Climate change is a global environmental issue driven by anthropogenic greenhouse gas emissions, resulting in an increase in average surface temperatures. In marine ecosystems, this leads to a range of serious impacts, including rising sea surface temperatures, sea level rise, and ocean acidification. Coastal zones are particularly vulnerable, as they are closely connected to human activities and experience intensified impacts from climate change. Therefore, it is essential to understand the influence of climate change–related warming on marine coastal environments and to use this knowledge to predict the future impacts of ongoing climate change. 

This dissertation focuses on the coastal sediment environment of the Baltic Sea. Specifically, all studies included in this dissertation were conducted within a two-bay system. One bay has been subjected to long-term warming for more than 50 years, while the other represents the current natural conditions of the Baltic Sea. This system serves as a predictive model to investigate the potential effects of future climate change in the Baltic Sea. Comparisons between the two bays revealed that long-term warming has reduced microbial cell abundance in the sediment and caused geochemical zones to shift closer to the sediment surface. Correspondingly, this warming related shift in geochemical zones has increased the potential for methane release from the sediment. Additionally, sediment eukaryotic activities are significantly affected by long-term warming, with increased metabolic activity observed in the heated bay. Moreover, the reversibility of the microbial community under prolonged warming was tested through a sediment reciprocal translocation experiment. The results indicated incomplete recovery within a one-year timescale. Together, this dissertation offers comprehensive insights into the adaptations of coastal sediments to climate change related warming, providing a basis for predicting future ecosystem responses to future climate scenarios.