Abiotic and biological differences in ballast water uptake and discharge samples.

During the type approval process of ballast water management systems (BWMS) performance tests need to be conducted according to the BWMS Code (previously Guidelines G8) of the International Maritime Organization (IMO). The shipboard tests previously included a control experiment with untreated ballast water to evaluate the BWMS performance by comparing test results of treated and untreated water. Biological results and abiotic parameters of 97 control water tests conducted during the last >10 years during ballast water uptakes and corresponding discharges were summarized. In general, a strong decline of organisms in ballast tanks was observed, especially during the first few days of the holding time. The IMO validity criteria for uptake water phytoplankton in shipboard control tests were met in 82.5% of all tests. Phytoplankton numbers below the validity criteria occurred predominantly in winter and/or when the water was taken up offshore. For zooplankton the validity criteria were always met. The TSS and POC content in our ballast water uptake samples was frequently much higher than required during IMO BWMS type approval tests so that the current testing requirements do not represent a challenge to BWMS. With this a risk is taken that type approved BWMS fail in water conditions which occur frequently in the real world.