Although represented by only a low species diversity, marine bacteria are the most abundant plankton group in all marine climates. Therefore it might be assumed that there is no need to further filtrate ballast water samples for the analysis of the bacterial load. Nevertheless these samples have to be filtered to eliminate plankton organisms larger than the bacteria, which might negatively impact on the analytical methods for bacterial detection.
There is hardly any other plankton organism, which is smaller than 2µm. The size of most of the marine microalgae (Phytoplankton) range from 16µm down to about 4µm and there are only very few zooplankton organisms that are smaller than 4µm.
In this respect any ballast water sample needs to be filtered through a 2µm size pore filter (membrane filtration) to make sure any other plankton organism, which could negatively impact on the chemical analysis, is eliminated from the sample.
After the 2µm filtration step the sample still has to be further processed with respect to the analytical method, that is applied to the sample.
For the classical incubation method the filtrate after the 2µm filtration step is ready to be applied to the respective agar plates.
In case a different analytical method will be applied, e.g. detection of the ATP concentration, the filtrate after the 2µm filtration step has to pass another filtration step through a pore filter (membrane) or glass fiber filter with an approximate pore size of 0,7µm. The bacteria will be retained on the filter material and directly enter the analytical cycle then.