Glossary - Freshwater Ecology: Concepts and Environmental Applications - page 501

Vicariant event, an atmospheric or geophysical
event resulting in disturbance or fragmenta-
tion of a previously constant distribution
and thus leading to allopatric speciation and
radiation
Viscosity, the resistance of a fluid to change, an
internal friction; two types of viscosity can
be considered—the dynamic viscosity (in-
trinsic property of the fluid) and the viscous
force (a property of scale)
Volcanic, related to action of volcanoes
Voltinism, relating to the number of genera-
tions produced annually by an organism
(e.g., bivoltine )
VPOM, very fine ( ultrafine ) particulate organic
matter (0.45-53

channel into one place and allow for mea-
surement of discharge
Well casing, a pipe used to keep a well from
collapsing
Wetland, areas inundated or saturated by sur-
face or groundwater at a frequency and du-
ration sufficient to support a prevalence of
vegetation adapted for life in saturated soil
conditions
Wet meadow, a meadow without open water
but saturated
Wet prairie, a shallow soil wetland with grass
Whitefish, a fish species that migrates to elude
severe conditions or, alternately, a type of
North American coreogonid fish
Whitewater, streams that carry large quantities
of suspended solids and appear muddy or
silty or large quantities of dissolved inor-
ganic solids and are slightly alkaline or cir-
cumneutral; also sections of rivers or
streams with extensive rapids and entrained
air causing the water to appear white
Wind, a natural movement of air of any velocity
Winterbourne, upper reach of a chalk stream
that channels water in fall after dry summer
periods when aquifer levels decrease
Withdrawal, taking something out (abstraction)
Yield, the amount of fish that are taken per
unit time from a fishery
z, depth; vertical distance from surface
z

m diameter)
Warm monomixis, a period of total circulation
during the cold time of year in a lake with-
out ice; temperature stratification occurs in
summer
Warm thereimixis, summer mixing or cold
monomixis with water warmer than 4°C
Water abstraction, removal of water to be used
for drinking water, irrigation, industry, etc.
from rivers, lakes, aquifers, and other
sources
Watershed, area above a point in a stream that
catches the water that flows down to that
point (in Europe, called catchment, and the
watershed is the high point that divides
catchments)
Water table, the top of the groundwater or sat-
urated zone
Water velocity, speed of water in any small re-
gion or channel
Water yield, the amount of water (depth) per
unit time from a specific area
Weathering, dissolution of materials from rocks
Weir, a device to concentrate all the fluid in a

, mean depth of a body of water; V/A
z m , maximum depth of a body of water
Z SD , the maximum depth of Secchi disc visibil-
ity; usually expressed in meters
Zoophyte, an organism growing on an animal
Zooplankton, suspended invertebrates, gener-
ally multicellular and generally smaller than
1 mm