Benthic communities: comparison among ecological indices
Concerning diatoms, the comparison between the two sampling campaigns is
reported in Figure 4. From 2005 to 2020 we observed a slight increase in
species richness and diversity, despite not being statistically relevant
(Figure 4 a,b,c). Diatom indices also (Fig 4 d,e,f,g) depict a
substantial unchanging situation, except for the Trophic Index TI, which
is significantly higher in 2020 (Wilcoxon paired test V= 11.5,
p<0.05). An increase in TI corresponds to a decrease in the
environmental quality as this index increases along a gradient of human
impact. The worsening in TI concerns, in particular, Valnontey, Orco,
Noaschetta and Nampio.
Fig4
In both years, diatom communities were mainly composed of alcalobiontic
and alcalophilous taxa, adapted to mean conductivity and very high
oxygen saturation levels (99.5%). Concerning tolerance to organic
matter and nutrients, both diatom communities were dominated by
oligosaprobous taxa with a small proportion of mesosaprobous ones, which
slightly increased in 2020; oligotrophic (more than 90%) and
oligo-mesotrophic taxa. In terms of ecological guilds, diatom
assemblages of 2005 were in mean dominated by low-profile species
(76.5%), which are favored in nutrient-poor and high-disturbance
habitats; 13.4% of the taxa belonged to the high-profile guild,
developing in nutrient-rich sites, more lentic flows and low disturbance
conditions; while only 1.9% was represented by motile species, mainly
epipelic taxa favored by high nutrient content and able to move within
periphyton and river bed toward suitable habitats. In comparison,
samples from 2020 were characterized by a higher proportion of
low-profile taxa (81.1%), which corresponded to a decrease in
high-profile diatoms (10.3%). Motile species also increased, reaching
3%.
In both campaigns, the genus Achnanthidium is by far the dominant
one in all the sites, with the three species Achnanthidium
lineare , Achnanthidium pyrenaicum and Achnanthidium
minutissimum , very common in the Alpine
rivers. Achnanthidium species are low profile, resistant to
current velocity thanks to physiological adaptation mechanisms which
allow them to tightly adhere to the substrate. Some other interesting
species reached consistent relative abundances, at least in some sites.
This is the case of Fragilaria arcus , a rheophilic species, very
sensitive to pollution and typical of low mineralized freshwaters, found
in high percentages in Forzo and Orco in 2005, well distributed but with
lower abundances in the 2020 campaign. Achnanthidium
thienemannii was found in Campiglia in 2020 with a 10.7% relative
abundance. Epithemia goeppertiana, a large motile species,
usually epiphytic, was exclusively found in Nampio in 2020.
Cymbella excisiformis , widely spread in headwater is the third species
in Leviona in 2020, while Encyonema silesiacum, a high-profile
species typical of siliceous rocks, was abundant in Roc.
It is worth noting that many of the species sampled in the GPNP are
included in the Red List of diatom species (Hofmann et al., 2018) as
endangered at different levels. In this sense, the sites sheltering the
highest number of endangered species were in 2005 Noaschetta and in 2020
Leviona and Vaudaletta. On average, 2005 samples hosted a lower
abundance (39.5%) of individuals belonging to endangered species than
2020 ones (50.2%).
For macroinvertebrates we computed seven metrics, none of which showed a
significant change between the two sampling years (Wilcoxon test
p>0.05; boxplots in Figure 5). On average,
macroinvertebrate community composition is characterized in both
sampling campaigns by a relatively small number of well-adapted,
orophilic and rheophilic taxa, with a mean number of taxa equal to 16, a
minimum value of 9 in Savara (2005) and a maximum of 23 in Campiglia
(2020). In all the sampling sites and for both years the stenoecious
taxa account for at least 50% of the total taxa (average 64%, minimum
47% in Campiglia 2020 and maximum 87.5% in Gran Nomenon 2005). Indeed,
these communities are mainly composed of orophilic and stenoxybiont
organisms strictly adapted to cold and harsh environments, such as
highly sensitive Diptera (namely Hapalotrix lugubris )
Ephemeroptera Heptageniidae (in particular, Epeorus alpicola ) and
Plecoptera Systellognatha, such
as Dictyogenus alpinus , Perlodes
intricatus and Siphonoperla montana .
No noticeable differences can be evidenced by comparing the two
campaigns. For example, the occurrence of stenoecious Plecoptera remains
constant, as in the case of Perlidae (58.3 % of stations on both
occasions), Chloroperlidae (41.7 %) and Perlodidae (66.7 %in 2005,
58.3 in 2020) were found. The same pattern of occurrence is reported for
Blephariceriidae, found in 75% of samples in 2005 and 83.3 % of
samples in 2020.
We applied the Spearman correlation test to verify the association
between HQA and biotic indices. HQA resulted as significantly and
positively correlated with the Shannon diversity index of diatoms and
macroinvertebrates (p<0.05) and with two macroinvertebrate
indices: EPT and IBE (p<0.05 as well).