# Projections of Climate Change Impact on Stream Temperature: A National-Scale Assessment for Poland
## 1 Introduction  
The temperature of streams plays a crucial role in shaping the health and vitality of freshwater ecosystems [1]. It profoundly influences the metabolic rates, growth, and reproductive behaviours of aquatic organisms, ranging from fish to invertebrates. It directly affects the solubility of oxygen in water, with warmer temperatures reducing the amount of dissolved oxygen available for aquatic life [2]. Furthermore, stream temperature influences the rates of biological and chemical processes, such as nutrient cycling and microbial activity [3]. Many species of fish and macroinvertebrates have specific temperature requirements for optimal development and survival, and deviations from these thresholds can lead to adverse effects on their populations [4, 5]. Elevated temperatures can exacerbate stress on aquatic organisms, potentially triggering shifts in species composition and disrupting the delicate balance of the ecosystem [6]. Monitoring and managing stream temperature are thus essential for the preservation and sustainable management of freshwater ecosystems, ensuring their resilience in the face of environmental changes. As climate change progresses, the consequences for our environment are becoming increasingly pronounced, reshaping the balance of Earth's natural systems and raising urgent concerns for the future. The anticipated effects of climate change on the environment, biodiversity, and freshwater ecosystems are poised to escalate, creating challenges for the balance of Earth's interconnected ecosystems [7]. The acceleration of rising global temperatures, coupled with altered precipitation patterns and an increased frequencyof extreme weather events, is expected to exacerbate these challenges [8]. This shift in temperature dynamics is expected to exert considerable stress on biodiversity, as many species may struggle to adapt or face habitat loss [9]. In freshwater ecosystems, the consequences could be particularly severe, with shifts in water availability, heightened incidences of droughs and floods, and the potential for widespread disruptions to aquatic habitats [10]. The scientific literature has demonstrated a notable emphasis on studying the impact of climate change on water discharge compared to the exploration of its effects on stream temperature [11; 12; 13; 14]. The heightened attention to water discharge is primarily attributed to its direct relevance in water resource management, ecosystem functioning, and human activities such as agriculture and water supplies [15]. While the importance of stream temperature in understanding aquatic ecosystems and water quality is acknowledged, it has historically received relatively less attention compared to water discharge [16]. Nevertheless, the projections of climate change impact on stream temperatures indicate a range of significant and far-reaching consequences for freshwater ecosystems. As global temperatures continue to rise, streams are expected to experience warmer water temperatures, which can disrupt the thermal balance critical for aquatic life [17; 18]. Increased air temperatures contribute to elevated stream temperatures, affecting the distribution and abundance of cold-water species, such as trout and salmon, as they may face reduced suitable habitats [19]. Moreover, higher temperatures can intensify thermal stratification, leading to decreased dissolved oxygen levels in deeper waters, adversely affecting fish and invertebrate populations [20]. These shifts in stream temperatures pose a threat to biodiversity, potentially triggering species migrations, declines, or even local extinctions [21]. Addressing the projected impacts of climate change on stream temperatures is imperative for the conservation and adaptive management of freshwater ecosystems in the face of this global environmental challenge. The projections of climate change impact on stream temperatures underscore the urgency of advancing our understanding and modelling capabilities to address challenges for freshwater ecosystems.