Abstract
The diatom Eucampia zodiacus is a harmful species that indirectly causes bleaching damage to nori (Pyropia) cultivation through the competitive utilization of nutrients during its bloom. In the present study, to develop an understanding of the population ecology of E. zodiacus and identify countermeasures to reduce the negative impacts to aquacultured nori, we investigated the cell density, cell size reduction and restoration of E. zodiacus at two sampling stations in northern Harima-Nada, eastern Seto Inland Sea, from April 2006 to March 2009. Vegetative cells of E. zodiacus were detected almost all-year round in the water column. Cell densities were generally higher at the northwest station (Stn. Aioi), where initial appearances of E. zodiacus blooming are often detected in Harima-Nada, than at the northeast site (Stn. Futami). The restoration of cell size occurred once every autumn with great regularity, and the cells with maximum size gradually decreased and reached the minimum size one year later at both sampling stations, suggesting that synchronous changes in cell size occurred throughout the whole area of Harima-Nada. Eucampia zodiacus is considered to adapt during the stratified period by reducing its cell size. In the future, predicting the occurrence timing of nori bleaching by E. zodiacus will probably be improved by using the data obtained at Stn. Aioi in addition to that at Stn. Futami.
