The in situ growth rates of some deep‐living oceanic dinoflagellates: Pyrocystis fusiformis and Pyrocystis noctiluca1

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Abstract
Division rates of Pyrocystis spp. were estimated for natural populations by determining the proportions of dividing cells produced each day. In Mona Passage (West Indies), P. fusiformis and P. noctiluca appeared to divide fastest at 80–100 m, the depth of their maximum cell concentration. Pyrocystis noctiluca divided at about the same rate in the mixed layer as in the thermocline, while P. fusiformis divided much more slowly in the mixed layer than below. In culture, reproductive stages of both species were negatively buoyant, but vegetative cells were positively buoyant. The effect of such buoyancy differences (and the rising and sinking rates which result from them) on the estimation of natural division rates was apparently negligible in the mixed layer and of some importance in the more stable waters of the thermocline.

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