Re-growth of potential invasive phytoplankton following UV-based ballast water treatment

Ballast water contains organisms which can survive the ship's journey and become established in the recipient water body when discharged. Phytoplankton species can become invasive and might be harmful by producing toxins or anoxic conditions following their blooms. Different technologies exist to treat ballast water in order to reduce the spread of invasive species. The effectiveness of a UV-based ballast water treatment system was tested in an incubation experiment over 20 days. After an initial decline in cell numbers, re-growth could be observed of certain phytoplankton taxa, namely the diatoms Thalassiosira, Skeletonema, Chaetoceros, Pseudo-nitzschia, and Nitzschia (order represents rank of abundance). The conclusion of this study is that a variety of taxa are able to survive UV-treatment. These may include harmful and potential invasive phytoplankton species. Long-term incubation experiments should be considered when testing the effectiveness of UV-based treatment systems. The dominant re-growing phytoplankton group was Thalassiosira which could be a suitable indicator organism for testing the efficiency of UV-units.