Chronically elevated sea surface temperatures revealed high susceptibility of the eelgrass Zostera marina to winter and spring warming

While it is well known that severe marine summer heatwaves can cause acute and dramatic die-offs of seagrass meadows, the effect of trans-seasonal warming and winter/spring heatwaves are yet poorly understood. This study simulated a 9-months warming scenario on the common seagrass Zostera marina from winter into summer, using outdoor mesocosms, which provided near-natural conditions. The relevance of the natural temperature pattern, as well as the 3.6°C warming, and their implications were further discussed in the context of a 22-yr temperature time series of the study region. Survival of plants was high in winter independent of temperature. In spring, however, heat-treated Z. marina flowered 1.5 months earlier and experienced high mortalities. Thereafter, plant survival, growth, and pigmentation were largely comparable between temperature regimes. Yet, a comparatively high mortality occurred in ambient plants, after an abnormally warm June. Final biomass was reduced by ~ 50% in heat-treated plants. These results imply that warm winter-to-spring conditions can have severe effects on vital seagrass traits. Warming accelerates consumption of energy reserves triggering advanced flowering, similar to many terrestrial plants. Although, surviving heat-treated plants were not able to re-stock energy reserves throughout the high-light summer as inferred from low plant biomass, these seemed rather resistant to summer heatwave events.