Benthic communities: multivariate analysis
For diatoms, we applied the NMDS to community species abundances.
Results are shown in Figure 6, where the 2020 samples outline an inner
ellipsoid nested in a wider ellipsoid including the 2005 data. This
distinction is confirmed by the PERMANOVA test which confirms a
significant difference between the 2005 and the 2020 assemblages (F
1,22=2.692, p<0.05). Moreover, the analysis of multivariate
homogeneity of group dispersions highlighted significant differences
between the two sampling years (F1.22 = 8.953; p = 0.007). The 2005
communities showed a higher heterogeneity among sites in the taxonomic
composition than the 2020 communities, as proved by a higher (and
statistically significant) distance from the centroid in 2005, while
2020 sites appear more homogenous and therefore graphically closer to
each other.
Fig6
The same analysis applied to macroinvertebrate communities (in this
case, on presence-absence data) did not detect a significant difference
between the two sampling years, with only a slightly higher distance
from the centroid in the 2005 assemblages (graphic not shown).
Figures 7 and 8 report the results concerning LCBD and SCBD applied to
diatom and macroinvertebrate data respectively. The two years did not
show a significant difference in terms of LCBD (Figure 7a and Figure
8a). For this reason, Figures 7b,7c, 8b and 8c report the single LCBD
and SCBD calculated for the most recent campaign.
Fig 7
Fig 8
A. minutissimum and A. lineare got the highest SCBD score
in 2020 (0.08 for both species). The other species contributing most to
ß-diversity were C. excisiformis, Achnanthidium thiennemanii, E.
silesiacum and Fragilaria pectinalis .
Concerning macroinvertebrates, S. montana among others got the
highest SCBD score, followed by several other taxa attaining similar
scores (e.g. Blephariceridae, Elmidae, Nemoura sp., Limoniidae).
Chironomidae got by far the lowest score.
Regarding the contribution of sites to local diversity, sites displaying
the highest LCBD in 2020 based on diatom community composition were
Leviona, Roc and Gran Nomenon; Roc Entrelor, Savara and Campiglia
according to macroinvertebrate data. Averaging the LCBD values obtained
on the two communities, the sites that contribute most to local
diversity are Roc, Entrelor and Leviona.