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CHAPTER 5 3.1.13 – 3.1.14 3.1.13 PSI (Proportion of Sediment-sensitive

Invertebrates) and E-PSI (Empirically-weighted PSI)

PSI can be calculated for species (mixed taxonomic level)
The Proportion of Sediment-sensitive Invertebrates (PSI)
– is a proxy to describe the extent
(95)
or family data. Invertebrate taxa are assigned to one of
– Extence et al. (2011)
four groups indicating their adaptation to fine sediment
to which riverbeds are composed of, or covered by, fine
sediments. It is probably more accurately used as a measure
deposition (Table 5.7). PSI index values (scores) for each
taxon depend on their log 10 abundance category.
of impact of fine sediment, either natural or anthropogenic.
It is not intended to assess the ecological quality of the
sediment.
PSI is calculated as the sum of abundance-related
values (scores) for sensitive taxa as a percentage of the
abundance-related values for all taxa.
PSI – Proportion of Sediment-
sensitive Invertebrates Because the sensitivity ratings depend on abundance,
PSI should only be calculated for samples collected by the
standard RIVPAC methods described in Chapter 2.
This index is used as a measure of the impact of fine
sediments on invertebrate communities, either natural or
anthropogenically derived, for example, from soil erosion.

Table 5.7
Fine sediment sensitivity rating definitions and abundance-related values (scores) for PSI – from Extence et al. (2011). (95)

Fine sediment sensitivity rating
Group Abundance
(FSSR)

1–9 10–99 100–999 1000 +

A Highly sensitive 2 3 4 5

B Moderately sensitive 1 2 3 4

C Moderately insensitive 1 2 3 4
D Highly insensitive 2 3 4 5

PSI is currently used in the Environment Agency’s Table 5.8
Hydroecological Validation (See Section 3.2.5). Interpretation of PSI – from Extence et al. (2011). (95)

The empirically-weighted E-PSI is a relatively minor
refinement of PSI that can be used for the same purposes, PSI Riverbed condition
principally to assess the impacts of sediment pressure
on freshwater invertebrate communities. E-PSI can be 81–100 Minimally sedimented/un-sedimented
calculated for species (mixed taxonomic level), (Turley et al.
2015 (96) ) or family data (Turley et al. 2016 (97) ). 61–80 Slightly sedimented

The main difference between PSI and E-PSI is that in E-PSI 41–60 Moderately sedimented
weights are given to taxa belonging to the same FSSR (Fine
sediment sensitivity rating) (Table 5.7) that vary according to 21–40 Sedimented
empirical data, although the FSSRs are unchanged to retain 0–21 Heavily sedimented
the biological basis used in the original PSI. Taxa originally
identified as moderately to highly sensitive are assigned
a range between 0.5 and 1.0 whereas taxa identified as
moderately to highly insensitive are assigned a range
between 0.0 and 0.49. Sensitive species are assigned Like PSI, E-PSI should only be calculated from samples
higher weightings because they are considered to be more collected by the standard RIVPACS methods because
important for identifying sediment pressures. its sensitivity values depend on abundance.

234 | Freshwater Biology and Ecology Handbook
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