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may vary both within the reference sites, and
between the reference sites and the monitoring site.
This can result in pronounced differences in the
habitat types within the reference sites, increasing
perceived natural variation, and thus reducing the
sensitivity of the index.
assessment. The index had a scoring system based
on the number of rare species found compared with
the number of alien species found, and therefore
was a measure of (native) species diversity.
The MTR system was applied in Portugal,
though it functioned satisfactorily only in highly
oligotrophic and highly eutrophic rivers. This
was thought to result from the low number of
scoring species found (80 out of a potential 120)
(Szoszkiewicz
et al
., 2006). The Indice de Macr ofitos
(IM) (Suarez
et al
., 2005) is another plant scoring
system developed for macrophytes in Spain. This
index had a low correlation when regressed against
log orthophosphate concentrations (r
2
Non-causal relationships
Demars and Edwards (2009) criticized macrophyte
monitoring methods that assign scores to
species since the approach assumes that all
the macrophytes at a specific site are limited by,
or are responding to, the pressure gradient along
which they are scored, which is unlikely to be
true (Barendregt and Bio, 2003). Although the
averaging of scores may produce good correlations
across a Member State, on a site by site basis
the score may be more associated with natural
variation than with any pressure. For example, a
small patch of light created by a fallen tree may
permit vascular plants to grow in a natural river
section that would normally have only bryophytes.
These small-scale habitat changes are difficult
to predict even when there is a sophisticated
modelling process in place, such as Discriminant
Function Analysis (Wright
et al
., 1998), to identify
site-specific reference conditions.
The probable reason why so many macrophyte
metrics for monitoring use species scores along
a pressure gradient (e.g. nutrient enrichment) is
that natural variation caused by other factors
(such as small-scale physical habitat variation)
can be reduced by the sampling or modelling
approach employed, thus dampening, though not
eliminating the influence of natural background
variation.
=
0.165).
Dodkins
et al
. (in press) used the method of Lavoie
et al
. (2006) that had been applied to diatoms
in Canada. Macrophyte species were scored based
on their position along the main gradient in
species change as represented by the first axis
in a Detrended Correspondence Analysis (DCA).
The advantage of this method is that the scores
measure the main changes in the community
between sites, and not only nutrient enrichment.
The rank correlation with a nitrate gradient within
the whole of Portugal was high (r
2
=
0.602);
however, the index failed to show a significant
separation between reference and impaired sites
within southern river types.
The BEnthic Assessment of SedimenT (BEAST)
method was originally used to measure species
similarity between reference conditions and an
impaired site using invertebrates (Reynoldson
et al
., 1995). Aguiar
et al
. (2011) applied the
same approach to macrophytes in Portugal. The
Spearman's rank correlation of the index with
nutrient gradients was less than 0.35 (equivalent
to a rank r
2
of 0.12). The BEAST approach
is not expected to correlate well with a single
human pressure, since the index responds to
multiple pressures, but this was a much lower
correlation than found with the DCA metric of
Dodkins
et al
. (in press), and the BEAST approach
also included terrestrial species. A RIVPACS-type
approach (Wright
et al
., 1984) attempted by
Aguiar
et al
. (2011) using macrophytes instead of
invertebrates was also compared to the macrophyte
BEAST
Previous macrophyte indices in
Iberia
Several different types of indices have been used
within the Mediterranean region. The Indice
de Conserva¸ ao Macrofitico (Ferreira, 1994) was
the earliest used in Portugal, although it was
developed to assess the conservation potential of
different reaches rather than for ecological quality
method,
but
it
also
performed
poorly
