General warming of the Baltic Sea with frequent summer heatwaves has been noted and studied over the past decades. Many coastal species have been shown to experience negative effect through heat stress induced by general warming of a water body. For example, if the species richness in those heated systems is significantly lower than in comparable non heated ecosystems, this can have an effect on the degradation of organic matter and the food web in general. To further investigate the effect of warming on coastal communities the heat effects on the community composition and abundance of macroinvertebrates in two Baltic Sea bays were studied. One of those bays has been artificially heated for decades by the reflux of cooling water from the nearby nuclear power plant. The other bay functioned as a control system. Macroinvertebrates were collected over three sampling occasions from December 2023 to April 2024 at 12 different stations. The artificially heated bay had a significantly lower abundance and diversity of macroinvertebrates than the control bay. Furthermore, the functional feeding group of shredders was significantly more abundant in the control bay compared to the heated bay. Lastly the specimens of Gammarus spp. and Asselus aquaticus were significantly larger in the control than in the heated bay. The vegetation in the two bays differed, while the heated bay mostly featured a filamentous macroalgae the dominant genus in the control bay was Fucus spp.. Furthermore, the heated bay had a higher oxygen concentration and a higher salinity, probably due to less freshwater input. Temperature is likely to be the driving factor, since the oxygen saturation in the control bay was still high. Additionally, marine taxa were mainly foundin the control bay even though the salinity was significantly lower, than in the heated bay. This leads to the conclusion that heating of a water body will have a negative impact on the abundance and composition of macroinvertebrates and possibly their reproductive health. In general, further climate warming will reduce the biodiversity  and abundance of many cold-water macroinvertebrate communities and with that the degradation of organic matter, by macroinvertebrate shredder, might be taken over by microbes. The degradation by microbes will produce more CO2 in that process, leading to further problems such as hypoxia.