Environmental Engineering Reference
In-Depth Information
Groundwaters can be oxic or anoxic depending on the relative supply
of organic C to heterotrophs, the scale that is considered, the residence
time of the water in the aquifer, and the dissolved O
2
concentration in the
incoming water. Many pristine groundwater systems are oxygenated,
whereas human activities lead to anoxia. However, row-crop agriculture
actually may decrease organic C input into aquifers leading to higher O
2
concentrations (Fig. 11.15A). Septic systems, feedlots, spills of organic
chemicals, and subsurface disposal of sewage effluent can lead to anoxic
groundwaters (Madsen and Ghiorse, 1993). If groundwaters become
anoxic, the biogeochemical cycling is altered and many species of inverte-
brates and microbes cannot exist in the aquifer. Thus, biodiversity of
groundwaters can be controlled in part by anoxia.
Additional small-scale anoxic habitats exist that are important in a va-
riety of aquatic habitats. Organic materials may have anoxic zones associ-
ated with them. For example, decaying
leaves may have anoxic zones at their sur-
face and inside, even though they are in com-
pletely oxygenated waters (Fig. 11.15B). The
digestive systems of many animals are
anoxic, and a distinct microflora forms in
these locations. The role of animal
digestive tracts in biogeochemical cycling
has not been well researched.
In lighted sediments, the algal commu-
nity that is present can cause great varia-
tions in O
2
concentration with time. Upon
darkening, respiration rapidly consumes O
2
;
there is a measurable decrease within 1 s
(Fig. 11.13C). Organisms living in such a
habitat must be adapted to rapid changes in
O
2
concentration.
Sidebar 11.2.
Fish Kills Result from Anoxia in
Streams and Lakes
When organic carbon is high and exchange
with the atmosphere is low, habitats can tem-
porarily become anoxic and fish die. This oc-
curs when total respiratory demand exceeds
input of photosynthetic and atmospheric O
2
.
High temperatures often exacerbate the prob-
lem because the metabolic rate of heterotrophs
is greater and O
2
solubility is lower in warmer
water (Cooper and Washburn, 1949).
Fish kills can occur during the summer in
eutrophic lakes. Kills occur in hypereutrophic
lakes when a highly productive system experi-
ences a series of calm, cloudy days. Under
these conditions, algal blooms have high total
respiration rates and little input of atmospheric
O
2
occurs, leading to anoxia and fish kills. Such
summer kills can be common in many areas
and similar kills can occur year-round in the
tropics.
Fish kills also occur in lakes in the winter
when an ice cover prevents O
2
transport into
the water. If snow covers the ice, the light
transmission and photosynthetic O
2
production
are low. However, a eutrophic lake contains a
significant amount of biomass so respiration
continues and O
2
concentrations decrease.
Fish kills occur in such situations unless the
fish can find an inlet stream or O
2
is bubbled
into the lake.
SUMMARY
1. Materials in water can be dissolved,
colloidal, or particulate (gravitoidal).
2. Conductivity (total dissolved ions) and
pH are chemical properties of water
that are important descriptive
parameters used by aquatic ecologists
because they can control the
distribution and activity of organisms.
3. Redox potential is another important
parameter that controls chemical and
biochemical processes in aquatic
ecosystems. The way chemicals are
transformed in the environment is
determined partially by the redox
potential of the environment. Redox
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