Cyst-forming dinoflagellates in a warming climate.

Many phytoplankton species, including many harmful algal bloom (HAB) species, survive long periods between blooms through formation of benthic resting stages. Because they are crucial to the persistence of these species and the initiation of new blooms, the physiology of benthic stages must be considered to accurately predict responses to climate warming and associated environmental changes. The benthic stages of dinoflagellates, called resting cysts, germinate in response to the combination of favorable temperature, oxygen-availability, and release from dormancy. The latter is a mechanism that prevents germination even when oxygen and temperature conditions are favorable. Here, evidence of temperature-mediated control of dormancy duration from the dinoflagellates Alexandrium catenella and Pyrodinium bahamense-two HAB species that cause paralytic shellfish poisoning (PSP)-is reviewed and presented alongside new evidence of complementary, temperature-based control of cyst quiescence (the state in which cysts germinate on exposure to favorable conditions). Interaction of the two temperature-based mechanisms with climate is explored through a simple model parameterized using results from recent experiments with A. catenella. Simulations demonstrate the importance of seasonal temperature cycles for the synchronization of cysts' release from dormancy and are consistent with biogeography-based inferences that A. catenella is more tolerant of warming in habitats that experience a larger range of seasonal temperature variation (i.e., have higher temperature seasonality). Temperature seasonality is much greater in shallow, long-residence time habitats than in deep, open-water ones. As warming shifts species' ranges, cyst beds may persist longer in more seasonally variable, shallow inshore habitats than in deep offshore ones, promoting HABs that are more localized and commence earlier each year. Recent field investigations of A. catenella also point to the importance of new cyst formation as a factor triggering bloom termination through mass sexual induction. In areas where temperature seasonality restricts the flux of new swimming cells (germlings) to narrow temporal windows, warming is unlikely to promote longer and more intense HAB impacts-even when water column conditions would otherwise promote prolonged bloom development. Many species likely have a strong drive to sexually differentiate and produce new cysts once concentrations reach levels that are conducive to new cyst formation. This phenomenon can impose a limit to bloom intensification and suggests an important role for cyst bed quiescence in determining the duration of HAB risk periods.