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Photopigment radiolabelling as a tool for determining in situ growth rates of the toxic dinoflagellate Karenia brevis (Dinophyceae)

Determination of growth rates of harmful algae is critical to understanding bloom dynamics. However, we have few reliable methods of directly determining in situ growth rates on natural populations. One available method is photopigment radiolabelling, where C-specific growth rates are based on synthesis rates of chlorophylls and carotenoids using 14C-bicarbonate as a tracer. Here, we examined radiolabelling of the biomarker pigment gyroxanthin-diester as a tool for determining in situ growth rates of the toxic dinoflagellate Karenia brevis. We also characterized growth responses of K. brevis to various nitrogenous (N) nutrient sources and the presence or absence of a natural plankton community (including grazers) at two irradiances. We found that gyroxanthin radiolabelling may be used successfully to determine C-specific growth rates of K. brevis during monospecific blooms. However, the approach may be of limited use in mixed assemblages when there is low Karenia biomass because low concentrations of gyroxanthin per cell and low turnover rates result in poor signal for the gyroxanthin pigment. Growth rates of K. brevis generally ranged between 0.1 and 0.4 d−1. Cells in batch culture grew equally well on inorganic and organic forms of N, so they may be physiologically capable of using the major N forms present in agricultural run-off, atmospheric deposition, and other anthropogenic inputs. This has implications for the control of K. brevis blooms in nutrient-impacted coastal waters in that organic N inputs should be considered (along with inorganic N) in nutrient management strategies. Our highest C-specific growth rates (0.7 d−1) were measured in the presence of microzooplankton grazers, also indicating that the release of dissolved compounds may play a role in stimulating the growth of K. brevis in nature.