Difficulties in obtaining representative samples for compliance with the Ballast Water Management Convention

► Difficulties in obtaining representative samples for ballast water compliance. ► Tetraselmis suecica was added to a storage tank at a known density and sampled at discharge. ► Holding period, cell density and sampling interval affected discharge cell density. ► Most samples underestimated the density of cells present. ► Samples from a tank containing 100 cells ml−1 showed <10 cells ml−1 at discharge.