Global warming has increased the frequency and severity of climatological events, including marine heatwaves, which are periods of at least five consecutive days where sea temperatures exceed the 90th percentile of seasonal averages over a baseline of 30 years. These events depending on their intensity, frequency and length can be detrimental to marine species and ecosystems, such as Zostera marina, a seagrass that relies partly on annual temperature patterns for its life cycle. This study investigated the effects of Spring marine heatwaves on the germination of Zostera marina seeds, a period before their normal germination. Seeds were extracted from reproductive shoots and subjected to controlled simulations of marine heatwaves, including moderate, severe, and extreme scenarios. Despite high numbers of observations of coat rupture without further development across all conditions, the overall germination remained extremely low, with the extreme heatwave showing the highest germination rate at 3.82% (SD= 3.18; SE = 1.84). And for the time to event no trends could be found. No significant impacts on germination were detected. These findings suggest that, under current temperature conditions, Zostera marina seed germination is not significantly affected by Spring marine heatwaves tested in the current experiment. Future studies should consider the impact of stronger marine heatwaves due to rising temperatures on the germination.
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