Aerobic

With at least some oxygen present but how much is not precisely defined. The usual sense assumes that enough is present to maintain iron compounds in the oxidised (Fe³⁺) state

Algae

A general term applied to photosynthetic organisms that are generally aquatic, may be microscopic or very large (seaweeds), may have a bacteria-like cell structure, or one like all other organisms, contain chlorophyll a and often a variety of other pigments that give them characteristic colours (blue-green, red, green, yellow-green etc, and which, when they reproduce sexually, produce sex cells in structures that are not bounded by sterile (non-reproducing) cells. They evolved early, but are incorrectly described as 'simple, or primitive plants'

Algal blooms

A much misused term. Blooms are surface scums of blue green algae (q.v.) which accumulate under calm weather conditions from populations that were previously distributed through the water. They are formed only by those blue green algae that have mechanisms that give them positive buoyancy and the bloom may be a reflection of breakdown of a mechanism that otherwise maintains these algae at depths optimal for their growth. However, the term has been much used to describe any large growth of algae, and, increasingly, any, even moderate, growth of almost any microorganism in waters. It is thus rapidly becoming meaningless

Allelopathy

Production of substances by one organism that inhibit the growth, activity or reproduction of another

Alternative stable states

Potential existence of markedly different biological communities under the same external environmental conditions

Anaerobiosis

Absence of oxygen

Biomanipulation

Deliberate alteration of the biological community to achieve a desirable and planned change in environment

Blue green algae

A group of largely microscopic, photosynthetic organisms with a bacterial structure (prokaryote), but containing chlorophyll a and a photosynthesis biochemistry unlike other bacteria but similar to that of other algae and higher plants. Alternative names are blue-green bacteria, cyanophytes, cyanobacteria, or, probably the most suitable, cyanoprokaryotes. Blue and red pigments, contained within them, give an often characteristic colour

Botulism

Lethal disease caused by an anaerobic bacterium, Clostridium botulinum. Type C affects birds, including ducks and gulls, foraging in anaerobic mud or decomposing plant material cast up on shorelines. Such conditions are often associated with eutrophication, but birds foraging in waste tips are also frequently affected

Buffer mechanisms

Devices which tend to preserve one of a series of alternative stable states against the action of switches(qv) tending to change the present state to an alternative one

Buffer zones

Areas of natural, semi-natural or uncultivated vegetation bordering streams, rivers or lakes or otherwise intercepting water being delivered from the catchment to the water body

Catchment area

The land (and including the streams, rivers, wetlands and lakes) from which water runs off to supply a particular location in a freshwater system. One might thus talk of the catchemnt of a particular point along a river, or at its mouth, or of a particular lake. In North America, the term watershed is often used instead of catchment area. In the UK, watershed means the line separating two adjacent catchments

Charophytes

A group of green algae, visible to the naked eye, with a characteristic structure in which the 'stems' are very large single cells, from which whorls of similarly constructed branches emerge. Charophytes are anchored in sediments by branching cellular systems, not roots. They often deposit marl (calcium carbonate) giving them a rough texture and the common name of 'stoneworts', though not all do this. They also have a characteristic smell, which some people describe as 'garlicy'

Chlorophyll a

The major photosynthetic pigment of algae and plants. Often used as a measure of the biomass of the former

Cladocera

Water fleas. A group of crustaceans up to a few mm long, which either filter particles from water for food or grasp larger particles such as smaller animals. The best known genus is Daphnia

Cohort

A generation of animals all born at more or less the same time so that stages in their life histories are near synchronised and that their progress can be followed as a group. Many temperate fish, for example are born in May or June in a single annual cohort

Copepods

A group of crustaceans more diverse in the sea than freshwaters. Some species filter particles for food, others grasp larger particles such as smaller animals. The life history comprises six successively larger naupliar and then five copepodite stages before the sexually reproducing adults are formed as the twelfth stage

Cyprinid fish

Fish of the carp family, including roach, bream, goldfish, common carp, crucian carp, grass carp, rudd, tench, barbel, bitterling, chub, dace, bleak and goldfish. They do not have teeth on the jaws but grind their food with teeth born on bones in the back of the throat

Denitrification

Conversion by bacteria of nitrate ions to nitrogen gas. The nitrate is used as an oxidising agent to release energy from organic matter

Diatoms

A group of algae, brown or yellow coloured, that is very common in natural waters. The cell wall is made of polymerised silicate, forming a sort of glass and is readily preserved in sediments, when the organic part of the organisms decays. Because of a long interest in this group by naturalists, its ecology is reasonably well known and diatoms in fossil deposits can be used to interpret changes in past environments

Diffuse sources

Supplies of nutrients or other pollutants that come from a myriad of small-sized locations (e.g the land surface) as opposed to a clearly located source (e.g. a sewage treatment works) that is individually usually comparatively large

Dinoflagellate

Algae of the Division Dinophyta, very common in the sea but often important also in freshwaters. They have flagella and can move actively in the water column, sometimes transferring nutrients in their cells from deeper to shallower layers

Eddy currents

Small, random, wind-generated movements of water that keep small particles in suspension. Also called eddies

Eutrophication

The process in which there is an increase in the rate of addition of substances of nitrogen and phosphorus to a natural system, usually aquatic. The term is currently being used by some people rather more loosely to emphasise the problematic consequences of this process. However, although there are always at least subtle biological consequences of addition of even small quantities of nutrients, this may not result necessarily in a perceived problem. The term 'nutrification' has been proposed to cover the addition of nutrients without causing problems but, as well as being a somewhat ill-sounding word, it is unnecessary when the proper use of 'eutrophication' already includes this

Export Coefficient Modelling

A technique for calculating nutrient loadings and concentrations in a stream or lake from a knowledge of land use, numbers of stock and number of people in the catchment, stream discharge and the rates at which the nutrients are leached or excreted from the various sources

Floodplain

Full extent of a river bed. For part of a year a river may only occupy a central smaller channel, but when flows are high the full extent of the channel will be required. the floodplain has been misperceived by many human societies as land at risk of flooding rather than as river bed that is sometimes dry. As a result, many floodplains have been artificially isolated from the dry season channel by embankments and the many values of the floodplain in flood storage, improvement in water quality, groundwater recharge and wildlife support have been lost

Flushing

Replacement of the water in a lake by incoming stream, river or ground water

Green algae

Algae of the Division Chlorophyta, containing the same pigments as land plants, which have evolved from them. Common in lakes as phytoplanton, periphyton or loose filamentous masses

Inorganic carbon equilibria

Relationships between carbon dioxide, water, carbonic acid, bicarbonate, hydrogen ions and carbonate in natural waters that determine the proportions of the available inorganic carbon able to exist as carbon dioxide, bicarbonate or carbonate at particular pH values. The proportions are also influenced by the total ion content of the water, but to a smaller extent

Internal loading

Release of nutrients within a water body from sources such as the sediment, decomposition of litter or carcasses and excretion, as opposed to external loading where the nutrients come from the catchment or atmosphere. Internal loading includes the concept of recycling of nutrients that have ultimately entered as an external load

Isoetid

Strictly an aquatic plant of the genus Isoetes, a group of early involved fern associates. The term is usually used to refer to a broader group of small submerged plants, including Lobelia and Littorella, which thrive in clear, base- and inorganic carbon-poor waters on sandy sediments. They derive carbon dioxide and nutrients from the sediments and have waxier cuticles than other submerged plants. This allows them to control carbon losses to the water through diffusion. Such plants, being small, are very vulnerable to eutrophication

Key nutrient

Element required by algae or plants in natural waters that is likely to be scarce in supply in relation to need and hence may limit growth rate or the accumulation of biomass. many elements may limit growth rate at least temporarily but biomass is most commonly limited in freshwaters by phosphorus, nitrogen or both together. Silicon and iron may also be important for particular groups or species

Limnology

Science of natural freshwaters. Strictly the term should include only lakes (Gk limnos, lake) but running waters have always been included

Loading

The amount of nutrient supplied to a water body in a given time (e.g. kg nitrogen per year). It may also be expressed per unit area (e.g. kg phosphorus per metre squared per year) and may be attributed to a particular source (e.g effluent loading) as well used for the total supply

Macroinvertebrates

Small animals from a few mm in size to a few cm, of a varietyof Phyla (e.g. flatworms, many crustaceans, roundworms, snails, insects) that do not posses strengthening, as cartilage or bone along their dorsal surfaces (backs). This distinguishes them from microinvertebrates (protozoa, many threadworms) which are simply very small and not easily sampled by conventional methods and from the usually bigger chordates, which include the vertebrates (fish, amphibians, reptiles, birds, mammals)

Macrophyte

A large aquatic plant (Gk macros, large, phytos, plant). Term coined to distinguish the bulkier plants from the microscopic forms of the algae (microphytes). In practice there is usually no confusion if the terms 'plant' and 'alga' are used. There are some large filamentous algae and some algal genera like Chara, which are functionally macrophytes, however. In the sea, the large red, brown and green seaweeds are macrophytes whilst being algae in a strict botanical sense. Some observers object to the use of the word plant because they say algae are plants. This is not strictly true. 'Algae' is a general term covering a huge range of organisms, most of which photosynthesise, but many of which also feed on dissolved organic compounds or even particles. This is a fascinating area of biology, but further discussion is unlikely to be enlightening except to afficionados. In practice, 'plant' to denote 'big' and 'alga' to denote 'small' seems to work very well for most people

Metabolism

Chemical functioning of living organisms by which energy is manipulated, structural substances synthesised, wastes excreted, and procreation ensured

Microaerophilic

Conditions of finite but very low oxygen concentration

Nitrogen fixation

Conversion by some bacteria and blue green algae of molecular nitrogen to combined nitrogen, ultimately as amino groups. The process usually requires microaerophilic or anaerobic conditions and is the main pathway by which nitrogen enters terrestrial ecosystems, and therefore ultimately freshwater systems

Nutrients

Substances absolutely required by living organisms. In the context of this article, the term is largely confined to substances of nitrogen and phosphorus, which are generally scarce in available forms in relation to need, but substances of about 20 elements are ultimately needed for growth and maintenance. Most are in relatively abundant supply. See also 'key nutrient'

Nymphaeid

Aquatic plant, commonly called water lily, with leaves produced from a rhizome in the sediment and at least some coming to float at the water surface

Palaeolimnology

Study of the past history of lakes (GK, limnos) from an investigation of the contents of their sediments. 'Limnology' includes also the study of the current characteristics of freshwaters

Periphyton

Complex of attached algae, bacteria, Protozoa, organic detritus and marl (qv) which develops on plant surfaces underwater. The term is increasingly used to include similar complexes on any underwater surface (e.g rocks, sediments, beer bottles). Because 'phytos' is very definitely Greek for plant, this reflects inadequacies in the linguistic training of current generations. (See also 'eutrophication')

Phytoplankton

Community of largely microscopic algae suspended or floating in natural waters. Most species are denser than water and tend to sink, but are maintained in suspension by wind-generated water currents. Some species have flagella, with which they move and are able to counteract their tendency to sink. Only a few species have buoyancy mechanisms and actually float. These are primarily the bloom (qv)-forming blue green algae

Piscivore

Animal (often another fish, a bird or mammal), that catches and eats fish. Not generally applied to anglers, however

Point sources

Supplies of nutrients or other pollutants that come from well-defined clearly located origins (e.g. a sewage treatment works) that are individually usually comparatively large compared with a myriad of small -sized locations (e.g the land surface). See 'Diffuse source'

Precambrian

Geological Era extending from the origin of Earth, between 4.6 and 5 x 109 years ago and 600 x 106 years ago. For most of this period, Earth was anaerobic or microaerophilic, becoming aerobic after about 2 x 106 years ago when photosynthesis using water as a hydrogen donor, and hence releasing oxygen, evolved. The blue-green algae were the major group to evolve this trait

Refuge concept

Provision of habitat by aquatic plants in which predation rates of fish on zooplankton, or fish on other fish, are reduced compared with those in the open water

Restoration

Process of re-establishing former pristine conditions and diversity to a habitat. This is very rarely achievable but the term is widely used as an alternative to 'rehabilitation', in which a partial restoration is achieved

Rotifers

A group of small, multicellular animals which feed on small particles in natural waters, often by a ring of cilia whose individual movements give the impression of a rotating wheel. The currents so created bring a supply of particles to the mouth of the animal

Secchi disc

White circular plate lowered into waters to measure visibility through the water column (transparency)

Site of Special Scientific Interest (SSSI)

An area in the UK identified by the statutory conservation agencies as containing features (biological or physiographical or geological) worthy of conservation. Sites are graded in value, Grade 1 sites being equivalent to National Nature Reserves

Stonewort

See 'Charophyte'

Switches

Mechanisms which determine movement from one state to another in systems capable of supporting alternative stable states (qv)

Thermal stratification

Heating of the surface waters of lakes under relatively sheltered conditions may convert a state of complete mixing to the bottom by wind to one in which the surface, warmer waters form a layer floating on deeper, colder waters. The upper stratum or layer is called the epilimnion, the lower, the hypolimnion. Particles falling into the hypolimnion are trapped there until the lake is mixed again when cooler, windier conditions return. The water chemistry of the two layers, separated by a thermal gradient, the thermocline, may become very different as nutrients are regenerated in the hypolimnion, but used up in the epilimnion. The hypolimnion may become anaerobic

Transparency

A measure of light penetration into a water body, determined by viewing the depth to which a white disc, lowered from the surface, disappears from view

Washout rate

Synonym for flushing rate (units are time-1), describing the rate at which the water mass of a lake is replaced from the inflows and rainfall. The inverse is turnover time, which describes the average time that a portion of water will spend in the lake

Water flea

See Cladocera

Wetland

Aquatic habitat in which plants, as opposed to microalgae are predominant. Includes swamps, marshes, bogs, and shallow lakes. Sometimes shallow waters down to 6m (the limit of penetration of diving ducks) are included, even if not dominated by plants

Young-of-the-year

Cohort (qv) of fish born in a given year. The cohort is known as 0+. Beyond their first birthday they are known as 1+ until their second birthday, when they become 2+ and so on

Zooplanktivore

Consumer of zooplankton. Usually fish or large invertebrates, but can be birds

Zooplankton

Animal community free swimming or suspended in the open water