Alexandrium catenella cyst dynamics in a coastal embayment : temperature dependence of dormancy, germination, and bloom initiation

Abstract

Blooms of the dinoflagellate Alexandrium catenella cause paralytic shellfish poisoning syndrome and present an expanding public health threat. They are inoculated through the germination of benthic cysts, a process regulated by internal and environmental factors, most importantly temperature. Less understood is the effect of temperature conditioning on cyst dormancy cycling, which inhibits germination for long periods. This thesis characterizes the temperature-dependence of both dormancy and germination in natural A. catenella cyst populations from Nauset Marsh (Cape Cod, MA, USA), a small estuarine embayment, and relates these processes to the phenology of blooms there. Through laboratory germination assays, it is shown that dormant A. catenella cysts require a quantifiable amount of chilling to exit dormancy and attain quiescence (i.e. become germinable). A series of experiments compares germination rates of quiescent cysts across a range of temperatures through laboratory experiments and field incubations of raw sediment using plankton emergence traps (PETs). Emergence rates of A. catenella germlings measured by PETs increased linearly with temperature and were comparable to germination under constant laboratory conditions. Total emergence fluxes were much lower than expected, suggesting that germination occurs in a much shallower layer of sediments than typically assumed. The results are synthesized to develop a temperature-dependent model to examine the sensitivity of A. catenella bloom phenology to dormancy-breaking by winter chilling. Notably, the chilling-alleviated dormancy model accurately predicted the timing of quiescence (January) and the variable bloom phenology from multiple blooms in Nauset. Once cysts became quiescent and began to germinate, however, temperatures were typically too cold for growth to exceed losses so there was a several-week lag until bloom development. Years with warmer winters and springs had shorter lag periods and thus significantly earlier blooms. Ecologically, dormancy-breaking by a chilling threshold is advantageous because it prevents the mismatch between conditions that are favorable for germination but not for the formation of large blooms. Synchronized germination after winter chilling also promotes promotes efficient conversion from the cyst seedbed to the spring bloom inoculum. The dormancy mechanism characterized here may be present in other cyst-forming dinoflagellates, but there is likely plasticity that reflects the temperature regime of each habitat.