4.3 Influence of season on the distribution of feeding types
For two of the functionl feeding groups (FFG), we observed an influence
of sampling time on the relative proportion of FFG, partly confirming
our third hypothesis. Shredders showed a large increase from summer to
winter and a decrease afterwards. As shredders feed on allochthonous
material such as decomposed leaf litter, the increase of the relative
proportion of shredders in winter is likely due to an increase in their
food after fallen leaves accumulate in the stream (Cummins, 1974;
Cummins et al., 1989). Although we expected to detect a higher number of
grazers in the warmer months, we did not detect an effect of sampling
day on the relative proportion of grazers. As the number of detected
species was in general extremely low in the summer samples, the lack of
seasonal effect on the proportion of grazers detected is likely the
result of the extremely dry and warm summer in 2018 reducing the
abundance of temperature sensitive taxa. Since we only looked at
presence/absence data, it is possible that abundance data would have
shown higher proportions of grazers in summer. Proportion of parasites
was highest in summer and their occurrence was likely linked to their
hosts. The exclusively parasitc larvae of Sisyra terminalis ,
which was frequently found in the spring and summer but not winter
samples, is known to parasite on sponges and the species Spongilla
lacustris and Eunapius fragilis , which were also present in our
dataset, are known to form colonies during warm seasons and produce
gemmules over the cold seasons (Gugel, 2001; Weißmair, 1994).
Additionally, also the Trichoptera Ceraclea nigronervosa , which
feeds on Spongillidae (Graf et al., 2002), was detected in the summer
and spring month but not in the winter months, further encouraging the
influence of the abundance of sponges on the detection of their
hosts/predators. These results are promising for future eDNA based
biomonitoring, as we could detect shifts in the relative proportion of
different FFG and therefore changes in functional composition of the
macroinvertebrate community over time, which could help to determine
changes in ecosystem functioning (Minshall et al., 1992).