Oxygen-Debt — A phenomenon that occurs in an orga- 

 nism when available oxygen is inadequate to supply 

 the respiratory demand. During such a period the 

 metabolic processes result in the accumulation of 

 breakdown products that are not oxidized until suf- 

 ficient oxygen becomes available. 



Parasite — An organism that lives on or in a host or- 

 ganism from which it obtains nourishment at the ex- 

 pense of the latter during all or part of its existence. 



Pelagic Zone — The free-water region of a sea. (Pelagic 

 refers to the sea, limnetic refers to bodies of fresh 

 water. ) 



Periphyton — The association of aquatic organisms at- 

 tached or clinging to stems and leaves of rooted plants 

 or other surfaces projecting above the bottom. 



Photosynthesis — The process by which simple sugars 

 and starches are produced from carbon dioxide and 

 water by living plant cells, with the aid of chlorophyll 

 and in the presence of light. 



Phototropism — Movement in response to a light gradi- 

 ent; for example, a movement towards light is positive 

 phototropism. 



Phytoplankton — Plant plankton that live unattached 

 in water. 



PisciciDE — Substances or a mixture of substances in- 

 tended to destroy or control fish populations. 



Plankton (Plankter) — Organisms of relatively small 

 size, mostly microscopic, that have either relatively 

 small powers of locomotion or that drift in that water 

 with waves, currents, and other water motion. 



Platyhelmenthes (^see Flatworms). 



PoiKiLOTHERMic ANIMALS {see Cold-Blooded Animals). 



Pool Zone — The deep-water area of a stream, where 

 the velocity of current is reduced. The reduced veloc- 

 ity provides a favorable habitat for plankton. Silt 

 and other loose materials that settle to the bottom of 

 this zone are favorable for burrowing forms of benthos. 



PORIFERA (see Sponges). 



PoTAMOLOGY — The study of the physical, chemical, geo- 

 logical, and biological aspects of rivers. 



Producers — Organisms, for example, plants, that syn- 

 thesize their own organic substance from inorganic 

 substances. 



Production (Productivity) — A time-rate unit of the total 

 amount or organism grown. 



Profundal Zone — The deep and bottom-water area 

 beyond the depth of effective light penetration. All of 

 the lake floor beneath the hypolimpion. 



Protozoa — Organism consisting either of a single cell 

 or of aggregates of cells, each of which performs all 

 the essential functions in life. They are mostly micro- 

 scopic in size and largely aquatic. 



Rapids Zone — The shallow-water area of a stream, where 

 velocity of current is great enough to keep the bottom 

 clear of silt and other loose materials, thus providing 

 a firm bottom. This zone is occupied largely by 

 specialized benthic or periphytic organisms that are 

 firmly attached to or cling to a firm substrate. 



Redd — A type of fish-spawning area associated with run- 

 ning water and clean gravel. Fish moving upstream 

 sequentially dig a pocket, deposit and fertilize eggs, 

 and then cover the spawn with gravel from the next 

 upstream pocket. Fishes that utilize this type of spawn- 

 ing area include some trouts, salmons, and minnows. 



Red Tide — A visible red-to-orange coloration of an area 

 of the sea caused by the presence of a bloom of certain 

 "armored" flagellates. 



Reducers — Organisms that digest food outside the cell 

 wall by means of enzymes secreted for this purpose. 

 Soluble food is then absorbed into the cell and re- 

 duced to a mineral condition. Examples are fungi, 

 bacteria, protozoa, and nonpigmented algae. 



Rheotropism — Movement in response to the stimulus 

 of a current gradient in water. 



Riffle — A section of a stream in which the water is 

 usually shallower and the current of greater velocity 

 than in the connecting pools; a riffle is smaller than a 

 rapid and shallower than a chute. 



Rotifers (Rotatoria) — Microscopic aquatic animals, pri- 

 marily free-living, fresh water forms that occur in a 

 variety of habitats. Approximately 75 percent of the 

 known species occur in the littoral zone of lakes and 

 ponds. The more dense populations are associated with 

 a substance of submerged aquatic vegetation. Most 

 forms ingest fine organic detritus for food, whereas 

 others are predaceous. 



Scavenger — An organism that feeds upon decomposing 

 organic matter. 



Scuds (Amphipods) — Macroscopic aquatic crustaceans 

 that are laterally compressed. Most are marine and 

 estuarine. Dense populations are associated with 

 aquatic vegetation. Great numbers are consumed by 

 fish. 



Secchi Disc — A device used to measure visibility depths 

 in water. The upper surface of a circular metal plate, 

 20 centimeters in diameter, is divided into four quad- 

 rants and so painted that two quadrants directly op- 

 posite each other are black and the intervening ones 

 white. When suspended to various depths of water 

 by means of a graduated line, its point of disappearance 

 indicates the limit of visibility. 



Seiche — A form of perodic current system, described as 

 a standing wave, in which some stratum of the water 

 in a basin oscillates about one or more nodes. 



Sessile Organisms — Organisms that sit directly on a 

 base without support, attached or merely resting unat- 

 tached on a substrate. 



Shellfish Poison (Mussel Poison) — ^A poison present in 

 shellfish that have fed upon certain small marine 

 phytoplankters in which the toxic principles exist. The 

 shellfish concentrates the poison without harmful ef- 

 fects to itself, but man is poisoned through consump- 

 tion of the toxic flesh. 



Species (Both Singular and Plural) — A natural popula- 

 tion or group of populations that transmit specific 

 characteristics from parent to offspring. They are re- 

 productively isolated from other populations with 

 which they might breed. Populations usually exhibit a 

 loss of fertility when hybridizing. 



Sphaerotilus — A slime-producing, nonmotile, sheathed, 

 filamentous, attached bacterium. Great masses are 

 often broken from their "holdfasts" by currents and 

 are carried floating downstream in gelatinous flocks. 



Sponges (Porifera) — One of the sessile animals that 

 fasten to piers, pilings, shells, rocks, etc. Most live 

 in the sea. 



Spore — ^The reproductive cell of a protozoan, fungus, 

 alga, or bryophyte. In bacteria, spores are specialized 

 resting cells. 



Spring Overturn — A physical phenomenon that may 

 take place in a body of water during the early spring. 

 The sequence of events leading to spring overturn in- 

 clude: (1) Melting of ice cover, (2) warming of sur- 

 face waters, (3) density change in surface waters pro- 



109 



