Effects of nutrients on growth of the red-tide dinoflagellate Gyrodinium instriatum Freudenthal et Lee and a possible link to blooms of this species

We investigated the impact of different nitrogen (N) and phosphorus (P) compounds and concentrations on the growth of Gyrodinium instriatum Freudenthal et Lee in laboratory experiments, and possible links to blooms of this species at Hakozaki Fishing Port, Fukuoka, Japan. G. instriatum utilized only inorganic N compounds as N sources for growth. In contrast, G. instriatum utilized many inorganic and organic phosphorus compounds. We used the Monod equation to describe the growth rate of G. instriatum in N- or P-limited batch cultures as a function of ambient nutrient concentrations. Kinetic growth parameters for maximum specific growth rate (μ_max) and half-saturation nutrient concentration (K_S) were 0.57 divisions d^-1 and 14.2 μmol l^-1, respectively, under N-limitation and 0.65 divisions d^-1 and 1.75 μmol l^-1, respectively, under P-limitation. Compared with these K_S values, all in situ average dissolved inorganic nitrogen (DIN) concentrations in Hakozaki Fishing Port were higher than K_S for N, but all in situ average dissolved inorganic phosphorus (DIP) concentrations were lower than K_S for P, whether a red tide occurred or not bloom. Moreover, average DIP concentration in April (a month critical to red-tide genesis) of 2004 (a non-red-tide year) was less than half those in 2002 and 2003 (red-tide years). Thus, differences in DIP concentrations may be an important factor controlling blooms of G. instriatum in Hakozaki Fishing Port.