Effects of various ballast water treatment methods on the survival of phytoplankton and bacteria

Aquatic invasive species are one of the major threats to the world’s oceans. One of the main transport pathways for these invasive species is ships ballast water. Because of this the International Maritime Organization formulated regulations limiting the number of organisms allowed to be in ballast water. To meet these regulations ballast water treatment systems were developed by various manufacturers. These treatment systems use various methods such as filtration, chlorination or UV-light to remove or kill the organisms. In this thesis the effectiveness of these methods was tested, as well as the potential of organisms to re-grow after treatment. Additionally different analysis methods were tested for identifying the re-growing species. The ballast water treatment systems tested all reduced organism numbers to below the international standards, but for all treatment systems organisms could re-grow. Using a combination of flow cytometry, microscopy and DGGE/sequencing these re-growing organisms were identified. The re-growing organisms were similar for different treatment systems using the same type of treatment, but different when the systems used different treatment types. Re-growth was especially observed for organisms <10 µm, this size of organisms is not included in the international ballast water regulations. This indicates a gap in the international regulations which should be corrected.