In situ life-cycle stages of Alexandrium fundyense during bloom development in Northport Harbor (New York, USA)

Knowledge of the specific life-cycle dynamics during harmful algal bloom (HAB) development is essential for understanding and forecasting the onset, evolution and future occurrence of these events. Life-cycle stages of the toxic dinoflagellate Alexandrium fundyense were monitored both in the water column and in the sediments from the onset to the decline of a bloom in Northport Harbor (New York, USA). Moreover, excystment and encystment were investigated in situ through the deployment of emergence and sediment traps, respectively. The bloom, the largest ever recorded on the east coast of the US south of Massachusetts, persisted for 6 weeks between April and June 2008, and reached maximum vegetative cell abundances of 1.3 × 106 cells l−1. Resting cysts in the surface sediments were quantified at the onset of the bloom, all of which germinated during the development of the bloom. Excystment of these resting cysts provided inoculum of vegetative cells for bloom development. In the water column, first detection of planozygotes occurred during the exponential phase of the bloom at vegetative cell abundances of ∼104 cells l−1. Nonmotile planozygotes and resting cysts were observed in the sediment traps before the first peak of vegetative cells, coinciding with the detection of planktonic planozygotes. The estimated encystment and planozygote percentages were relatively low, indicating that a small proportion of the vegetative cell population was involved in sexual reproduction. However, encystment was a crucial process for replenishing the cyst stock of the A. fundyense population in the sediments as cyst densities were low before the bloom but high after it. For the first time, formation of pellicle cysts in the field by A. fundyense was observed, which coincided with high vegetative cell abundances in the water column during the bloom.

► We quantified in situ life-cycle stages and transitions of Alexandrium fundyense during a bloom event.
► All mature resting cysts in surface sediments germinated during bloom development.
► Planozygotes were detected during exponential bloom phase at ∼104 vegetative cells l−1.
► Encystment involved a small proportion of vegetative population, but sufficient for replenishing cyst stock.
► Formation of pellicle cysts in the field at high vegetative cell abundances.