Environmental Engineering Reference
In-Depth Information
Ta b l e 3 . 1 2 Diatom Index and River Water Quality. a
Diatom Index ( I d )
River Water Quality
>
4.5
No pollution, the best biological quality.
4.0-4.5
Very slight pollution, near normal communities.
3.0-4.0
Toxic pollution at moderate level or nutrient enrichment (eutrophication).
Community changes apparent and sensitive species disappearing.
2.0-3.0
Heavy pollution. Only pollution-resistant species abundant or dominant. Sensitive
species severely reduced.
1.0-2.0
Severe pollution. Only a few tolerant species survive. Very reduced diversity.
Source : Descy, 1979.
a Water quality considered in relation to toxic pollution (e.g. heavy metal content) or nutrient enrichment (eutrophication).
This very simple index is assessing environmental
impact on a single species. It assumes that site 1 is
the typical one for that river and that changes in the
numbers of the selected species will occur at other
sites (e.g. a decline with downstream pollution).
Palmer's index (Palmer, 1969) This, applied to
river diatoms, is derived from an extensive literature
survey and is based on the occurrence of 20 common
diatom species, listing them according to their toler-
ance to organic pollution. The sequence, from least
polluted to most polluted waters, includes
Multiple taxon indices
Fragilaria capucina
Achnanthes minutissima
Most diatom indices are based on multiple taxa (gen-
era or species). They are determined either in terms
of the presence/absence of key indicator species (e.g.
Palmer's index) or are based on the weighted average
equation of Zelinka and Marvan (1961):
Cocconeis placentula
Diatoma vulgare
Surirella ovata
Gomphonema parvulum
Synedra ulna
Nitzschia palea.
As with the more general version of Palmer's index
(Section 3.2.3, Table 3.5), diatom species are rated
on a scale 1-5 (intolerant to tolerant) and the index is
calculated by summing up the scores of all relevant
taxa present within the sample. Values
n
j =1
a j s j v j
Index =
(3.2)
n
j =1
a j v j
>
20 indicate
high levels of organic pollution.
where a j is the relative abundance (proportion) of
species j in the sample, s j the pollution sensitivity of
species j , v j the indicator value of species j and n the
number of species counted in the sample.
Different indices adapt this equation in different
ways, and the performance of a particular index
depends on which taxa are used, the number of taxa
and the values given for the constants s and v for each
taxon. The values of individual indices based on this
equation vary from 1 to a maximum value equal to the
highest value of s . Commonly used indices include
the following.
Descy's index (Descy, 1979) A frequently used
index. Values of s j range from 1 to 5 and v j from 1
to 3 (Equation 3.2) giving index values from 1 to 5.
These can then be related to water quality (Table 3.12;
Descy, 1979).
Example. The example of Descy index ( I d ) calcu-
lation shown in Table 3.13 is taken from the River
Semois in Belgium (Round, 1993). The part of the
river investigated was heavily polluted with domes-
tic sewage and contained the key diatom species N.
accomoda , Nitzschia palea , Achnanthes lanceolata ,
 
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