Commercial vessels utilized seawater in ballast tank to maintain the stability of the vessels. Ballast system used ballast pump to distribute the seawater inside and outside the ballast tank. The disposal of ballast water from commercial vessels affected the aquatic environment which was entered by commercial vessels. This study is aimed to assess the quality of commercial vessels ballast water in Port of Tanjung Emas Semarang (PTES) in the west moonson based on phytoplankton saprobity. Certain phytoplankton had tolerance to the dark environment, the lack of oxygen, and the polluted water. Phytoplankton could be used as bioindicator for the changes in aquatic environment. The adaptation of phytoplankton in the ballast tank of commercial vessels could be witnessed by the discovery of eight phytoplankton classes, they were Bacillariophyceae, Dinophyceae, Euglenoidea, Conjugatophyceae, Cyanophyceae, Charophyceae, Chrysophyceae, Conjugatophyceae and Chlorophyceae. The dominant genus in the commercial vessels in order were Skeletonema followed by Chaetoceros and Nizschia. The research resulted in the domination and survivability of Skeletonema sp. against the changing environment. Saprobity value index (SI) of ballast water of commercial vessels anchored in PTES was ranging from 0.4 to 2 with the lowest value held by GW vessel and the highest value held by A vessel. SI value of commercial vessel showed ballast water in the oligosaprobic state or has not been polluted yet. To prevent the spread of phytoplankton through commercial vessel ballast tank based on International Maritime Organization (IMO) in the Convention of Ballast Water Management in 2004, commercial vessels must obey basic standard D1 that said the vessels must change ballast water in the middle of ocean.

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