Environmental DNA-based detection has become an attractive method for determining biodiversity as it can provide more detailed information than traditional morphology-based approaches. Despite the availability of several guidelines, the workflow for marine environments has yet to be standardised. This complicates the comparison between samples across different monitoring programmes and projects. This project focused on the first step of the workflow – determining and standardising the minimum reliable sample volume needed to detect phytoplankton diversity. A test with four sample volumes (1000, 500, 200, 20 mL) was conducted with samples collected from the Baltic and North Seas and NW Atlantic Ocean. Across the studied samples, 200 mL was identified as a sufficient volume to detect various species. This represents an important first step towards standardising the eDNA-based approach.