Geoscience Reference
In-Depth Information
0.7
2
R = 0.990
0.6
diatom-
macrophyte
combined
metric
0.5
diatom
metric
0.4
0.3
macrophyte
metric
0.2
0.1
0
0
50
100
150
200
250
300
350
400
450
number of species in metric
Figure 11.1
The number of species in the combined diatom-macrophyte metric plotted against the r
2
-value when the
metric (with this number of species) was regressed against the impact (combined nitrate and BOD). The fitted
regression curve is a second order polynomial. The locations of the diatom only metric (281 species) and macrophyte
only metric (105 species) are also shown. The line is not continued below 25 species since at this point many sites had
no scoring species.
biological site scores, calculated as a weighted
average of the species scores, had a high correlation
(r
2
numbers of species in an index that uses species
scoring can increase the correlation with a pressure
gradient, although indices should still be designed
such that there is evidence of a causal relationship
between the species and the pressures.
There are several benefits of combining
macrophytes and diatoms. First, there are more
macrophyte species in high discharge rivers than
in low discharge rivers, whereas the converse is
true for diatoms (Figure 11.2). The same pattern
was also found in high and low nutrient rivers in a
study by Camargo and Jim enez (2007). Also, many
macrophytes are not easily detected outside the
late spring and summer growth period, whereas
diatoms can be monitored throughout the year.
Thus, the number of species that will be found at a
particular site is more stable.
The response of diatoms over much shorter
time periods (Dixit
et al
., 1992) than macrophytes
=
0.620) with the pressure gradient in the
whole of Portugal. The index performed better in
the northern region (r
2
=
0.711) than the southern
region (r
2
=
0.346). When the index values were
calculated separately for diatoms and macrophytes,
diatoms had a higher correlation with the nutrient
pressure gradient (r
2
=
0.510) than macrophytes
(0.381).
The correlation of the combined diatom-
macrophyte index against the pressure gradient
was also assessed with consecutive and random
removal of species from the index. Figure 11.1
shows that the performance of the metric had
a clear relationship with the numbers of species
in the index (r
2
=
0.990). Also, per species, the
separate diatom index performed worse than the
separate macrophyte index. Thus, increasing the
