It is a principle of marine planktology that the abundance of plankton organisms increases with decreasing size of the individuals.
The smaller IMO organism class >10µm<50µm may be sampled by the same technique and procedures, i.e. filtration through a nylon mesh. However, the abundance of this size class is considerably higher than the abundance of the larger size class >50µm and thus in most cases the concentration of the sample by filtration is not necessary.
Subsequently the ballast water sample is taken with an adequate volume and directly enters the analytical procedures.