Phytoplankton are a major concern in the introduction of marine invasive species by ships. The current ballast water performance standard (Regulation D2) under the Ballast Water Convention prescribes the number of viable organisms allowed in discharged ballast water. For organisms ≥10 μm and ≤50 μm in minimum dimension, the D2 allows <10 viable organisms per ml, a size category including mostly phytoplankton. However, many phytoplankton are narrower than 10 μm but much longer than 10 μm. This discrepancy causes two major problems: firstly, ballast water management systems will not be examined for their performance at removing organisms larger than regulated by D2. Secondly, this will require higher concentrations of plankton in the challenge water to meet the required test water condition of >103 ml−1 for organisms of sizes as stipulated in the guidelines. Examination of the seasonal distribution of local Korean fresh and sea water shows that phytoplankton communities are frequently dominated by cells <10 μm in width but >10 μm in length. In addition, review of over 600 freshwater diatoms of elongated shapes reveals that more than 300 are >10 μm long, but <10 μm wide. We suggest consideration of these phytoplankton in land-based or shipboard treatment procedures and to report their treatment efficacy, in addition to reporting the performance of ballast water management systems in accordance with the D2 regulation.

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