Glass slips were submerged in the sea (1 m depth) off Taira-cho, Nagasaki, Japan for 1, 2, 3 and 4 weeks every month from Jul 2010 to Oct 2012, and the dry weights, chlorophyll contents and diatom community structures of the marine biofilms were investigated. Glass slips immersed in the sea acquired biofilms that consisted mainly of diatoms. Young thalli of Ulva compressa also occurred in the biofilms from Apr to Dec. Marine biofilms increased in mass with longer immersion periods, indicating that growth of marine biofilms should be looked into for up to a month. Colonization on glass slips by invertebrate macroorganisms occurred from Jul to Sep, consequently making the estimation of biofilm biomass inaccurate during this period. Dry weights of biofilms were linearly correlated with the amounts of chlorophylls-a and -c, indicating that these pigments can be used as indices to estimate biomass growth of marine biofilms, which primarily consist of periphytic microalgae. Chlorophyll-a contents and diatom densities of biofilms both showed temporal and seasonal variations, whereas chlorophyll-c contents of biofilms showed seasonal variation. Decrease in the chlorophyll (-a and -c) contents and diatom densities of biofilms in Jul to Sep may probably be due to the disturbance of biofilms caused by the attachment of invertebrate macroorganisms. Navicula almost always was the dominant diatom in established marine biofilms.

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Chlorination of seawater is a common practice to prevent biofouling on maritime structures. To enable control and reduce the environmental impacts of chlorine we measure the residual chlorine in wastewaters. In routine measurements of residual chlorine using N,N-diethyl-p-phenylenediamine (DPD), we found significant color development by DPD in non-chlorinated waters when large-sized phytoplankton appeared in high abundance. Through screening 27 microalgal isolates in laboratory culture, the diatom Coscinodiscus wailesii was shown to be the only species that causes the DPD color development in the procedure for total residual chlorine, although not for free residual chlorine, despite no chlorine additions to the culture medium. The result was also confirmed in field samples. Since two other species of genus Coscinodiscus did not show a similar result, this is considered to be highly species-specific. The color development was prevented by removing the C. wailesii cells using a 100 µm mesh cell strainer. Coscinodiscus wailesii did not react with 3,3′-dimethylbenzidine (o-tolidine), an alternative reagent to measure residual chlorine. We conclude that C. wailesii produces some materials which can oxidize DPD to produce the red color, and C. wailesii blooms in the field interfere with measurements of residual chlorine concentrations in natural and waste seawater.

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