to exploit a broad prey base, covering multiple zonation regions. Like invertebrates, fish feeding habits can be categorized into guilds. In the pelagic zone,
Lentic food webs
As noted in the previous sections, the lentic biota are linked in complex web of trophic relationships. These organisms can be considered to loosely be associated with specific trophic groups (e.g. primary producers, herbivores, primary carnivores, secondary carnivores, etc.). Scientists have developed several theories in order to understand the mechanisms that control the abundance and diversity within these groups. Very generally,
Community patterns and diversity
Local species richness
The biodiversity of a lentic system increases with the surface area of the lake or pond. This is attributable to the higher likelihood of partly terrestrial species of finding a larger system. Also, because larger systems typically have larger populations, the chance of extinction is decreased.[19] Additional factors, including temperature regime, pH, nutrient availability, habitat complexity, speciation rates, competition, and predation, have been linked to the number of species present within systems.[2][10]
Succession patterns in plankton communities – the PEG model
Phytoplankton and zooplankton communities in lake systems undergo seasonal succession in relation to nutrient availability, predation, and competition. Sommer et al.[20] described these patterns as part of the Plankton Ecology Group (PEG) model, with 24 statements constructed from the analysis of numerous systems. The following includes a subset of these statements, as explained by Brönmark and Hansson[2] illustrating succession through a single seasonal cycle:
Winter
1. Increased nutrient and light availability result in rapid phytoplankton growth towards the end of winter. The dominant species, such as diatoms, are small and have quick growth capabilities.
2. These plankton are consumed by zooplankton, which become the dominant plankton taxa.
Spring
3. A clear water phase occurs, as phytoplankton populations become depleted due to increased predation by growing numbers of zooplankton.
Summer
4. Zooplankton abundance declines as a result of decreased phytoplankton prey and increased predation by juvenile fishes.
5. With increased nutrient availability and decreased predation from zooplankton, a diverse phytoplankton community develops.
6. As the summer continues, nutrients become depleted in a predictable order: phosphorus,
7. Small-sized zooplankton become the dominant type of zooplankton because they are less vulnerable to fish predation.
Fall
8. Predation by fishes is reduced due to lower temperatures and zooplankton of all sizes increase in number.
Winter
9. Cold temperatures and decreased light availability result in lower rates of primary production and decreased phytoplankton populations.
10. Reproduction in zooplankton decreases due to lower temperatures and less prey.
The PEG model presents an idealized version of this succession pattern, while natural systems are known for their variation.[2]
Latitudinal patterns
There is a well-documented global pattern that correlates decreasing plant and animal diversity with increasing latitude, that is to say, there are fewer species as one moves towards the poles. The cause of this pattern is one of the greatest puzzles for ecologists today. Theories for its explanation include energy availability, climatic variability, disturbance, competition, etc.[2] Despite this global diversity gradient, this pattern can be weak for freshwater systems compared to global marine and terrestrial systems.[21] This may be related to size, as Hillebrand and Azovsky[22] found that smaller organisms (protozoa and plankton) did not follow the expected trend strongly, while larger species (vertebrates) did. They attributed this to better dispersal ability by smaller organisms, which may result in high distributions globally.[2]
Natural lake lifecycles
Lake creation
Lakes can be formed in a variety of ways, but the most common are discussed briefly below. The oldest and largest systems are the result of
Natural extinction
All lakes and ponds receive sediment inputs. Since these systems are not really expanding, it is logical to assume that they will become increasingly shallower in depth, eventually becoming wetlands or terrestrial vegetation. The length of this process should depend upon a combination of depth and sedimentation rate. Moss[7] gives the example of Lake Tanganyika, which reaches a depth of 1500 m and has a sedimentation rate of 0.5 mm/yr. Assuming that sedimentation is not influenced by anthropogenic factors, this system should go extinct in approximately 3 million years. Shallow lentic systems might also fill in as swamps encroach inward from the edges. These processes operate on a much shorter timescale, taking hundreds to thousands of years to complete the extinction process.[7]
Human impacts
Acidification
Eutrophication
Eutrophic systems contain a high concentration of phosphorus (~30 µg/L), nitrogen (~1500 µg/L), or both.[2] Phosphorus enters lentic waters from sewage treatment effluents, discharge from raw sewage, or from runoff of farmland. Nitrogen mostly comes from agricultural fertilizers from runoff or leaching and subsequent groundwater flow. This increase in nutrients required for primary producers results in a massive increase of phytoplankton growth, termed a "plankton bloom." This bloom decreases water transparency, leading to the loss of submerged plants.[23] The resultant reduction in habitat structure has negative impacts on the species that utilize it for spawning, maturation, and general survival. Additionally, the large number of short-lived phytoplankton result in a massive amount of dead biomass settling into the sediment.[7] Bacteria need large amounts of oxygen to decompose this material, thus reducing the oxygen concentration of the water. This is especially pronounced in stratified lakes, when the thermocline prevents oxygen-rich water from the surface to mix with lower levels. Low or anoxic conditions preclude the existence of many taxa that are not physiologically tolerant of these conditions.[2]
Invasive species
See also
- Freshwater environmental quality parameters
- Lake aeration
- Limnology
- Man-made lentic water bodies of Maharashtra
References
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- ^ Smriti, Saifun Nahar (2023-10-05). "Adaptation of Phytoplankton to Float in Water". GreenLeen.Com. Retrieved 2023-10-05.
- ^ ISBN 0521739675.
- ^ a b Gliwicz, Z. M. "Zooplankton", pp. 461–516 in O'Sullivan (2005)
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- ^ a b Jónasson, P. M. "Benthic Invertebrates", pp. 341–416 in O'Sullivan (2005)
- ^ Winfield, I. J. "Fish Population Ecology", pp. 517–537 in O'Sullivan (2005)
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Sources
- O'Sullivan, Patrick; Reynolds, C. S. (2005). The Lakes Handbook: Lake Restoration and Rehabilitation. Wiley. ISBN 978-0-632-04795-6.