Lakes and Watercourses

Benthic fauna
Introduction
Benthic fauna is an important element in the biological diversity of lakes
and watercourses. These organisms are also an important food resource
for fish and play an important part in breaking down organic matter.
Methods of classifying and assessing lakes and watercourses based on
benthic fauna are in general use.
The model criteria given here cover a selection of parameters for
various types of habitat and various quality aspects. Diversity (here
calculated as Shannon-Wiener’s diversity index) is high if there are many
species and several of them are dominant, and low if there are few
species, one or more being very dominant. The ASPT index is a ”clean
water index”, mainly indicating the presence of sensitive groups (high
values) or tolerant ones (low values). The impact on fauna of eutrophication
and organic pollutants is assessed using the Danish fauna index
(watercourses and the littoral zone of lakes), the BQI index and the O/C
index (profundal zone in lakes). The Danish fauna index is used to
ascertain whether animals belonging to various key groups with varying
tolerance are present in the samples. Depending on the groups occurring,
samples are classified according to a scale from high (sensitive species) to
low values (tolerant species). The BQI index is a quality index based on
chironomid species. High values indicate that species preferring clean
water and well-oxygenated water dominate; low values indicate the
presence of tolerant species. The O/C index expresses the relationship
between the number of oligochaete worms and chironomid midge larvae
and is a further measure of oxygen conditions and the degree of organic
load on lake beds. A high index (ratio) shows a predominance of oligochaete
worms, ie, relatively low oxygen levels and/or high organic load.
Finally, acidification is reflected by the acidity index, which is based on
the occurrence of species possessing differing degrees of pH tolerance.
High values indicate predominantly species sensitive to acidification. The
methods of calculating the various indices are shown in Appendix 2.
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