Between heatwaves: recovery debt, thermal memory, and recurrent-event vulnerability in marine zooplankton and protistan grazers

Marine heatwaves can occur again before organisms and communities have recovered from an earlier event. However, most plankton experiments end when warming stops and describe resistance more clearly than recovery. Here, I distinguish recovery debt (a residual physiological, reproductive, demographic, or community deficit after temperature returns toward reference conditions) from thermal memory (any persistent effect of prior exposure on a later response, whether protective or detrimental). A structured Web of Science search returned 50 candidate records; 9 met the eligibility criteria, and citation chaining added 7 more, giving 16 focal studies. Nine concerned copepods, and, to my knowledge, none followed an isolated free-living heterotrophic protist after heating. Only one study, a mesocosm experiment, applied two finite heatwaves, separated by 5 days and without a Heatwave-2-only treatment. No study manipulated the inter-event interval. Two copepod experiments compared naive and previously exposed lineages under the same later heatwave, in both cases across generations. The available studies show that post-event trajectories can differ among traits, that delayed reproductive or molecular effects can persist after cooling, and that community function can return without restoration of composition. This evidence does not establish an interval effect, but it suggests a testable hypothesis: the response to a second heatwave depends on how far protective regulation has decayed and how far energetic repair, reproduction, and demographic turnover have progressed since the first. Testing this requires factorial event histories, repeated sampling during recovery, and intervals expressed in biological as well as chronological time.