Why was the 2012 bloom so early? Using chilling and warming metrics to resolve interannual variability in the timing of Alexandrium fundyense bloom initiation
Why was the 2012 bloom so early? Using chilling and warming metrics to resolve interannual variability in the timing of Alexandrium fundyense bloom initiation
Abstract:
The Nauset Marsh System (NMS) on Cape Cod (MA, USA) has recurrent Alexandrium fundyense blooms that have caused nearly annual shellfishing closures due to paralytic shellfish poisoning. Blooms were observed over a multi-year period (2009-2015) to examine the effects of seasonal cooling and warming on the transition from resting cysts to vegetative cells in the plankton. The life cycle processes of cyst dormancy, germination, and vegetative cell growth are all uniquely sensitive to temperature, which can translate to changes in bloom initiation phenology. Bloom initiation (>100 cells/l-1) occurred as early as 14 February 2012, and as late as 15 April 2015. To quantitatively examine the mechanisms responsible for this two-month range, laboratory studies were performed. In experiments mimicking winter’s onset, mature cysts were exposed to chilling temperatures (2-8°C), and at regular intervals the germination potential of cyst cohorts was evaluated. Next, in experiments mimicking a range of late-winter, early-spring temperatures, the time to germination was observed for cold-conditioned cysts. To account for the interannual temperature variability in the NMS and enable comparison to laboratory studies, we calculated growing degree-days and chilling-units, both metrics that tabulate accumulated temperature exposures. Here we pair laboratory studies with seven years of bloom data to present a conceptual model of three temperature-dependent phases of bloom initiation for A. fundyense:
1) Winter dormancy. As temperatures cool, cysts enter a state of dormancy during which germination is physiologically inhibited, until they experience a threshold of winter chilling.
2) Quiescence. Cysts are physiologically able to germinate, but require a specific amount of heat, oxygen, and light.
3) Growth. Germling cells transform to vegetative cells, which divide asexually as a function of heat to create the bloom.
These results help to explain differences in bloom timing between years, and provide insights into potential responses of A. fundyense to climate change.