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• First model to quantify the role of mixotrophy in a coastal Karenia brevis bloom. • Mixotrophy is more prominent at bloom edges, potentially helping offshore spread. • Mixotrophic grazing helps K. brevis outcompete oligotrophic-adapted Synechococcus . While laboratory experiments show Karenia brevis graze on Synechococcus to supplement nutrition, the ecological role of mixotrophy in K. brevis blooms on the West Florida Shelf remains unquantified. This study employs a three-dimensional coupled hydrodynamic-biogeochemical model to investigate the contribution of mixotrophic feeding to the K. brevis growth as well as the role of mixotrophic grazing in suppressing prey population and delivering a competitive advantage to the mixotroph. It is shown that mixotrophy moderately increased the growth rate of K. brevis in the bloom center but was important along the offshore margin of the bloom region, where the prey-to-predator ratio was high. With the digestion rate reaching over 200 prey cells K. brevis -1 day -1 , mixotrophy could support a heterotrophic growth rate of up to 0.16 day -1 in the offshore region and may facilitate the offshore expansion of K. brevis blooms. Mixotrophic feeding also played a significant role in sustaining the K. brevis bloom during the late spring and summer periods when the inorganic nutrient concentrations were low. The grazing pressure exerted on Synechococcus was found to be vital for the slow-growing K. brevis to outcompete the oligotrophic-adapted prey. These model results demonstrate that mixotrophy not only functions as an additional nutrient source but also as an ecological mechanism that can reshape interspecific competition and harmful algal bloom dynamics.
Chen et al. (Wed,) studied this question.