Examples of monomictic lakes
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Monomictic lakes are holomictic lakes that mix from top to bottom during one mixing period each year. Monomictic lakes may be subdivided into cold and warm types.
Cold monomictic lakes are lakes that are covered by ice throughout much of the year. During their brief "summer", the surface waters remain at or below 4 °C. The ice prevents these lakes from mixing in winter. During summer, these lakes lack significant thermal stratification, and they mix thoroughly from top to bottom. These lakes are typical of cold-climate regions (e.g. much of the Arctic). [1] An example of a cold monomictic lake is Great Bear Lake in Canada. [2]
Warm monomictic lakes are lakes that never freeze, and are thermally stratified throughout much of the year. The density difference between the warm surface waters (the epilimnion) and the colder bottom waters (the hypolimnion) prevents these lakes from mixing in summer. During winter, the surface waters cool to a temperature equal to the bottom waters. Lacking significant thermal stratification, these lakes mix thoroughly each winter from top to bottom. These lakes are widely distributed from temperate to tropical climatic regions. [1] One example is South Australia's Blue Lake, where the change in circulation is signaled by a striking change in colour.
The identification and categorization of monomictic lakes relies on the formation of both an epilimnion (warmer, less dense water) and hypolimnion (cooler, more dense water) separated by a thermocline a majority of the year. [3] The distinct separation of these layers of the water column are collectively referred to as the thermal and density strata. Thermal and density stratification is a critical factor influencing the composition of the water column. Composition often refers to the presence of or lack of nutrients and organisms. [4] In both cold and warm monomictic lakes, the epilimnion and hypolimnion are separated for a majority of the year. In warm monomictic lakes, the water is in a uniform, liquid form; in cold monomictic lakes, the body contains a layer of ice and is cooler in temperature. Concerns and solutions pertaining to both warm and cold monomictic lakes are explored below.
As warm monomictic lakes are entirely liquid, warmer in temperature, and highly productive, summer stratification commonly leads to eutrophication. This summer stratification is especially long in warm monomictic lakes. During eutrophication, excess nutrients are produced and depleted in a lake at opposite, vertical ends of the water column. This in turn dictates the growth and maturation of populations of organisms which tend to influence water oxygen and nutrient levels. In warm monomictic lakes, thermal stratification lends to oxygen depletion in the hypolimnion; a lack of mixing prevents the introduction of oxygen from the atmosphere into the water. This measure is known as dissolved oxygen (DO). When DO is lowered in the hypolimnion, nutrients like ammonium, nitrate, and phosphates tend to dominate. When oxygen levels are extremely low, the water is considered hypoxic and cannot support many forms of life. A lack of oxygen also limits natural chemical processes like the conversion of ammonium to nitrate. [4]
A mixture of ammonium and nitrates is required to sustain plant growth; an overabundance of ammonium is linked to poor plant growth and productivity. [5] In a lake, the overabundance of ammonium also indicates anaerobic and acidic conditions. This lack of oxygen modifies a lake’s oxidation-reduction potential (ORP). The higher a lake’s ORP, the higher the levels of oxygen present in the water. Ideal ranges are between 300 and 500 millivolts. Ideally, higher levels of oxygen aid resident bacteria and microorganisms in the decomposition of organic matter and dispersal of necessary nutrients into the water column. [6] Conversely, a low ORP and low oxygen drives the release of sediment phosphorus via diffusion along concentration gradients through a process known as internal loading. [7] Together, the increases in phosphorus, ammonium, and nitrate can drive the production of toxic algal blooms. Such blooms create a positive feedback loop of depleting nutrients and oxygen, and the subsequent release of nutrients needed to support their continued growth. Eutrophication can be both a natural and an anthropologic process; anthropogenic inputs are typically through sewage and waste water, or agricultural soil erosion and run-off. [8]
A rather new hypothesis is a link between residence time of water and seasonal stratification in monomictic lakes leading to eutrophication. Increased residence time leads to longer periods of stratification, reduced water mixing, and increased eutrophication in the epilimnion. Some propose the development of interventions personalized to lakes to reduce these conditions. Such personalization refers to the manipulation of a lake’s residence time to combat internal loading and eutrophication by reducing the length of a stratification time period. Current models utilize the redirection of water flow into and out of monomictic lakes to assist in overturn and the physical “flushing” of phytoplankton and excess nutrients. Such methods have shown to reduce residence time and stratification by days. While these time frames are limited in scope, they show potential to be lengthened for greater results in future studies and various lake models. [9]
Hypolimnetic aeration and oxygenation aims to directly address lowered DO levels in a given lake which leads to eutrophication. By increasing oxygen levels in the hypolimnion, one aims to increase the ORP and reduce the rate and incidence of internal loading. Aerators are utilized to introduce oxygen, pure or atmospheric, directly into the water column. This is an especially expensive intervention given the electrical demands required to power such equipment. These costs make these aerators rather unsustainable as they are economically costly, and production of electricity can have environmental implications. Ecological threats have also been demonstrated. Use of aerators correlates to increased prevalence of gas bubble disease amongst fish. Yet, other organisms, such as zooplankton and fish, benefit from this process as increased aerobic conditions expand their territory in a lake. [10]
Hypolimnetic withdrawal involves the withdrawal of water from a eutrophic lake at the hypolimnion at peaks of seasonal stratification. This water is removed to indirectly remove phosphorus. Upon addition of this water back into the hypolimnion, cyanobacteria growth is limited. This addition to the hypolimnion also reduces mixing of the water column and dispersal of nutrients to feed epilimnion algae. The physical removal of water can be either passive or active and is typically limited to minimize quality impacts on the water level. This water can also be discharged downstream and can have unintended effects. The low quality water rich in toxins and nutrients removed from the hypolimnion when transferred to other lakes can destabilize their water columns. In some cases, lakes treated via hypolimnetic withdrawal may also experience undesirable water-level reductions and overall increases in average water temperature followed by mixing. [10]
Lastly, sediment dredging pertains to the direct collection and removal of sediment at the bottom of the lake. Removal of the top layer of the sediment aims to remove organic matter containing undesired nutrients. This method has measurable impacts on benthic organisms. It can take up to three years to restore the benthic organisms removed by dredging. Such organisms are essential to nutrient cycling in lakes and aquatic environments. [10]
The largest factor that controls water temperature in a given lake is air temperature. [4] Current changes and trends in global temperatures year round are a formidable threat to aquatic ecosystems. Current studies support that the combination of increased air temperatures and reduced precipitation impact shallow, monomictic lakes. In particular, their mixing may increase; this mixing lends to increased nutrient dispersal, anoxic conditions, and algal blooms. Southern regions may also see increases in salinity. [10] Warm monomictic lakes that have experienced historically warm winters demonstrate greater thermal stability. This stability reduces mixing interactions and the oxygenation of waters. Furthermore, cold monomictic lakes may experience less cool conditions year-round leading to increased mixing and changes in thermal stratification otherwise unseen. [11]
Limnology is the study of inland aquatic ecosystems. The study of limnology includes aspects of the biological, chemical, physical, and geological characteristics of fresh and saline, natural and man-made bodies of water. This includes the study of lakes, reservoirs, ponds, rivers, springs, streams, wetlands, and groundwater. Water systems are often categorized as either running (lotic) or standing (lentic).
The hypolimnion or under lake is the dense, bottom layer of water in a thermally-stratified lake. The word "hypolimnion" is derived from Ancient Greek: λιμνίον, romanized: limníon, lit. 'lake'. It is the layer that lies below the thermocline.
The epilimnion or surface layer is the top-most layer in a thermally stratified lake.
A thermocline is a distinct layer based on temperature within a large body of fluid with a high gradient of distinct temperature differences associated with depth. In the ocean, the thermocline divides the upper mixed layer from the calm deep water below.
Lake stratification is the tendency of lakes to form separate and distinct thermal layers during warm weather. Typically stratified lakes show three distinct layers: the epilimnion, comprising the top warm layer; the thermocline, the middle layer, whose depth may change throughout the day; and the colder hypolimnion, extending to the floor of the lake.
Water aeration is the process of increasing or maintaining the oxygen saturation of water in both natural and artificial environments. Aeration techniques are commonly used in pond, lake, and reservoir management to address low oxygen levels or algal blooms.
A lake ecosystem or lacustrine ecosystem includes biotic (living) plants, animals and micro-organisms, as well as abiotic (non-living) physical and chemical interactions. Lake ecosystems are a prime example of lentic ecosystems, which include ponds, lakes and wetlands, and much of this article applies to lentic ecosystems in general. Lentic ecosystems can be compared with lotic ecosystems, which involve flowing terrestrial waters such as rivers and streams. Together, these two ecosystems are examples of freshwater ecosystems.
Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen. The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. Anoxic waters can be contrasted with hypoxic waters, which are low in dissolved oxygen. This condition is generally found in areas that have restricted water exchange.
A dimictic lake is a body of freshwater whose difference in temperature between surface and bottom layers becomes negligible twice per year, allowing all strata of the lake's water to circulate vertically. All dimictic lakes are also considered holomictic, a category which includes all lakes which mix one or more times per year. During winter, dimictic lakes are covered by a layer of ice, creating a cold layer at the surface, a slightly warmer layer beneath the ice, and a still-warmer unfrozen bottom layer, while during summer, the same temperature-derived density differences separate the warm surface waters, from the colder bottom waters. In the spring and fall, these temperature differences briefly disappear, and the body of water overturns and circulates from top to bottom. Such lakes are common in mid-latitude regions with temperate climates.
The Trophic State Index (TSI) is a classification system designed to rate water bodies based on the amount of biological productivity they sustain. Although the term "trophic index" is commonly applied to lakes, any surface water body may be indexed.
Freshwater biology is the scientific biological study of freshwater ecosystems and is a branch of limnology. This field seeks to understand the relationships between living organisms in their physical environment. These physical environments may include rivers, lakes, streams, ponds, lakes, reservoirs, or wetlands. Knowledge from this discipline is also widely used in industrial processes to make use of biological processes involved with sewage treatment and water purification. Water presence and flow is an essential aspect to species distribution and influences when and where species interact in freshwater environments.
An Olszewski tube is a pipe designed to bring oxygen-poor water from the bottom of a lake to the top. This tube was first proposed by a Polish limnologist named Przemysław Olszewski in 1961 and helps combat the negative effects of eutrophication, high nutrient content, in lakes. The basic concept behind the Olszewski tube is the reduction of nutrient concentration and destratification; the more specific goal is hypolimnetic withdrawal.
Freshwater environmental quality parameters are those chemical, physical or biological parameters that can be used to characterise a freshwater body. Because almost all water bodies are dynamic in their composition, the relevant quality parameters are typically expressed as a range of expected concentrations.
Hypoxia refers to low oxygen conditions. For air-breathing organisms, hypoxia is problematic but for many anaerobic organisms, hypoxia is essential. Hypoxia applies to many situations, but usually refers to the atmosphere and natural waters.
Deep-water aeration, also known as hypolimnetic aeration, describes the provision of oxygen from the atmosphere to meet oxygen demand in deep water without disrupting the natural stratification of the water above. This process promotes the development of aerobic conditions in deep water, leading to a significant reduction in phosphate dissolution and an improvement in sediment mineralization. Scientific studies support the effectiveness of implementing technical ventilation measures to maintain year-round aerobic conditions in the deep water, thereby restoring the natural balance of lakes.
Chemical phosphorus removal is a wastewater treatment method, where phosphorus is removed using salts of aluminum, iron, or calcium. Phosphate forms precipitates with the metal ions and is removed together with the sludge in the separation unit.
Lake metabolism represents a lake's balance between carbon fixation and biological carbon oxidation. Whole-lake metabolism includes the carbon fixation and oxidation from all organism within the lake, from bacteria to fishes, and is typically estimated by measuring changes in dissolved oxygen or carbon dioxide throughout the day.
Benthic-pelagic coupling are processes that connect the benthic zone and the pelagic zone through the exchange of energy, mass, or nutrients. These processes play a prominent role in both freshwater and marine ecosystems and are influenced by a number of chemical, biological, and physical forces that are crucial to functions from nutrient cycling to energy transfer in food webs.
Stratification in water is the formation in a body of water of relatively distinct and stable layers by density. It occurs in all water bodies where there is stable density variation with depth. Stratification is a barrier to the vertical mixing of water, which affects the exchange of heat, carbon, oxygen and nutrients. Wind-driven upwelling and downwelling of open water can induce mixing of different layers through the stratification, and force the rise of denser cold, nutrient-rich, or saline water and the sinking of lighter warm or fresher water, respectively. Layers are based on water density: denser water remains below less dense water in stable stratification in the absence of forced mixing.
Lake Lacawac is located at the very middle of Lacawac's Sanctuary Field Station in Pennsylvania and has been deemed the "southernmost unpolluted glacial lake in North America." Lake Lacawac has proven to be invaluable to researchers and students to conduct field experiments in order to learn more about the limnology of the lake.
