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3.1 River health indices: Indices in this report
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The South African Scoring System (SASS) for aquatic invertebrate fauna
A variety of invertebrate organisms (e.g. snails, crabs, worms, insect larvae, mussels, beetles) require specific habitat types and conditions for at least part of their life cycles. Changes in the structure of aquatic invertebrate communities are a sign of changes in overall river conditions. As most invertebrate species are fairly short-lived and remain in one area during their aquatic life phase, they are particularly good indicators of localised conditions in a river over the short term.
The SASS is based on the presence of families of aquatic invertebrate fauna and their perceived sensitivity to water quality changes. SASS has been tested and is used widely in South Africa as a biological index of water quality. SASS results are expressed both as an index score (SASS score) and the average score per recorded taxon (ASPT value).
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The Fish Assemblage Integrity Index (FAII)
Fish, being relatively long-lived and mobile, are good indicators of
long-term influences on a river reach and the general habitat
conditions within the reach. The numbers of species of fish that
occur in a specific reach, as well as factors such as different
size classes and the health of fish, can be used as indicators
of river health.
This index categorises fish populations according to an
intolerance rating which takes into account trophic
preference and specialisation, requirement for flowing water
during different life-stages, and association with habitats
with unmodified water quality. Results of the FAII are
expressed as a ratio of observed conditions versus conditions
that would have been expected in the absence of human
impacts. Although this index has been applied and published,
it is being further developed and refined under the leadership
of Dr Neels Kleynhans of the Institute for Water Quality Studies.
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The Riparian Vegetation Index (RVI)
Healthy riparian zones maintain channel form and
serve as filters for light, nutrients and sediment.
Changes in the structure and function of riparian
vegetation commonly result from changes in the
flow regime of a river, exploitation for firewood, or
use of the riparian zone for grazing or ploughing.
The RVI determines the status of riparian vegetation
within river reaches based on the qualitative
assessment of a number of criteria in the riparian
zone. These criteria are vegetation removal,
cultivation, construction, inundation, erosion,
sedimentation and alien vegetation. The output is
expressed as percentage deviation from natural or
unmodified riparian conditions.
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Index of Habitat Integrity (IHI)
Habitat availability and diversity are major determinants of aquatic community structure. Adverse changes in biological communities may be attributed either to deterioration in water quality or to habitat degradation, or to both. Loss of habitat is regarded as the single most important factor that has contributed towards the extinction of species in the last century. The destruction of a particular type of habitat will result in the disappearance of certain species. Examples of river habitat types are pools, rapids, sandbanks, stones on the riverbed, and vegetation fringing the water’s edges. The availability and diversity of habitat are major determinants of whether a given system is acceptable to a specific suite of biota or not. Therefore knowledge of the availability and quality of habitats is very important in an overall assessment of ecosystem health.
The IHI has been developed to assess the impact of major disturbances on river reaches. These disturbances include water abstraction, flow regulation, and bed and channel modification. This index accounts for both the condition of the riparian zone and the in-stream habitats.
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