Bloom termination of the toxic dinoflagellate Alexandrium catenella: Vertical migration behavior, sediment infiltration, and benthic cyst yield

New resting cyst production is crucial for the survival of many microbial eukaryotes including phytoplankton that cause harmful algal blooms. Production in situ has previously been estimated through sediment trap deployments, but here was instead assessed through estimation of the total number of planktonic cells and new resting cysts produced by a localized, inshore bloom of Alexandrium catenella, a dinoflagellate that is a globally important cause of paralytic shellfish poisoning. Our approach utilizes high frequency, automated water monitoring, weekly observation of new cyst production, and pre‐ and post‐bloom spatial surveys of total resting cyst abundance. Through this approach, new cyst recruitment within the study area was shown to account for at least 10.9% ± 2.6% (SE) of the bloom's decline, ∼ 5× greater than reported from comparable, sediment trap based studies. The observed distribution and timing of new cyst recruitment indicate that: (1) planozygotes, the immediate precursor to cysts in the life cycle, migrate nearer to the water surface than other planktonic stages and (2) encystment occurs after planozygote settlement on bottom sediments. Near surface localization by planozygotes explains the ephemerality of red surface water discoloration by A. catenella blooms, and also enhances the dispersal of new cysts. Following settlement, bioturbation and perhaps active swimming promote sediment infiltration by planozygotes, reducing the extent of cyst redistribution between blooms. The concerted nature of bloom sexual induction, especially in the context of an observed upper limit to A. catenella bloom intensities and heightened susceptibility of planozygotes to the parasite Amoebophrya, is also discussed.