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Daily Estuary: An estuary in which tidal movements cause substantial changes in salinity at any one location on a daily basis.

Decomposer: Heterotrophic microorganisms (usually bacteria and fungi) that break down nonliving organic matter and release nutrients, which are then available for reuse by autotrophs.

Deep Layer: The layer extending from the lowest part of the thermo cline to the bottom.

Deep Percolation: The movement of water below the maximum effective plant root zone.

Deep Scattering Layer (DSL): Well-defined horizon in the ocean that reflects sonar; indicating a layer of organisms, usually consisting of fishes, squid, or other larger zooplankton.

Defense Mechanism: A physical part of or a process in a plant or animal that helps protect it against attack or injury.

Deforestation: The removal of trees and vegetation from land; the most common cause of soil erosion.

Deionization: A process for filtering tap water before it is added to the aquarium. It comes with either separate or mixed-bed resins. The mixed-bed resins are disposable when they are exhausted, whereas separate resins can be recharged, though that requires working with caustic chemicals.

Deionizer: A filtration device used to purify tap water before it is introduced into the aquarium. They are normally composed of many chemical and mechanical filtration media.

Demersal Fish: Fish living near and on the bottom.

Demographic: Referring to numerical characteristics of a population (e.g., population size, age structure).

Denitrification: Breakdown of nitrates by anaerobic bacteria into other forms.

Denitrifying Bacteria: In the process of nitrification of wastewater, the two key bacteria of ecological importance are nitrosomonas and nitrobacteria. These bacteria facilitate "catalyze" the reactions. Nitrosomonas results in the removal of three pairs of electrons from ammonia facilitating the formation of nitrite and nitrobacteria removes to electrons from nitrite to form nitrate. The bacteria responsible for denitrification are autotrophic and heterotrophic facultative anaerobes. Monitoring for denitrifying bacteria is typically done to monitor the performance of denitrification systems.

Density: (seawater) Grams of sea water per milliliter of fluid.

Density-Dependent Factors: Factors, such as resource availability, that vary with population density.

Dependent Relationship: A type of relationship in which one thing needs or relies on another for its continued existence.

Deposit Feeder: An organism that derives its nutrition by consuming some fraction of a soft sediment.

Detritus: Dead and decomposing animal or plant material that collects on the bottom of fish tanks. It is often noticeable as a layer of oily stuff or gunk that builds up in mechanical filter systems or under gravel filters.

Detrivores: Animals that eat detritus. Common detrivores are urchins, stars, hermits, etc.

Diaphragm Pump: The most common type of air pump. A great variety of brands and styles are available which produce different amounts of air, with differing amounts of noise.

Diatom: A microscopic unicellular alga (Protists) possessing an external skeleton of silica, consisting of two interlocking valves. They appear like a golden powder coating the tank to the naked eye.

Diatom Filter: This purely mechanical filter uses diatomaceous earth to remove very fine particles from the water. They clog quickly and are only used occasionally as water polishers rather than continuously. See diatomaceous earth.

Diatomaceous Earth: A filter media made up of skeletons of Diatoms, used in Diatom filters, able to filter particles down to microns in size.

Dichotomous: Dividing into two equal branches.

Diffusion: A process where heat or chemicals are transported in response to differences in chemical concentration or temperature. Movement is from high concentration (or temperature) to low concentration (or temperature). This process could involve liquids, gases or solids.

Digestion Efficiency: The fraction of living food that does not survive passage through a predator's gut.

Dinoflagellate: Dominant planktonic algal form, occurring as a single cell, and is often biflagellate; possesses characteristics of both plants and animals.

Dioecious: Having male or female flowers on separate plants.

Directional Selection: Preferential change in a population, favoring the increase in frequency of one allele over another.

Discharge Area: An area where groundwater moves toward or is delivered to the soil surface. Groundwater can flow into springs, or seeps; contribute base flow to streams; or provide supplemental water for plant use.

Discus: A group of fish belonging to the family Cichlidae and originate from South America. They are somewhat delicate, large round flat fish, often with blue or red marbled coloring.

Disease: Literally the lack of ease; a condition of the body which presents particular symptoms and sets the condition apart as abnormal. Disease can be acute (rapid onset) or chronic (slow onset and lasting for a long time), and caused by hereditary factors, trauma or injury, infectious agents (bacteria, viruses, fungi), chemicals (from within the body or contaminants from outside the body) and parasites.

Dispersion: The process whereby a chemical, contained in water, deviates from the path that would be expected due to bulk flow. In the process the chemical is mixed with surrounding liquids, causing its concentration to be reduced.

Dissolved Organic Matter: Dissolved molecules derived from degradation of dead organisms or excretion of molecules synthesized by organisms.

Dissolved Oxygen: Oxygen that is available in water.

Dissolved Substance: A material that has been mixed in liquid to form a solution.

Distal: Near or toward the free end of a structure; opposite of basal.

Distillation: A two-stage water treatment method: 1) the liquid is boiled, producing water vapor; 2) the water vapor is condensed, leaving most contaminants behind. Distillation can be used to remove inorganic chemicals, some non-volatile organic chemicals, and bacteria.

Disturbance: A rapid change in an environment that greatly alters a previously persistent biological community.

Diverse: Of different kinds, types, or species.

Diversity: The state of having different kinds, types or species; the state of being diverse. Diversity is a parameter describing, in combination, the species richness and evenness of a collection of species. Diversity is often used as a synonym for species richness.

Diversity Gradient: A regular change in diversity correlated with a geographic space or gradient of some environmental factor.

Division: Division of the vegetative point of Rosette plants into two or more parts for propagation.

DIY: An abbreviation for Do-It-Yourself, which usually refers to projects that you can do at home for cheaper than purchasing the commercial version.

DKH: Abbreviation for Degrees of Carbonate Hardness.

DLS (double-layered spiral): A material that is made by rolling up a polyester pad and plastic wire mesh. It is used in both biological and mechanical filters.

Dog Salmon: Nickname for the Chum Salmon. See also Chum Salmon.

Dolomite: A limestone gravel with a small pH buffering ability.

Dormant Period: Interruption of growth in an effort to adjust to seasonal periods of stress.

Dorsal Fin: Single fin mounted on top of the fish. located between the head and the tail. Some species have two, one behind the other.

Dorsal: On or relating to the back surface.

Dorsal Surface: The top part of the fish.

Dosing Pump: A pump that serves to maintain a specific water level in an aquarium. They are used to add a constant supply of additives or trace elements, much like a hospital IV. The most common type is a peristaltic pump.

Double-Layered Spiral (DLS): A material made by rolling up a polyester pad and plastic wire mesh. It is used in both biological and mechanical filters.

Drainage: The process of transporting surface water over a land area to a river, lake or ocean (surface drainage), or removal of water from a soil using buried pipelines that are regularly spaced and perforated (subsurface drainage).

Drawdown: A lowering of the groundwater surface caused by withdrawal or pumping of water from a well. It is the difference between the static water level and the pumping water level in a well pumped at a constant flow rate.
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