Factors influencing the initiation of blooms of the raphidophyte Heterosigma akashiwo and the diatom Skeletonema costatum in a port in Japan

We investigated how environmental factors initiate Heterosigma akashiwo and Skeletonema costatum blooms from resting stages in bottom sediments in a shallow port over 2 yr. Using field-collected sediments, we also conducted laboratory experiments on how light intensity affects germination of resting stages and growth of the germinated cells. Both phytoplankton species bloomed only in summer, when water temperature and solar radiation were high enough for growth. All three blooms of H. akashiwo and the earliest bloom of S. costatum in a year occurred right after transmission of strong light (.200 mmol quanta m22 s21) to the bottom layer and a peak occurred in dissolved inorganic phosphorus (DIP). In the laboratory, resting stages of H. akashiwo and S. costatum germinated even in dim light (20 and 65 mmol quanta m22 s21, respectively), but germinated cells required stronger light of .130 and 280 mmol quanta m22 s21, respectively, for rapid growth. This value is much higher than the threshold for survival, and is higher than the half-saturating light intensity for growth of vegetative cells. Abundance of the resting stages of both species in the sediments rapidly increased during blooms and logarithmically decreased during nonbloom periods, suggesting that resting stages are continuously consumed. For both species, our results suggest that blooms initiate when transmission of sufficient light permits: first, germination of cells from the sediment; second, rapid growth of these germinated cells. Temperature and DIP must also exceed a facilitating threshold. Many phytoplankters have resting stages in their life cycle. The resting stages usually inhabit bottom sediments when conditions are unfavorable for the survival and growth of vegetative cells (Anderson and Wall 1978). Germination of resting stages is usually regulated by environmental factors such as water temperature (Anderson and Rengefors 2006) and light (McQuoid and Hobson 1996), and studies have suggested that the release of environmental constraints allows germination of the resting stages and often initiates a bloom (Anderson and Wall