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abyssal zone

The ocean between about 4,000 and 5,000 meters (13,000 and 16,500 feet) deep.

acid rain

Rain containing acids and acid-forming compounds such as sulfur dioxide and oxides of nitrogen.

active transport

The movement of molecules from a region of low concentration to a region of high concentration through a semipermeable membrane at the expense of energy.


An inheritable structural or behavioral modification. A favorable adaptation gives a species an advantage in survival and reproduction. An unfavorable adaptation lessens a species' ability to survive and reproduce.


The kingdom to which multicellular heterotrophs belong.

aphotic zone

The dark ocean below the depth to which light can penetrate.

artificial system of classification

A method of classifying an object based on attributes other than its reason for existence, its ancestry, or its origin. Compare natural system of classification.


An organism that makes its own food by photosynthesis or chemosynthesis.

bathyal zone

The ocean between about 200 and 4,000 meters (700 and 13,000 feet) deep.

benthic zone

The zone of the ocean bottom. See also pelagic zone.

biogeochemical cycle

Natural processes that recycle nutrients in various chemical forms from the nonliving environment to living organisms and then back to the nonliving environment.

biological factor

A biologically generated aspect of the environment, such as predation or metabolic waste products, that affects living organisms. Biological factors usually operate in association with purely physical factors such as light and temperature.


The mass of living material in a given area or volume of habitat.

carbon cycle

The movement of carbon from reservoirs (sediment, rock, ocean) through the atmosphere (as carbon dioxide), through food webs, and back to the reservoirs.


The synthesis of organic compounds from inorganic compounds using energy stored in inorganic substances such as sulfur, ammonia, and hydrogen. Energy is released when these substances are oxidized by certain organisms.


A pigment responsible for trapping sunlight and transferring its energy to electrons, thus initiating photosynthesis.


A way of grouping objects according to some stated criteria.


The phylum of animals to which corals, jellyfish, and sea anemones belong.


A heterotrophic organism.

convergent evolution

The evolution of similar characteristics in organisms of different ancestry; the body shape of a porpoise and a shark, for instance.

denitrifying bacteria

Bacteria capable of converting nitrite or nitrate to gaseous nitrogen.


The movement—driven by heat—of molecules from a region of high concentration to a region of low concentration.

disphotic zone

The lower part of the photic zone, where there is insufficient light for photosynthesis.

dissolved organic carbon (DOC)

Organic (carbon-containing) molecules dissolved in water.

dissolved organic nitrogen (DON)

Nitrogen-containing organic molecules dissolved in seawater. Most DON is in the form of protein.

divergent evolution

Evolutionary radiation of different species from a common ancestor.


An organism incapable of generating and maintaining steady internal temperature from metabolic heat and therefore whose internal body temperature is approximately the same as that of the surrounding environment; a cold-blooded organism.


An organism capable of generating and regulating metabolic heat to maintain a steady internal temperature. Birds and mammals are the only animals capable of true endothermy. A warm-blooded organism.


The capacity to do work.


A measure of the disorder in a system.

epipelagic zone

The lighted, or photic, zone in the ocean.

euphotic zone

The upper layer of the photic zone in which net photosynthetic gain occurs. Compare photic zone.


Change; the maintenance of life under constantly changing conditions by continuous adaptation of successive generations of a species to its environment.


An organism capable of tolerating extreme environmental conditions, especially temperature or pH level.


General term for organic molecules capable of providing energy to heterotrophs when combined with oxygen during biochemical respiration.

food web

A group of organisms associated by a complex set of feeding relationships in which the flow of food energy can be followed from primary producers through consumers.

hadal zone

The deepest zone of the ocean, below a depth of 5,000 meters (16,500 feet).


An organism that derives nourishment from other organisms because it is unable to synthesize its own food molecules.


Grouping of objects by degrees of complexity, grade, or class. A hierarchical system of nomenclature is based on distinctions within groups and between groups.

hydrostatic pressure

The constant pressure of water around a submerged organism.


Referring to a solution having a higher concentration of dissolved substances than the solution that surrounds it.


Referring to a solution having a lower concentration of dissolved substances than the solution that surrounds it.


Referring to a solution having the same concentration of dissolved substances as the solution that surrounds it.


The largest category of biological classification. Five kingdoms are presently recognized.

limiting factor

A physical or biological environmental factor whose absence or presence in an inappropriate amount limits the normal actions of an organism.

Linnaeus, Carolus

Carl Linnaeus (1707-1778). Swedish "father" of modern taxonomy.

littoral zone

The band of coast alternately covered and uncovered by tidal action; the intertidal zone.

mass extinction

A catastrophic, global event in which major groups of species perish abruptly.


A complex structure of proteins and lipids that forms boundaries around and within the cell. It is usually semipermeable, allowing some kinds of molecules to pass through but not others.

metabolic rate

The rate at which energy-releasing reactions proceed within an organism.


A heritable change in an organism's genes.

natural selection

A mechanism of evolution that results in the continuation of only those forms of life best adapted to survive and reproduce in their environment.

natural system of classification

A method of classifying an organism based on its ancestry or origin.

neritic zone

The zone of open water near shore, over the continental shelf.

nitrifying bacteria

Bacteria capable of fixing gaseous nitrogen into nitrite, nitrate, or ammonium ions.

nitrogen cycle

The cycle in which nitrogen moves from its largest reservoir (the atmosphere) through the ocean, ocean sediments, and food webs, and then back to the atmosphere.


Any needed substance that an organism obtains from its environment except oxygen, carbon dioxide, and water.

oceanic zone

The zone of open water away from shore, past the continental shelf.


The diffusion of water from a region of high water concentration to a region of lower water concentration through a semipermeable membrane.

pelagic zone

The realm of open water. See also benthic zone.

photic zone

The thin film of lighted water at the top of the world ocean. The photic zone rarely extends deeper than 200 meters (660 feet). Compare euphotic zone.


The smallest unit of light energy.


The process by which autotrophs bind light energy into the chemical bonds of food with the aid of chlorophyll and other substances. The process uses carbon dioxide and water as raw materials and yields glucose and oxygen.

physical factor

An aspect of the physical environment that affects living organisms, such as light, salinity, or temperature.


The kingdom to which multicellular vascular autotrophs belong.


An organism consumed by a predator.

primary consumer

Initial consumer of primary producers. The consumers of autotrophs; the second level in food webs.

primary producer

An organism capable of using energy from light or energy-rich chemicals in the environment to produce energy-rich organic compounds; an autotroph.

primary productivity

The synthesis of organic materials from inorganic substances by photosynthesis or chemosynthesis; expressed in grams of carbon bound into carbohydrate per unit area per unit time (gC/m2/yr).

scientific name

The genus and species name of an organism.

second law of thermodynamics

Disorder (entropy) in a closed system must increase over time. If disorder decreases, it does so at the expense of energy. Because the universe as a whole may be considered a closed system, it follows that an increase in order in one part must result in a decrease in order in another.

secondary consumer

Consumer of primary consumers.


The formation of new species. Charles Darwin suggested that this is accomplished through isolation and natural selection.


Any group of actually or potentially interbreeding organisms reproductively isolated from all other groups and capable of producing fertile offspring. (Note: The word species is both singular and plural.)

species diversity

Number of different species in a given area.

sublittoral zone

The ocean floor near shore. The inner sublittoral extends from the littoral (intertidal) zone to the depth at which wind waves have no influence; the outer sublittoral extends to the edge of the continental shelf.

supralittoral zone

The splash zone above the highest high tide; not technically part of the ocean bottom.

surface-to-volume ratio

A physical constraint on the size of cells. As a cell's linear dimensions grow, its surface area does not increase at the same rate as its volume. As the surface-to-volume ratio decreases, each square unit of outer membrane must serve an increasing interior volume.


In biology, the laws and principles covering the classification of organisms.

top consumer

An organism at the apex of a trophic pyramid, usually a carnivore.

trophic level

A feeding step within a trophic pyramid.

trophic pyramid

A model of feeding relationships among organisms. Primary producers form the base of the pyramid; consumers eating one another form the higher levels, with the top consumer at the apex.


Division or province of the ocean with homogeneous characteristics.

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