Estuarine heat waves are projected to exert profound impacts on marine communities.  Many of these impacts are thought to be negative.  In 2019, 2020, and 2021, estuarine heat waves in the lower Chesapeake Bay impacted the initiation, magnitude and duration of summertime blooms of Margalefidinium polykrikoides, a species of harmful algae that blooms there. We observed that the timing of heat waves with respect to the biological processes involved in bloom initiation could both positively and negatively affect the magnitude and duration of blooms in the lower Chesapeake Bay.  For example, in 2019, an estuarine heat wave occurred in June during the period when M. polykrikoides blooms routinely initiate in shallow sub-tributaries of the lower Chesapeake Bay.  This resulted in a failure of the bloom to initiate. In contrast, in 2020 and 2021, heat waves occurred in July, after blooms of M. polykrikoides had initiated and been transported to the lower James River and Chesapeake Bay. During these years, M. polykrikoides populations continued to thrive outside of the super-heated shallow estuaries where blooms had initiated.  This suggests that excystment and bloom development processes are temperature sensitive and that summertime warming of shallow waters where blooms of this organism initiate could actually reduce bloom formation or create conditions conducive for the emergence of new phytoplankton populations.
 

Heat waves,,Margalefidinium polykrikoides