4.4 Influence of water temperature and discharge on species and
OUT richness
We neither detected an influence of temperature on overall species nor
on OTU richness, thus indicating that other factors like sampling time
or season had a greater effect on eDNA community composition. In
contrast, we found a negative effect of increasing discharge on
hololimnic taxa, Annelida, and Ephemeroptera species richness. For
Ephemeroptera with a p-value close to 0.05 this should be interpreted as
a trend and future studies are needed to verify the effect of discharge.
For hololimnic species, where Annelida were the most species-rich group,
a possible explanation could be that the possibility of many Annelida
specimens being swept away with high discharge is low as most annelids
are embedded within the sediment and are potentially moving even deeper
into the sediment with high discharge. This could lead to a higher
dilution effect and therefore lower annelid DNA concentrations as
specimens are not washed away.
In the summer months, an extremely low species and OTU richness was
detected for all groups with August 2018 being the month with the lowest
richness. The summer 2018 was outstandingly warm with low rainfall and
high sun exposure. The exceptionally warm temperatures probably affected
abundance and growth rates of the more sensitive species and therefore
causing a decrease in detection rates but also increased DNAse activity
or microbial break DNA down leading to faster DNA degradation rates
could be an influencing factor.
Conclusion Our data show how eDNA-based time-series data on a stream
macroinvertebrate community can support ecological research and
biomonitoring of rivers, as we detected seasonal differences and small
temporal shifts in community composition and key functional feeding
groups. Based on our results it is difficult to predict eDNA detection
probability of different sampling positions in general, as we only
sampled three positions in one location of one stream for 15 months. As
a clear advantage of eDNA metabrcoding, our study detected many species
of often morphologically neglected taxa such as annelids and chironomids
that are often underrepresented in other studies. By including such taxa
in the analysis, eDNA metabarcoding could function as a complementary
tool for long-term monitoring when assessing differences in
macroinvertebrate community composition on a highly resolved temporal
scale. This could help to disentangle variations in community
composition due to recent and extreme changes from gradual changes, as
seen with our “seasonal clock” data which showed gradual shifts in
temporal beta-diversity, and help deriving guidelines for future
eDNA-based monitoring of rivers and streams.