Environmental Engineering Reference
In-Depth Information
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Taxonomic richness
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Taxonomic composition
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Ta xonom ic diversit y
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Feeding groups
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Habits
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Pollution tolerance
The taxonomic richness refers to the number of distinct taxa within a sample, while the diversity
compares all taxa and the distribution of organisms among taxa. Healthy communities should be
both rich and diverse. Also, speciic taxa (the composition and pollution tolerance) serve as indica-
tors of aquatic health, both positive and negative. For example, the presence of pollution-tolerant
taxa and/or the absence of nontolerant species would represent a degraded condition. Changes in
the expected feeding groups or habitats from a reference condition would also represent an impact.
As illustrated by the RCC (Figure 7.2), certain functional feeding groups may be representative of
speciic habitats or stream orders. A change, such as from a periphyton-dominated to a planktonic-
dominated algae community, along with the associated changes in feeding groups, would represent
a negatively impacted stream segment perhaps following the loss of riparian vegetation or some
other cause. Also, a loss of organisms representative of a certain habitat could be an indication of
degradation, such as could result from burial due to excess sedimentation.
In addition to biological conditions, the WSA also considered the chemical, physical, and bio-
logical indicators of aquatic stress. The chemical indicators included:
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Total phosphorus
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Total nitrogen
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Salinity
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Acidiication
Excess nutrients can result in excessive algal growth, hypoxia, and a variety of other problems,
as discussed in Chapter 5. Freshwater organisms are generally intolerant of salinity, so high salinity
can cause stress, and acidiication can reduce or eliminate aquatic populations. The physical indica-
tors of aquatic stress included in the WSA (USEPA 2006) are:
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Relative stability of streambed sediments
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Instream habitat
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Riparian vegetative cover
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Riparian disturbance
The relative stability of streambed sediments is the ratio of the observed mean streambed par-
ticle diameter to a “critical diameter” representing the largest particle size that the stream can move
as bedload during storm lows, where the critical diameter is a function of the physical stream char-
acteristics (size, slope, etc.; Kaufmann et al. 1999). A low ratio would indicate that the streambed
is unstable, such as could occur as a result of excess sedimentation, an increase in lows, or other
causes. Similarly, changes in habitat such as burial by ine sediments, or changes in riparian habitat
could indicate aquatic stress.
The WSA (USEPA) also included an evaluation of the biological causes of aquatic stress. One of
the causes of stress results from invasive species, which will be discussed in the following section.
The result of the WSA (USEPA 2006) indicated that 45% of the wadeable streams (measured in
stream lengths) in the western ecoregions were in good condition, while only 8% of stream lengths
was in good condition in the eastern ecoregions. More than one-half of the stream lengths in eastern
ecoregions was in poor condition (Figure 7.12). The plains and lowland ecoregions were intermedi-
ate, with nearly 30% in good condition and 40% in poor condition (USEPA 2006).
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