2. Re-analysis of the time-series changes in eDNA particle size
distribution
Our meta-analyses provided new insights into the relationship between
eDNA persistence and its state. We then re-analysed the dataset from a
previous tank experiment (Jo et al., 2019b) to estimate mitochondrial
and nuclear eDNA decay rates at multiple size fractions and water
temperature levels. The results of the re-analysis appeared to be
generally consistent with those of the meta-analyses; as indicated by
the meta-analyses, eDNA persistence depended on the interactions between
its size fraction, type of the target gene, and water temperature (Table
3; Figure 4). In particular, a significant interaction between filter
pore size and temperature indicated that inflow of the degraded,
larger-sized eDNA into smaller size fractions could buffer the effect of
temperature on eDNA degradation in these smaller size fractions, as
described in previous sections. The dependence of eDNA degradation on
water temperature would likely be smaller when targeting smaller-sized
eDNA or using a smaller pore size filter.
Some recent studies attempted to estimate species biomass and abundance
by integrating quantitative eDNA analysis and hydrodynamic modelling,
allowing the consideration of eDNA dynamics, such as its production,
transport, and degradation (Carraro et al., 2018; Tillotson et al.,
2018; Fukaya et al., 2020). For a more accurate estimation,
environmental parameters affecting these eDNA dynamics may be included
in the statistical modelling framework. The effect of temperature on
eDNA degradation can be minimized during statistical modelling by
considering eDNA particles at smaller fraction sizes, which will allow
simplification of the modelling procedure while retaining its accuracy
and reliability. However, considering the apparent suppression of eDNA
degradation in smaller size fractions, owing to the inflow of the
degraded larger-sized eDNA, it is possible that such smaller-sized eDNA
yield ‘older and less fresh’ biological signals than the larger-sized
eDNA. Such non-fresh eDNA signals can result in false-positives during
eDNA detection (Yamamoto et al, 2016; Jo et al., 2017), in which case
the use of eDNA particles in the smaller size fractions would be
disadvantageous for eDNA-based biomass or abundance estimation. The
applicability of smaller-sized eDNA for such estimations can be verified
by comparing the correlation between eDNA quantification and species
biomass and abundance, and the availability of longer eDNA fragments
among the filter pore sizes or eDNA particle sizes, for which
meta-analyses such as the present study may be suitable.