Cessation of diel vertical migration by an inshore dinoflagellate bloom under prey deprivation

Many dinoflagellates are mixotrophic and regulate their vertical position to navigate dynamic gradients in light, nutrients, and prey. Here, it is shown that the obligate kleptoplastidic mixotroph, Dinophysis acuminata, transitions from diel vertical migration to formation of a stationary, sub-surface thin layer in response to prolonged prey deprivation. An inshore bloom within a salt marsh kettle pond was recorded through continuous in-situ imaging, automated oxygen and fluorescence depth profiling, and targeted water chemistry measurements. During the blooms initial development, D. acuminata cells were photosynthetically active and divided vegetatively while vertically migrating. As photosynthesis and growth slowed, vertical migration ceased and cells formed a stable thin layer that promoted conditions for local acidification and nitrogen remineralization. Surface avoidance by the thin layer drove selective retention of cells within the relatively deep kettle hole. Together, these findings illustrate linkage of metabolic state and swimming behavior in D. acuminata and show how swimming behavior can drive development of toxic blooms within inshore systems. They also illustrate how D. acuminata and other eurytolerant bloom-forming species can exploit and shape physicochemical gradients associated with coastal eutrophication.