Beyond warming: changes in copepod phenological timing in response to marine heatwaves before and during their blooms

Introduction In marine ecosystems, copepods play an essential role in linking primary producers to higher trophic levels. As ectothermic organisms with short life cycles, they are particularly sensitive to environmental changes, because their metabolism and development are closely tied to abiotic conditions. Consequently, changes in environmental conditions, including temperature, can affect copepods and potentially lead to phenological shifts that may disrupt entire food webs. Whilst such shifts have been linked to global warming, the specific impact of marine heatwaves (MHWs) on copepod phenology remains unclear. Methods We used the Helgoland Roads long-term time series (German Bight, North Sea), one of the world’s richest marine datasets, to examine changes in the phenological timing of key copepod taxa in response to MHWs. We computed yearly bloom traits using an algorithm adapted from phytoplankton bloom analyses and assessed their relationships with MHW components (i.e., mean temperature intensity, proportion of MHW days, maximum temperature decline rate and maximum temperature increase rate) both before and during the bloom. Results With the exception of the calanoid copepod Acartia spp., we found no evidence of consistent phenological shifts in response to MHW components occurring before the bloom. However, MHWs occurring during copepod blooms led to significant changes in bloom duration. For most taxa, bloom duration decreased (contraction) with increasing proportion of MHW days (relative to the bloom duration) and with faster rates of temperatures increase within the bloom. Interestingly, during their blooms, copepods experienced increases in both mean and variance of proportion of MHW days and temperature intensity over the time, suggesting a shift in selective pressure linked to MHW exposure patterns. Discussion These findings highlight the complex influence of MHWs on copepod phenology and the potential consequences for marine food webs, including mismatches with fish larvae that may affect fish recruitment. Understanding these dynamics is crucial, as MHWs are expected to intensify in duration, frequency, and magnitude in future climates.