Journal of Oceanography, Vol. 65 (No. 6), pp. 885-891, 2009

Diatoms Grow Faster Using Ammonium in Rapidly Flushed Eutrophic Dokai Bay, Japan

Kuninao Tada¹*, Marut Suksomjit¹, Kazuhiko Ichimi², Yui Funaki¹, Shigeru Montani³, Machiko Yamada⁴ and Paul J. Harrison⁵

¹Faculty of Agriculture, Kagawa University, Miki-cho, Kita-gun, Kagawa 761-0795, Japan
²Aji Marine Station, Seto Inland Sea Regional Research Center, Kagawa University, Aji, Takamatsu, Kagawa 761-0130, Japan
³Graduate School of Fisheries Sciences, Hokkaido University, Sapporo 060-0813, Japan
⁴Kitakyushu City Institute of Environmental Sciences, Tobata-ku, Kitakyushu 804-0082, Japan
⁵AMCE Program, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

(Received 22 March 2009; in revised form 28 July 2009; accepted 29 July 2009)

Abstract: The importance of the nitrogen source for phytoplankton growth in a highly eutrophic embayment, Dokai Bay, was investigated. The DIN concentration often exceeded 100 μM of which 40-70% was NH₄⁺. During two incubation experiments, the natural assemblage of mainly diatoms took up NH₄⁺ instead of NO₃^-. The growth of two Skeletonema species isolated in Dokai Bay were significantly faster on NH₄⁺ (1.86 and 1.27 div. d^-1 respectively) than on NO₃^- (1.55 and 1.04 div. d^-1 respectively). Our results indicated that these diatoms could grow faster by using NH₄⁺ compared to NO₃^- in this eutrophic bay.