Discussion
In freshwater environments, biodiversity loss is dangerously menaced by
several anthropogenic pressures acting on streams and river ecosystems
both at local and global scales. This study is the first to examine
possible temporal changes in hydro-morphology and aquatic biodiversity
in an alpine protected area. Our study demonstrated that, overall, the
hydro-morphology of the examined stretches within the Gran Paradiso area
did not significantly vary over the last 15 years, as demonstrated by
the indices derived from the RHS. HQA and HMS values indicated the
presence of some modifications in the bank structure and the
re-sectioning of the river course, and the strong lotic character which
in some cases reduces the habitat diversification. The slight decrease
in HQA or increase in HMS recorded in a few sites is mainly due to a
decrease in the river flow and channel heterogeneity, as in the case of
Leviona, or to bank reinforcements, as in Forzo, Roc, and Nampio. In
some cases, HMS improved thanks to the removal of artificial structures
such as small bridges or weirs. The stronger loticity highlighted by LRD
Descriptor in 2020 confirms the highest percentage of erosive habitats
detected during the last survey. Indeed, only 28% out of 194 monitored
spot-checks showed erosive bank features in 2005 (such as local erosion,
eroding cliff, or eroding bank), while 66% in 2020.
Water quality highlighted a small increase in the nitrate concentration
in 2020 with a mean value of 0.760 mg l-1 that still corresponds to a
first quality class according to the Italian legislation (DM 260/2010)
and that is in line with nitrate concentrations in streams originating
from ablating glacier (Brighenti et al., 2019). This increase in
nitrates could be related to the ice melting as glaciers accumulated
several atmospheric delivered chemical compounds and nitrate is easily
eluted from melting glacier ice and snowpack (Saros et al., 2010).
Indeed, the climatic data, although limited as temporal series, seem to
indicate a trend towards an increase in maximum temperature: in recent
years the summer temperature reach values close to 26 °C, and also in
the winter months the values are always higher than in the 1999-2005
period. Also, the annual trend of the average monthly rainfall shows
clearly visible differences, with higher values almost always in the
years preceding the 2005 samplings. These data are in line with what has
been highlighted by studies on climatic trends in the Alps, which are
identified as an area particularly affected by climate change (Doretto
et al., 2018; Gobiet et al., 2014).
Overall, our study shows a picture of few changes in local
hydro-morphological impacts, accompanied by slight variations in water
quality in a period affected by visible consequences of climate change,
How the stream benthic communities, and especially diatoms, respond to
these changes is evidenced by the multivariate analysis of the community
composition rather than by the traditional indices. The NMDS ordination
technique performed on diatom community composition showed that the 2020
samples are nested in a wider ellipsoid including the 2005 data and
resulted in significantly less heterogeneous. The species turnover
between the two years mainly concerns co-generic species, mostly within
the genera Navicula and Nitzschia. A. lineare ,
which showed a positive trend from 2005 to 2020, and A.
minutissimum have the highest SCBD scores: the two species however have
different ecological preferences: A. lineare is more sensitive to
anthropic pressures, while A. minutissimum, despite being very
common in mountain streams, has a wide tolerance range towards both
water pollution and physical alterations (Brighenti et al., 2019). It is
important to notice that in 2005 species classified as “early alert”
in the diatom Red List represented on average 6.6% of the total
abundance, while only 3.1% in 2020. Of the 11 taxa belonging to this
category, F. arcus showed the most significant decrease from a
mean abundance of 5.8% in 2005 to 2.2% in 2020. Another species
deserving attention is Psammothidium subatomoides, recorded in 8
samples out of 12 in 2005 and disappeared in 2020 inventories. The
results obtained in this research confirm the need to preserve habitat
conditions favoring the recovery of these species. When looking at
species not yet classified in the Red List because of insufficient data,
we focused on those species following a decreasing trend over the two
sampling periods. Among them, Fragilaria delicatissima , detected
in 2005 was not present in any of the 2020 inventories
while Encyonopsis minuta frequency showed a decrease. Being both
very sensitive species, particular attention to their temporal and
spatial distribution pattern should be paid. We found a general positive
trend between the increasing habitat quality (HQA), diatom species
richness and Shannon diversity, confirming that habitat heterogeneity is
one of the main drivers for taxa richness (Bona et al. 2008). At the
same time, this trend was negative (although not significant)
considering habitat modification score, which means that poor
communities are typical of the most impacted sites. Moreover, according
to Ossyssek, Hofmann, Geist, & Raeder (2022), we found a good
correlation between total diatom richness and endangered species.
Among the biomonitoring indices, a significant worsening of the
ecological status was detected only by the TI index which on average
indicated oligotrophy in 2005 and mesotrophy in 2020. The IPS remains in
the class of oligotrophy and the ICM indicates a high ecological quality
for both years. TI is the most sensitive to nutrient levels of natural
origin (Rott et al., 1999) and it is reputed for its suitability in
alpine rivers.
Macroinvertebrate communities showed no significant variations between
the two campaigns. Orophilic and stenoxybiont organisms, strictly
adapted to cold and harsh environments, characterized these communities
on both occasions. It is interesting to notice that, regardless of the
sampling year, macroinvertebrate biomonitoring indices showed a greater
correlation with hydro-morphological alteration than those based on
diatoms.
These differences in the response to anthropogenic stress confirm what
has been reported by previous studies which compared the two indicators
(Bona et al., 2008; Mykrä et al., 2012; Piano et al., 2019): diatoms
respond to water quality and habitat features at different scales
respect to macroinvertebrates, discouraging the use of one group as a
substitute for the other. Their different responses are undoubtedly
linked to bio-ecological factors, such as their different trophic role,
and the difference in size and length of the life cycle. Metrics related
to diversity are certainly influenced by the taxonomic level and the
qualitative or quantitative nature of the data collected as well.
Indeed, indices currently used for macroinvertebrates require the genus
as the finest taxonomic level and data collected in 2005 were expressed
as presence/absence. For diatoms, the metrics used are based on the
taxonomic level of species and their relative abundance and this could
have contributed to their more complex response highlighted by the
multivariate analysis.
Our study highlights the role of a large, protected area in preserving
freshwater habitats and biological communities from human impacts. The
ß- diversity partitioning identified sites to prioritize in protection
efforts, confirming its role in conservation planning (Ruhı´, Datry, &
Sabo, 2017).
Moreover, this research demonstrates that the creation of databases
relating to the ecological quality of rivers is crucial for tracking the
environmental evolution of lotic systems, creating historical series of
considerable management interest, and supporting decision-making
strategies also in rural or mountainous areas. In this context, the
presence of a natural park is essential for its role as a promoter of
research and data collection.
Indeed, the conservation of aquatic ecosystems is one of the main
objectives of protected areas and requires harmonization actions among
protected areas managers and stakeholders, who should be driven toward
biodiversity protection. In protected areas, water use for human
purposes (such as recreational or energetical) is generally minimal, and
catchment integrity is mostly maintained, so as spatial and temporal
river connectivity and habitat heterogeneity. All these features
significantly contribute to the maintenance of a high functional
richness of biological communities, as demonstrated by recent research
(Falasco, Piano, Doretto, Fenoglio, & Bona, 2018; Falasco, Doretto,
Fenoglio, Piano, & Bona 2020; Falasco, Bona, Risso, & Piano, 2021;
B-Béres, Stenger-Kovács, Török, & Török-Krasznai, 2021). It has been
recognized that high functional richness increases community resilience
against extreme events, which is expected to significantly increase
under a global climate change scenario (Mason, Mouillot, Lee, & Wilson,
2005; Doretto et al., 2020; B-Béres et al., 2021). However, not all
protected areas can play this role with the same effectiveness with
rivers. In fact, the GPNP covers the entire headwater and therefore
there are no impacts located upstream.
In conclusion, this study once again underlines the importance of
maintaining and, if necessary, enlarging the alpine protected areas
facing an era of rapid climate change (Finlayson & Pittock, 2018), as
they play a fundamental role in the preservation of the water resource
and the ecosystems that guarantee its quality.