Temperature (°C) –
thermal energy of the water.
Temperature
affects a number of biological processes and is a common
source of stress to organisms if it does not fall within a
range typical for organisms. Water temperature also affects
the ability of solutions to hold dissolved gases – gases
are more soluble in colder water than in warmer water. In
aquatic environments, this relationship is particularly
important with respect to dissolved oxygen (see below).
Obviously, water temperature varies seasonally with air
temperature, but it can also vary in response to shading
(or lack of shading) by terrestrial vegetation. In
addition, human effluent from power plants requiring
cooling water is frequently warmer than receiving water
bodies and can cause temperature to increase in streams,
rivers, and lakes.
Specific
conductance (μS/cm)
– ability of water to conduct an electrical current
standardized to a particular temperature (“specific” to
25°C); units are the inverse of resistance (microSiemens)
over a 1 cm path length.
Conductivity
is affected by dissolved substances in the water,
particularly salts. Conductivity values naturally vary
considerably depending on regional geology, proximity to
oceans, and rainfall. High amounts of dissolved salts in
freshwater can be stressful to aquatic life and indicate
possible contamination from a variety of human sources.
Examples include runoff from roads (particularly after
winter deicing), effluent from certain industries and
sewage treatment plants, mine drainage, and fertilized
cropland or lawns. In general, distilled water has a
conductivity of 14 μS/cm, and seawater has a conductivity
of 40,000 μS/cm. Fresh surface water typically has
conductivity in the range of 40-1000
μS/cm.
Depth
(m) –
water depth at the instrument based on pressure of water
column above sensors; instruments are suspended in
mid-water column.
Water
depth at these sensors is actually controlled, so this
measurement is a bit arbitrary. For river stage, which is
related to depth (although not a direct measure), check the
real-time river gage data maintained by the USGS
(http://waterdata.usgs.gov/pa/nwis/rt). Depth is generally
more important in non-flowing water
bodies
pH
–
activity of dissolved hydrogen ions in exponents of 10 (a
change of 1 pH unit represents a 10-fold difference in H+
ion activity), generally considered an indication of
acidity.
pH
controls a wide variety of physiological processes, many of
which are accustomed to a “neutral” pH (near 7). pH of
surface water is typically between 6 and 9, but pH could be
higher or lower depending on sources of acids or bases. pH
can fluctuate in response to changing concentrations of
carbon dioxide in water. For example, during daytime,
photosynthesis consumes carbon dioxide produced by
respiration and results in higher pH, but pH decreases at
night when respiration continues but photosynthesis stops.
Oxidation-reduction
potential (mV) –
the tendency of a solution to move electrons by accepting
them from or donating them to ionic chemicals in the
solution. Also called redox potential or reduction
potential.
Oxidation
of a substance occurs when it loses electrons, so a
substance in a solution with high ORP will likely be
oxidized. Reduction of a substance occurs when it gains
electrons, so a substance in a solution with low ORP will
likely be in reduced form.
ORP of a solution affects the state of many substances in
the water. For example, iron will rapidly oxidize to rust
in solutions with high ORP. In addition, many biological
processes are controlled by the redox potential of the
environment where the organisms live. ORP is affected by
the amount of dissolved oxygen in the water, along with the
concentrations of other dissolved solids (especially
metals).
Chlorophyll
(μg/L) –
the concentration of chlorophyll in the water column at the
sensor.
Chlorophyll
is the most abundant pigment responsible for photosynthesis
by aquatic plants. Its abundance in the water column
indicates productivity by algae. Algae in the water column
could be phytoplankton in the river, phytoplankton reaching
the river from lakes or reservoirs, or algae that had been
attached to rocks.
Turbidity
(NTU) –
the cloudiness of water caused by suspended particles, such
as clay, silt, algae, or organic particles, in
nephelometric turbidity units (NTU).
Turbidity
can clog gills of fish and aquatic invertebrates, interfere
with feeding mechanisms of filtering organisms, reduce prey
detection by fish and other visual predators, modify
substrate to affect benthic organisms, and reduce light
availability to algae. Particles that cause turbidity can
be generated from a number of sources, such as mining,
erosion of stream banks, runoff from agriculture or
developed land, and organic particles. Algal productivity
in the water column also contributes to
turbidity.
Dissolved
oxygen ODO
(mg/L) – concentration of dissolved oxygen in the water
column. ODOSat (%) – Dissolved oxygen concentration as a
percentage of the amount that could be dissolved in the
water at a certain temperature (see above) and atmospheric
pressure.
Dissolved
oxygen is perhaps the single most important property in
aquatic environments to organisms. Oxygen is required for
survival of all aerobic organisms (pretty much every
organism other than some bacteria). Dissolved oxygen is
related to temperature (see above), atmospheric gas
exchange (related to water velocity, depth, and
turbulence), and biological “metabolism” (i.e. balance of
photosynthesis and respiration by all organisms in the
ecosystem). Oxygen depletion below levels required to
support many organisms can occur if amounts of biological
respiration exceed replenishment from the atmosphere or
photosynthesis, particularly in warm water where oxygen
concentrations are already lower than in cold
water.
Interesting
relationships among parameters:
Chlorophyll, turbidity, dissolved oxygen
Dissolved oxygen, specific conductance, oxidation-reduction
potential
More
Information
Links
to other websites with useful descriptions of water quality
parameters and their measurement
http://ga.water.usgs.gov/edu/characteristics.html