Currents matter
This study provides indirect evidence of how marine eDNA suffers
susceptibility to currents disturbance: Madeira samples’ stations 1
(MmoM+01) and 2 (MmoM+02) are only 112 meters apart from each other, and
yet, in the first one (at the entrance of the cave frequented by
>8 seals) the molecular signal was the loudest of our eDNA
sample set, while in the latter no signal at all was found over 27
reactions (3 filters, 3 replicas, 3 markers). So, apparently along those
112 meters there is (or there was at the time of sampling) an invisible
barrier that prevents the two water masses from merging (Figure S6).
This also implicitly means that around station 1 there is a sort of
pocket or relatively unstirred water, where dated and recent monk seal
biological traces may accumulate overtime. This fortuitous circumstance
makes station 1 the ideal scenario where testing the differential
ability of three markers to perceive the temporal scale of eDNA
traces. The discrepancy in monk seal eDNA content between the two
closely adjacent sampling stations if, on one hand, highlights relevance
of currents in swiping away eDNA signals, on the other hand, it adds a
reassuring element on the fact that when a signal is indeed intercepted,
this probably reflects the presence of the animal in a limited
spatio/temporal range (as signals that disperse with currents become
shortly extremely diluted and therefore remain undetected).