Positive effect of filtration additives for increasing environmental DNA detections in summer and winter oceanic waters

Abstract Environmental DNA (eDNA) is a convenient biodiversity monitoring tool, and many studies have focused on river and coastal areas, while deep‐water oceanic surveys are rare. However, using eDNA techniques to learn about oceanic communities may also be a useful approach. We tested methods to enhance ocean eDNA collections adding four different materials (sediment, diatomite, zirconia bead, and molecular sieve) in combination with 0.45‐μm pore size filter, also compared 0.22‐μm pore size filter, using five 2‐L volume replicates. The experiments were conducted off the Pacific coast of Hokkaido, Japan, in summer and winter. The winter trial also included a larger water volume (10 L, duplicate) experimental treatment. The added diatomite showed the highest number of detected operational taxonomic units (OTU) in the summer trial, while sediment and beads showed higher numbers in winter. GLM analysis showed that pore size does not affect the number of OTUs, while season and the other four additive materials were significant effects. The added materials in the water likely trapped more eDNA and prevented it from passing through the filters. The 10‐L filtered sample had more OTUs than combining five replicates of 2‐L filtered samples. Comparison among the same trial revealed that DNA composition in the water is variable, and detected species differed even in the same water sample. Species richness and diversity were higher in winter than in summer, and species compositions differed between seasons. The greater numbers of species detected using filtration additives suggest these methods could be useful for eDNA ocean biodiversity surveys.