Ballast water-mediated transfer of aquatic invasive species is considered a major threat to marine biodiversity, marine industry and human health. A ballast water treatment is needed to comply with International Maritime Organization (IMO) ballast water discharge regulations. Didecyldimethylammonium chloride (DDAC) was tested for its applicability as a ballast water treatment method. The treatment of the marine phytoplankton species Tetraselmis suecica, Isochrysis galbana and Chaetoceros calcitrans showed that at 2.5 µL L(-1) DDAC was able to inactivate photosystem II (PSII) efficiency and disintegrate the cells after 5 days of dark incubation. The treatment of natural marine plankton communities with 2.5 µL L(-1) DDAC did not sufficiently decrease zooplankton abundance to comply with the IMO D-2 standard. Bivalve larvae showed the highest resistance to DDAC. PSII efficiency was inactivated within 5 days but phytoplankton cells remained intact. Regrowth occurred within 2 days of incubation in the light. However, untreated phytoplankton exposed to residual DDAC showed delayed cell growth and reduced PSII efficiency, indicating residual DDAC toxicity. Natural marine plankton communities treated with 5 µL L(-1) DDAC showed sufficient disinfection of zooplankton and inactivation of PSII efficiency. Phytoplankton regrowth was not detected after 9 days of light incubation. Bacteria were initially reduced due to the DDAC treatment but regrowth was observed within 5 days of dark incubation. Residual DDAC remained too high after 5 days to be safely discharged. Two neutralization cycles of 50 mg L(-1) bentonite were needed to inactivate residual DDAC upon discharge. The inactivation of residual DDAC may seriously hamper the practical use of DDAC as a ballast water disinfectant.
Keywords: DDAC; IMO D-2 standard; ballast water treatment; phytoplankton; zooplankton.