# Projections of Climate Change Impact on Stream Temperature: A National-Scale Assessment for Poland
## 4 Discussion
### Potential Consequences of Stream Temperature Increase for Freshwater Ecosystems  
The observed elevations in stream temperatures during the summer and winter seasons underline the urgency of addressing climate change-induced alterations in aquatic environments. The disparities between RCP4.5 and RCP8.5 scenarios emphasize the significance of emission trajectories in shaping the magnitude of these changes. The potential consequences of an increase in stream temperatures for freshwater ecosystems are multifaceted and can have far-reaching impacts on aquatic life and overall ecosystem health [7]. These consequences manifest across various dimensions, highlighting the interplay between temperature changes and ecosystem dynamics.  
Altered habitat suitability stands out as a primary concern, as the link between thermal conditions and stream temperatures can lead to shifts in the suitability of freshwater habitats [54]. These changes could significantly impact reproductive success, growth, and the survival of aquatic species [55]. Some species may face decline, while those adapted to warmer conditions might thrive, leading to substantial alterations in the overall composition of freshwater communities. For example, within the freshwater ecosystems of Poland, changes in stream temperatures could present considerable risks to various fish species. Coldwater inhabitants, including brown trout and Atlantic salmon, are especially sensitive to fluctuations in river temperatures [56]. The spread of invasive species adds yet another dimension to the challenges posed by changing stream temperatures. The creation of conditions conducive to invasive species, adapted to warmer environments, introduces the risk of increased competition for resources and potential disruptions to the ecological balance of native ecosystems [57].  
Beyond individual species, stream temperature plays a crucial role in broader ecosystem processes, such as nutrient cycling [58]. Changes in temperature can influence microbial activity and nutrient availability, potentially disrupting the balance of essential elements in freshwater ecosystems. Moreover, the reduction in dissolved oxygen levels due to elevated water temperatures poses a direct threat to aquatic organisms that rely on well-oxygenated water for respiration [59]. This holds particular significance within the Polish context. In the summer of 2022, the River Odra in Poland and Germany witnessed one of the most significant environmental disasters ever documented in Europe [60]. This event led to substantial fish mortality, attributed to a toxic algal bloom. Contributing factors to this disaster likely included the release of saline water into the river, low dissolved oxygen, and various forms of water pollution (such as industrial and municipal wastewater from specific sources and the runoff of nutrients and chemicals from agricultural zones), along with hydrometeorological conditions like elevated water temperatures and reduced river stages and flows. The following urgent query arises: could the frequency of such disasters increase in the future, given the findings presented in this study?  
The multifaceted consequences of increased stream temperatures necessitate a holistic and adaptive approach to freshwater ecosystem management. Understanding these complexities is crucial for developing strategies that not only mitigate the immediate impacts but also enhance the resilience of aquatic ecosystems in the face of ongoing climate change.