There is probably no ecosystem on Earth that is completely unaffected by thermal heating associated with climate change. Coastal ecosystems are of particular concern because they are at the intersection between land and sea, experience increased thermal stress, and contain essential organisms at the bottom of the food web that provide important ecosystem services. A key question is whether the negative consequences of climate change for biodiversity and ecosystem health are permanent or can be remediated.

This thesis focuses on the effects of climate change related heating on copepods, biofilms, and sediment microbes. To investigate this, I used a model system that consisted of a long-term heated Baltic Sea coastal bay, a nearby unaffected control bay, and a thermal gradient along a coastline in-between the two bays. I combined observational data obtained via field sampling with laboratory experiments, and introduction and translocation experiments performed in the field.

For the copepods, I found that long-term heating has resulted in a shift in phenology, with an earlier onset of population growth and abandoned production of dormant eggs. This may induce a mismatch between trophic levels and make copepod populations more vulnerable to disturbances. For the biofilms, the results indicated that their diversity, composition, and seasonal dynamics varied among and within the three environments, largely due to temperature, water chemistry, and wave exposure, with prokaryotes exhibiting stronger spatial heterogeneity and seasonal dynamics compared to micro-eukaryotes. For the functional groups of micro-eukaryotes, climate heating seems to decrease the seasonal fluctuations. Together, this indicated that climate warming may disproportionately impact different components of coastal biofilm communities, potentially decoupling key ecological processes and reducing community resilience. Finally, when substrates with biofilm and bottom sediment cores were reciprocally translocated between the heated and the control bays, it was found that while the community compositions shifted, legacy effects of past temperature conditions shaped the responsiveness to perturbations. Specifically, warming elicited faster responses than exposure to colder conditions, suggesting that microbial communities may not fully convert even if original environmental conditions are restored.