Herrera, S.; Govindarajan, A. F.; Andruszkiewicz Allan, E.; Francolini, R.; Frates, E.; McCartin, L.; Pittoors, N. C.; Sengthep, M.; Stover, S.; Vohsen, S.; Yang, N.

Environmental DNA (eDNA) surveys are increasingly used to assess marine biodiversity, but standard low-volume protocols inherited from coastal work may be inadequate for the deep sea. We compiled 841 eDNA samples from eight oceanographic expeditions across the North Atlantic, Wider Caribbean, and Pacific (surface to 4000 m) to quantify how recoverable eDNA scales with depth and surface productivity, and to derive depth- and productivity-aware sampling targets. Total eDNA concentration declined predictably with depth as a power law, with attenuation exponents (b) systematically modulated by surface productivity: most gradual in eutrophic waters (b = 0.67), intermediate in mesotrophic (b = 0.90), and steepest in oligotrophic systems (b = 1.25); volume-weighted models explained 66-88% of the variance. Required filtration volumes scaled steeply with depth and diverged by orders of magnitude across productivity regimes, with the greatest demands in deep oligotrophic systems. Among adjustable laboratory parameters, the assay-specific extract-concentration target exerted greater leverage on required volume than extraction efficiency or elution volume within realistic ranges. Conventional Niskin bottles, therefore, systematically undersample biodiversity in many deep-sea ecosystems, particularly mid- to low-productivity ones. Volume-aware sampling paired with optimized post-collection recovery should be a routine part of deep-sea eDNA surveys.