Extreme events and warming in the Baltic Sea: relevance for coastal benthic communities

Ongoing global climate changes leads to an increased water temperature and a higher probability of extreme events.

This thesis focuses on (i) the characterisation of heatwaves and cold-spells in the Kiel Fjord, (ii) using this characterisation for treatment development, (iii) running mesocosm experiments to unravel the importance of frequency, duration, and intensity of environmental stress (i.e., heatwaves, upwelling, and warming) for the impact on the two important species Asterias rubens and Zostera marina, and (iv) the coping mechanisms of these species with (succeeding) environmental stress.

Starfish were substantially impacted by heatwaves, with stronger and longer-lasting impacts in amplified and prolonged heatwaves. The impacts reached from 100% mortality (simulated future intensity) to decreased feeding rates, lower growth, and reduced activity (today’s intensity). Upwelling reduced the starfish's activity. However, the succession of extreme events led to an alleviated impact during the second event.

Eelgrass was not impacted by short heatwaves or upwelling alone but had less leaves when the heatwave was extended, or a short heatwave was followed by upwelling. The latter combination of stressors also reduced the number of eelgrass shoots. When exposed to long-term warming, aboveground and belowground biomass of non-heat-selected eelgrass was strongly reduced. However, eelgrass individuals from a potentially heat-selected population grew more than those from a non-heat-selected population.

Overall, this thesis highlights that extreme events are important drivers of temperate benthic ecosystems but that their impact depends on the nature of the event, their succession, duration, and intensity. Yet, species may have the potential to cope with recurring stress events via an ecological memory or via heat-selection in situ.


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