Environmental DNA Transport at an Offshore Mesophotic Bank in the Northwestern Gulf of Mexico

Accurately constraining the transport of environmental DNA (eDNA) after it is shed from an animal is vital to appropriately geolocate species detections and biodiversity measurements from eDNA sequencing data. Modeling studies predict the horizontal transport of eDNA at concentrations detectable using quantitative PCR over scales of tens of kilometers, but more limited vertical transport. Field studies routinely find that eDNA metabarcoding data distinguishes biological communities at small spatial scales, over scales of tens of meters. Here, we leverage the unique bathymetry of an offshore, mesophotic bank and the benthic invertebrate community that it supports to determine the extent to which vertical and horizontal eDNA transport may affect the interpretation of species detections from eDNA metabarcoding data. We found that in a stratified water column, eDNA from benthic invertebrates was vertically constrained to depths close to the seafloor in the thermocline versus in the surface mixed layer above. However, when using primers that are taxonomically specific to corals, we found evidence for the horizontal transport of coral eDNA at distances at least ~1.5 km from where they can be reasonably expected to occur. On the contrary, there was minimal evidence for horizontal transport of benthic eDNA in data generated using primers that broadly targeted sequences from eukaryotes. These results highlight the importance of horizontal transport as well as considering methodological details, like the taxonomic specificity of PCR primers, when interpreting eDNA sequencing data.