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Macroclimate — The general large-scale climate of a large area or country, as distinguished from Mesoclimate and Microclimate.

Macroinvertebrate — An animal without a backbone, large enough to see without magnification.

Macrophyte —
(1) A member of the macroscopic plant life, especially of a body of water.
(2) The macroscopic plants in the aquatic environment. The most common macrophytes are the rooted vascular plants that are usually arranged in zones in aquatic ecosystems and restricted in the area by the extent of illumination through the water and sediment deposition along the shoreline.

Makeup Water — Water added to the flow of water used to cool condensers in electric power plants. This new water replaces condenser water lost during passage of the cooling water through cooling towers or discharged in blowdowns.

Maximum contaminant level (MCL) — Maximum permissible level of a contaminant in water that is delivered to any user of a public water system. MCLs are enforceable standards established by the U.S. Environmental Protection Agency.

Maximum Sustainable Yield — The greatest amount of a renewable natural resource (e.g., forests or wildlife) that can be removed without diminishing the continuing production and supply of the resource.

Mayordomo — A Spanish word used in the Southwestern United States to identify the individual responsible for overseeing water allocation and maintenance of the water conveyance systems. Used synonymously for water commissioner or ditch rider.

Meander —
(1) The turn of a stream, either live or cut off. The winding of a stream channel in the shape of a series of loop-like bends.
(2) A sinuous channel form in flatter river grades formed by the erosion on one side of the channel (pools) and deposition on the other side (point bars).

Meander Amplitude — The distance between points of maximum curvature of successive meanders of opposite phase in a direction normal to the general course of the Meander Belt, measured between centerlines of channels.

Meander Belt — The zone along a valley floor that encloses a meandering river.

Meander Breadth — The distance between the lines used to define the Meander Belt.

Meander Length — The distance in the general course of the meanders between corresponding points of successive meanders of the same phase.

Meander Line — A line delineated by government survey for the purpose of defining the bends or windings of the banks of a stream or the shore of a body of water, and as a means for ascertaining the quantity of land embraced by the survey.

Median — The middle or central value in a distribution of data ranked in order of magnitude. The median is also known as the 50th percentile.

Meniscus — The curved surface of the liquid at the open end of a capillary column.

Meromictic Lake — A lake in which some water remains partly or wholly unmixed with the main water mass at circulation periods. The process leading to a meromictic state is called Meromixts. The perennially stagnant deep layer of a meromictic lake is the Monimolimnion. The part of the meromictic lake in which free circulation can occur is the Mixolimnion. The boundary between the monimolimnion and the mixolimnion is the Chemocline. Compare to Dimictic Lake.

Meromixis — A condition of permanent stratification of water masses in lakes.

Mesic — Refers to environmental conditions that have medium moisture supplies as compared to wet conditions (Hydric) or dry conditions (Xeric).

Mesotrophic (Water) — Pertaining to a lake or other body of water characterized by moderate nutrient concentrations such as nitrogen and phosphorous and resulting significant productivity. Such waters are often shallow, with algal blooms and periods of oxygen deficiency. Slightly or moderately eutrophic water can be healthful and support a complex web of plant and animal life. However, such waters are generally undesirable for drinking water and other needs. Degrees of Eutrophication typically range from Oligotrophic water (maximum transparency, minimum chlorophyll–a, minimum phosphorus) through Mesotrophic, Eutrophic, to Hypereutrophic water (minimum transparency, maximum chlorophyll–a, maximum phosphorus).

Metabolism — (Biology) The sum of the processes concerned in the building up of protoplasm and its destruction incidental to life; the chemical changes in living cells, by which the energy is provided for the vital processes and activities, and new material is assimilated to repair the waste. Metabolism may be considered as including two aspects or processes: constructive metabolism (termed Anabolism or Assimilation) or destructive metabolism (termed Catabolism or Dissimilation). Anabolism and Catabolism go on together, but one may predominate and obscure the other. Also see Zone of Net Metabolic Production.

Metalimnion — The middle layer of a thermally stratified lake or reservoir. In this layer there is a rapid decrease in temperature with depth. Also referred to as Thermocline.

Metamorphic Rock — (Geology) A sedimentary or igneous rock that has been changed by pressure, heat, or chemical action. For example, limestone, a sedimentary rock, is converted to marble, a metamorphic rock.

Metamorphism — A change in the constitution of rock; specifically a pronounced change effected by pressure, heat, and water that results in a more compact and more highly crystalline condition.

Meteoric Water — Groundwater derived primarily from precipitation and the atmosphere.

Methemoglobinemia — A blood disorder that impairs the ability of the blood supply to carry oxygen throughout the body. Also known as “blue baby syndrome”, it is frequently caused by high concentrations of nitrate in drinking water supplies. It primarily affects infants less than 6 months of age. Most instances of the problem can be traced to babies drinking milk formula mixed in water with very high nitrate levels.

Methylmercury — An organic compound that has known neurological toxicity effects that tend to biomagnify up the food chain in aquatic environments.. Biomagnification is a biological process wherein a contaminant’s concentration increases at each level up the food chain, including humans. Thus, the availability of such contaminants, even in the seemingly insignificant parts per trillion range, often are ecologically important. Typically, methylmercury is formed by the action of certain bacteria on available supplies of inorganic mercury in stream-bottom sediments containing low concentrations of dissolved oxygen. However, the reverse process, or demethylation also is known to occur and this “detoxifying” of methylmercury is the subject of ongoing research.

Microbe — Short for Microorganism. Small organisms that can be seen only with the aid of a microscope. The term encompasses viruses, bacteria, yeast, molds, protozoa, and small algae; however, microbe is used most frequently to refer to bacteria. Microbes are important in the degradation and decomposition of organic materials added to the environment by natural and artificial mechanisms. Also referred to as Germs.

Microbial Growth — The activity and growth of microorganisms such as bacteria, algae, diatoms, plankton, and fungi.

Microbial Load — The total number of bacteria and fungi in a given quantity of water or soil or on the surface of food. The presence of the bacteria and fungi may not be related to the presence of disease-causing organisms.

Microbiological Anaerobic Degradation — The use of Microbes, either already present at a site or introduced for a specific treatment process, to degrade and render harmless hazardous wastes and toxic compounds in soil and water. Under such conditions, the microbes are used to break down organic compounds in contaminated soil and groundwater in an environment of little or no oxygen.

Microbiota — The plants, animals, and microorganisms that can only be seen with the aid of a microscope.

Microclimate —
(1) The local climate conditions, brought about by the modification of general climatic conditions by local differences in elevation and exposure. The detailed climate of a very small area of the earth’s surface.
(2) Also, the localized climate conditions within an urban area or neighborhood.

Microcosm — A laboratory model of a natural Ecosystem in which certain environmental variables can be manipulated to observe the response. The model test results are not always applicable to an actual ecosystem because the microcosm is, of necessity, a simplified collection of selected physical, chemical, and biological ecosystem components.

Micrograms per liter (µg/L) — A unit expressing the concentration of constituents in solution as weight (micrograms) of solute per unit volume (liter) of water; equivalent to one part per billion in most stream water and ground water. One thousand micrograms per liter equals 1 milligram per liter (mg/L).

Microscopic Particulate Analysis (MPA) — (Water Quality) A process used to assess water treatment plant performance. This form of analysis compares type, size, and quantities of Bioindicators, or microbiota (1–600 µm) in particles found in Raw Water to those found in the Finished Water. This method is particularly effective in evaluating filtration efficiencies, as log reduction, of conventional treatment systems, as well as for on-site evaluation of alternate filtration technologies.

Miner’s Inch [Western United States] — The rate of discharge through an orifice one inch square under a specific head. An old term used in the western United States, now seldom used except where irrigation or mining water rights are so specified. The equivalent flow in cubic feet per second is fixed by state statute. One miner’s inch is equivalent to 0.025 cubic foot per second (1.5 cubic feet per minute, equivalent to one-fortieth of a second-foot) in Arizona, California, Montana, Nevada, and Oregon; 0.020 cubic foot per second in Idaho, Kansas, Nebraska, New Mexico, North and South Dakota, and Utah; 0.026 cubic foot per second in Colorado; and 0.028 cubic foot per second in British Columbia.

Minimum Flow Appropriation — An appropriation designed to preserve a specified minimum flow in a stream. When the flow in the stream drops to that which is specified in the appropriation, junior appropriations will be required to stop diverting water in order to maintain the minimum flow. See (Prior) Appropriation Doctrine.

Minimum Instream Flow (Streamflow) — The specific amount of water required to support aquatic life, to minimize pollution, or for recreation. It is subject to the priority system and does not affect water rights established prior to its institution. Also referred to as Minimum Instream Flow.

Moisture Tension — The equivalent negative pressure in the soil water. It is equal to the equivalent pressure that must be applied to the soil water to bring it to hydraulic equilibrium, through a porous permeable wall or membrane, with a pool of water of the same composition.

Monitoring —
(1) Sampling and analysis of air, water, soil, wildlife, and other conditions, to determine the concentrations of contaminants. (2) (Ecology) The component of Adaptive Management in which information is collected to track system behavior and its response to management.

Monomictic — Lakes or reservoirs which are relatively deep, do not freeze over during the winter, and undergo a single stratification and mixing cycle during the year (usually in the fall).

Moor — An extensive area of waste ground in high, poorly drained country, overlaid with peat, and usually more or less wet. In popular usage, the word is restricted to the European moors, in which heather is often the prevailing plant, but similar phytogeographical areas occur elsewhere. Sphagnum moss is always characteristic of high moors, and especially in North America various insectivorous (insect feeding) plants flourish in them.

Moraine — An accumulation of boulders, stones, or other debris carried and deposited by a glacier. Moraines, which can be subdivided into many different types, are deposits of Glacial Till. Lateral Moraines are the ridges of till that mark the sides of the glacier’s path. Terminal Moraines are the material left behind by the farthest advance of the glacier’s toe. Each different period of glaciation leaves behind its own moraines. Also see Recessional Moraine.

Morphology —
(1) The science of the structure of organisms.
(2) The external structure form and arrangement of rocks in relation to the development of landforms. River morphology deals with the science of analyzing the structural make-up of rivers and streams. Geomorphology deals with the shape of the Earth’s surface.

Most Probable Number (MPN) — (Water Quality)
(1) A statistically determined number which represents the number of bacteria most likely present in a sample, based on test data. Widely used in the evaluation of waters from a bacterial standpoint.
(2) An index of the number of coliform bacteria that, more probably than any other number, would give the results shown by the laboratory examination. It is not an actual enumeration. MPN is determined from the distribution of gas-positive cultures among multiple inoculated tubes.

Mutagenic — Causing mutation, or the abrupt change in the genotype of an organism.