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).