Growth dynamics of Heterosigma akashiwo (Raphidophyceae) in Hakata Bay, Japan

In Hakata Bay, Japan, from April to June 2006, Heterosigma akashiwo dynamics were investigated by daily to weekly sampling in relation to environmental parameters. To test how well the water in the samples supported H. akashiwo growth, bioassays of growth of a cultured strain of H. akashiwo were also conducted on these samples. In Hakozaki, a semi-enclosed fishing port, H. akashiwo had bloomed at almost all stations, one week after DIN and DIP concentrations increased by the end of May. Thereafter, H. akashiwo declined and fluctuated at low densities. The H. akashiwo population almost completely disappeared associated with a rapid decrease in salinity at the end of June. In the bioassays, addition of phosphate and nitrate often promoted H. akashiwo growth during the investigation, but 1 week before the H. akashiwo bloom, the growth rates in seawater samples without addition of these nutrients became close to those with nutrients. Addition of vitamins and metals barely affected growth rates during the investigation period. Moreover, even when some phytoplankton species also bloomed densely, H. akashiwo grew in the seawater samples at rates similar to that when phytoplankton abundance was low, indicating absence of significant allelopathic control by other phytoplankton. The present study shows that H. akashiwo growth is limited by N- and P-sources in this bay.