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Dissolved oxygen
All aquatic animals need dissolved oxygen, so it is vital to the health of aquatic ecosystems.
Waterways with consistently high dissolved oxygen levels are most likely healthy and stable
environments, and are capable of supporting a diversity of aquatic organisms. Oxygen enters
water through photosynthesis of aquatic plants and from the atmosphere. Stream riffles and
waves put more water in contact with the atmosphere and thereby increase the amount of
oxygen in water. Dissolved oxygen percent saturation is an important measurement of water
quality. Cold water can hold more dissolved oxygen than warm water.
Natural and human-induced changes to the aquatic environment can affect levels of dissolved
oxygen. Increased water temperature, due to stormwater runoff from hot pavement or
removal of streamside trees that shade water, can reduce the amount of dissolved oxygen the
water can hold. High levels of bacteria in water can deplete available dissolved oxygen. Also,
algal "blooms" can lead to decreased dissolved oxygen levels. This is because as the algae
decomposes, oxygen is used up by bacteria breaking it down. Algal "blooms" are caused by
excess nutrients in water, as from fertilizer runoff or animal waste.
pH
pH is a measurement of the acidic or basic (alkaline) quality of water. The pH scale ranges
from a value of 0 (very acidic) to 14 (very basic), with 7 being neutral. The pH of natural
water is usually between 6.5 and 8.2. Most aquatic organisms are adapted to a specific pH
level and may be harmed or even die if the pH of the water changes even slightly. The
reproductive success of brook trout, for example, which live in many of Virginia's streams, will
be inhibited if the pH falls below 5. The pH of water can be affected by acidic precipitation
("acid rain"), industrial waste, agricultural runoff or draining from improperly run mining
operations.
Water temperature
Temperature affects the level of dissolved oxygen in water, the rate of photosynthesis by
aquatic plants and the sensitivity of organisms to toxic wastes, parasites and disease. Thermal
pollution---the discharge of heated water from industrial operations or stormwater runoff from
hot pavement---can cause temperature changes that threaten the balance of aquatic systems.
Water temperature can also be affected when trees and other vegetation that shade streams
are removed.
Turbidity
Turbidity is the measure of the clarity of water. Turbid water is caused by suspended and
colloidal matter including clay, silt, organic and inorganic matter and microscopic organisms.
Turbid water may be the result of soil erosion, urban runoff, algal "blooms," and bottom
sediment disturbances that can be caused by boat traffic and abundant bottom-feeding fish.
(Turbidity should not be confused with color, since darkly colored water can still be clear and
not turbid.) Water that is turbid for long periods of time can be harmful to aquatic ecosystems.
Suspended particles can clog gills of fish and other animals. Animals like aquatic mussels that
filter feed can be harmed if large quantities of suspended material impair their filter-feeding
system. Suspended matter also cuts down on the amount of light that can penetrate into
water, so it can reduce photosynthesis in submerged aquatic plants. Also, when suspended
matter like silt settles to stream bottoms, it covers feeding and spawning areas for aquatic
animals, and also fills in spaces between rocks where aquatic animals live, thereby degrading
stream habitats.