A limnological tower is a structure constructed in a body of water to facilitate the study of aquatic ecosystems (limnology). They play an important role in drinking water infrastructure by allowing the prediction of algal blooms which can block filters and affect the taste of the water.
Limnological towers provide a fixed structure to which sensors and sampling devices can be affixed. [1] The depth of the structure below water level allows for study of the various layers of water in the lake or reservoir. [2] The management of limnological conditions can be important in reservoirs used to supply drinking water treatment plants. In certain conditions algal blooms can occur which can block filters, change the pH of the water and cause taste and odour problems. If the sensors extend to the bed level the tower can also be used to monitor the hypolimnion (lowest layer of water) which in some conditions can become anoxic (of low oxygen content) which may affect the lake ecology. [3]
Limnological towers have been constructed in reservoirs used to supply drinking water in the United Kingdom since algal blooms began causing problems with water quality. By providing data on water conditions and algae levels the towers can predict the behaviour of the algae and allow managers to make decisions to alter conditions to prevent algal blooms. These decisions may include altering water inflows (particularly where nutrient-rich intakes are considered), activating water jets to promote the mixing of different layers of water and altering the depth from which water is abstracted. These decisions can affect the behaviour of the reservoir over a period from a few hours to a few years. [3]
Six combined limnological and meteorological observation towers were established in the Great Lakes on the US-Canadian border in 1961. Three were installed in Lake Huron, two in Lake Ontario and one in Lake Erie by the Great Lakes Institute. These were innovative in design and cheap to construct, being built largely from 4-inch (100 mm) water pipe. Constructed in water depths of 7–63 metres (23–207 ft) the towers provided measurements of wind speed, air temperature and rainfall as well as water temperature and current flows at different depth. The shorter towers (in water less than 60 feet (18 m) of depth) were attached directly to the bed, towers in greater depths of water were floating units, with a submerged ballast tank, that were anchored to the lake bed by means of cables and weights. [4]
A further two limnological towers were constructed near Douglas Point in Lake Huron in the 1960s. One, 24 metres (79 ft) high was built 4 kilometres (2.5 mi) offshore in 1961 and a second 47 metres (154 ft) high in 1969. They are poles anchored to the lake bed by means of a gimbal and braced by tensioned cables and anchor guys. They featured a mobile thermistor sensor that could be moved to any depth on the tower as well as fixed thermometers at various depths and were intended to montor the temperatures of different water layers in the lake. [5]
A concrete limnological tower was installed at Rutland Water, England's largest reservoir by surface area, when it was built in the early 1970s. The design of the tower was influenced by consultation with the Water Research Centre and was intended to provide the best possible tools to monitor the ecological conditions of the reservoir so that it could be best managed by its operator (the Anglian Water Authority). The tower monitors water temperature, dissolved oxygen levels and water fluorescence (which is a measure of algal content) at 2m depth intervals. [3] The tower also has the ability to draw water samples for further testing from the various depths and also mounts an automatic weather station. [2] The data is continuous and displayed visually in real-time at the reservoir control centre, situated at the dam. The site of the tower was chosen to best suit the needs of the operator. The reservoir consists of two arms – northern and southern – and has been designed such that all nutrient-rich water enters the southern arm. The intention being that nutrients will be depleted before the water is abstracted for use at the eastern end of the site. The northern-arm is fed by nutrient-poor sources and should be relatively unaffected by algal blooms. A secondary outlet is available that draws solely from the northern arm, in cases that the southern arm is affected by algal growth. Additionally the operators are able to draw directly from the River Nene if the reservoir water is unusable. [3]
The Queen Mother Reservoir near London also has a limnological tower. [6]
An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term algae encompasses many types of aquatic photosynthetic organisms, both macroscopic multicellular organisms like seaweed and microscopic unicellular organisms like cyanobacteria. Algal bloom commonly refers to the rapid growth of microscopic unicellular algae, not macroscopic algae. An example of a macroscopic algal bloom is a kelp forest.
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytoplankton productivity". Water bodies with very low nutrient levels are termed oligotrophic and those with moderate nutrient levels are termed mesotrophic. Advanced eutrophication may also be referred to as dystrophic and hypertrophic conditions. Eutrophication can affect freshwater or salt water systems. In freshwater ecosystems it is almost always caused by excess phosphorus. In coastal waters on the other hand, the main contributing nutrient is more likely to be nitrogen, or nitrogen and phosphorus together. This depends on the location and other factors.
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).
Lake Okeechobee, also known as Florida's Inland Sea, is the largest freshwater lake in the U.S. state of Florida. It is the tenth largest natural freshwater lake among the 50 states of the United States and the second-largest natural freshwater lake contained entirely within the contiguous 48 states, after Lake Michigan.
The epilimnion or surface layer is the top-most layer in a thermally stratified lake. It sits above the deeper metalimnion and hypolimnion. It is typically warmer and has a higher pH and higher dissolved oxygen concentration than the hypolimnion.
A thermocline A distinct layer based on temperature within a large body of fluid A gradiant of distinct temperature differences associated with depth. In the ocean, the thermocline divides the upper mixed layer from the calm deep water below.
The spring bloom is a strong increase in phytoplankton abundance that typically occurs in the early spring and lasts until late spring or early summer. This seasonal event is characteristic of temperate North Atlantic, sub-polar, and coastal waters. Phytoplankton blooms occur when growth exceeds losses, however there is no universally accepted definition of the magnitude of change or the threshold of abundance that constitutes a bloom. The magnitude, spatial extent and duration of a bloom depends on a variety of abiotic and biotic factors. Abiotic factors include light availability, nutrients, temperature, and physical processes that influence light availability, and biotic factors include grazing, viral lysis, and phytoplankton physiology. The factors that lead to bloom initiation are still actively debated.
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, which may change depth throughout the day, and the colder Hypolimnion extending to the floor of the lake.
In biological oceanography, critical depth is defined as a hypothetical surface mixing depth where phytoplankton growth is precisely matched by losses of phytoplankton biomass within the depth interval. This concept is useful for understanding the initiation of phytoplankton 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.
Thin layers are concentrated aggregations of phytoplankton and zooplankton in coastal and offshore waters that are vertically compressed to thicknesses ranging from several centimeters up to a few meters and are horizontally extensive, sometimes for kilometers. Generally, thin layers have three basic criteria: 1) they must be horizontally and temporally persistent; 2) they must not exceed a critical threshold of vertical thickness; and 3) they must exceed a critical threshold of maximum concentration. The precise values for critical thresholds of thin layers has been debated for a long time due to the vast diversity of plankton, instrumentation, and environmental conditions. Thin layers have distinct biological, chemical, optical, and acoustical signatures which are difficult to measure with traditional sampling techniques such as nets and bottles. However, there has been a surge in studies of thin layers within the past two decades due to major advances in technology and instrumentation. Phytoplankton are often measured by optical instruments that can detect fluorescence such as LIDAR, and zooplankton are often measured by acoustic instruments that can detect acoustic backscattering such as ABS. These extraordinary concentrations of plankton have important implications for many aspects of marine ecology, as well as for ocean optics and acoustics. Zooplankton thin layers are often found slightly under phytoplankton layers because many feed on them. Thin layers occur in a wide variety of ocean environments, including estuaries, coastal shelves, fjords, bays, and the open ocean, and they are often associated with some form of vertical structure in the water column, such as pycnoclines, and in zones of reduced flow.
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.
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.
The deep chlorophyll maximum (DCM), also called the subsurface chlorophyll maximum, is the region below the surface of water with the maximum concentration of chlorophyll. The DCM generally exists at the same depth as the nutricline, the region of the ocean where the greatest change in the nutrient concentration occurs with depth.
A harmful algal bloom (HAB) is an algal bloom that causes negative impacts to other organisms by production of natural algae-produced toxins, mechanical damage to other organisms, or by other means. HABs are sometimes defined as only those algal blooms that produce toxins, and sometimes as any algal bloom that can result in severely lower oxygen levels in natural waters, killing organisms in marine or fresh waters. Blooms can last from a few days to many months. After the bloom dies, the microbes that decompose the dead algae use up more of the oxygen, generating a "dead zone" which can cause fish die-offs. When these zones cover a large area for an extended period of time, neither fish nor plants are able to survive. Harmful algal blooms in marine environments are often called "red tides".
A lake is a naturally occurring, relatively large body of water localized in a basin surrounded by land, with much slower-moving flow than any inflow or outflow streams that serve to feed or drain the lake. Lakes lie completely on land and are separate from the ocean, although, but like the much larger oceans, they form part of the Earth's water cycle by serving as large standing water reservoirs. Most lakes are freshwater, but some are salt lakes with salinities even higher than that of seawater.
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.
Freshwater phytoplankton is the phytoplankton occurring in freshwater ecosystems. It can be distinguished between limnoplankton, heleoplankton, and potamoplankton. They differ in size as the environment around them changes. They are affected negatively by the change in salinity in the water.
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.
Water clarity is a descriptive term for how deeply visible light penetrates through water. In addition to light penetration, the term water clarity is also often used to describe underwater visibility. Water clarity is one way that humans measure water quality, along with oxygen concentration and the presence or absence of pollutants and algal blooms.