Seasonal warming and marine heatwaves shape thermal acclimation and growth of juvenile European plaice in a coastal nursery habitat

Elena Tamarit-Castro, Francesca Molinari, Malva Burden, Mia Spiering, Felix Steinbrecher, Fredrik Jutfelt, Joachim Sturve, Diana Perry, Martin Gullström, Hans Linderholm

Global climate change is driving ocean warming and exposing shallow coastal nursery habitats to increasingly frequent marine heatwaves. The European plaice (Pleuronectes platessa) is a flatfish that settles in shallow sandy bays of the Northeast Atlantic in spring and remains until late summer migration, experiencing peak seasonal temperatures during a critical period for juvenile growth. We combined satellite-derived sea surface temperatures (SSTs), high-resolution in situ water temperatures, seasonal field sampling, and laboratory experiments to examine how seasonal warming and marine heatwaves influence thermal acclimation and growth of 0-group plaice between May and September 2025, in a nursery bay on the Swedish Skagerrak coast. Regional SST increased by 0.45°C decade-1 between 1982 and 2025, coinciding with increases in marine heatwave frequency, duration and intensity. We identified three marine heatwaves occurring near the nursery site during 2025, but in situ temperature records revealed longer lasting and more intense thermal extremes than regional SSTs. Wild plaice showed capacity to acclimate to seasonal temperatures, with Critical Thermal Maximum (CTmax) closely tracking seasonal warming. Also, plaice exposed to a simulated heatwave rapidly induced the expression of heat shock proteins. In contrast, plaice’s upper thermal tolerance did not increase during a natural marine heatwave, indicating limited acclimation capacity. When exposed to chronic warming, growth was maintained at 19-21°C, but declined at 24°C, a temperature repeatedly recorded within the nursery bay during summer. Together, these results show that newly settled plaice possess substantial physiological plasticity to buffer seasonal warming during the first growth season and to respond to acute heat stress with rapid molecular responses. However, this plasticity was insufficient to maintain length growth under prolonged warming. As marine heatwaves intensify in the North Sea, increasingly stressful temperatures may constrain juvenile fish growth and the quality of shallow coastal nursery habitats may decline.