Long time-series observations in population dynamics of the harmful diatom Eucampia zodiacus and environmental factors in Harima-Nada, eastern Seto Inland Sea, Japan during 1974-2008

Long-term variations of the harmful diatom Eucampia zodiacus, a causative organism for bleaching of aquacultured "nori" (Porphyra yezoensis), had been monitored at 19 sampling stations in Harima-Nada, eastern Seto In- land Sea, Japan for 35 years from 1974 to 2008. Over the study period, E. zodiacus cells were detected every year, and seasonal cell densities tended to be higher from January to April. The proportion of E. zodiacus to the total phytoplank- ton cell density has increased in recent years, because the abundances of E. zodiacus started to increase in the mid 1990s, whereas the total phytoplankton cell densities in the 1980s and thereafter appeared to be lower than those of the 1970s. During the 35-year period, there were two significant long-term changes, i.e. an increase in winter water temper- atures and a decrease in the concentration of dissolved inorganic nitrogen. The present results suggest that the shift of environmental conditions is more advantageous to the growth of E. zodiacus, which contributed to the domination of E. zodiacus abundance in recent years.