Assessment of ship hull-attached marine diatoms: species composition and reattachment

Ship biofouling is a primary pathway for the introduction and global spread of non-indigenous species, posing significant risks to marine ecosystems. However, the extent to which ship hull-attached diatoms survive and disperse following in-water hull cleaning (IwHC) remains poorly understood. Here, we analyzed the attached diatoms compositions in the in-water hull cleaning water (IwHCW) from three international ships entering Korea as well as benthic diatoms at nearby stations, using both morphological and molecular methods. In addition, we assessed the survival of ship hull-attached diatoms using reattachment analysis. Water temperature and salinity varied during sampling of the IwHCW of the ships, while the conditions at the four nearby stations were more consistent. The diatom Halamphora spp. was most abundant in the IwHCW of the ships, whereas Achnanthes species dominated at the nearby stations. Non-metric Multidimensional Scaling (NMDS) revealed significant variation in species composition between the IwHCW and the stations, suggesting limited dispersal of ship-associated diatoms or spatiotemporal influences. Survival and reattachment analyses revealed that Halamphora oceanica survived for over 21 days in unfiltered IwHCW, with no attached diatoms observed in IwHCW filtered through 5 and 32 μm filters. These findings indicate that diatoms in an unfiltered IwHCW may survive and reattach to substrates, highlighting the need for IwHC risk assessment and biofouling management in coastal waters.