International Journal of Aquaculture, 2024, Vol.14, No.4, 221-231 http://www.aquapublisher.com/index.php/ija 222 knowledge gaps and research needs that must be addressed to enhance our understanding and management of aquatic ecosystems under current and future climate change scenarios. 2 Impacts of Climate Change on Aquatic Ecosystems 2.1 Temperature variations and thermal stress 2.1.1 Effects on fish physiology and behavior Climate change has led to significant increases in water temperatures, which directly affect fish physiology and behavior. Elevated temperatures can alter fish metabolism, growth rates, and reproductive cycles. For instance, fish species in subarctic freshwater ecosystems are experiencing changes in their behavior, habitat use, and growth due to increased ambient water temperatures (Rolls et al., 2017). Fish are undergoing evolutionary adaptations to cope with temperature extremes, which include changes in tolerances to high temperatures and shifts in sex ratios in species with temperature-dependent sex determination (Scheffers et al., 2016). 2.1.2 Changes in metabolic rates Increased water temperatures result in higher metabolic rates in fish, which can lead to increased energy demands and altered feeding behaviors. Studies have shown that the overall effects of climate change on fish growth are predominantly negative, with higher temperatures leading to increased metabolic costs and reduced growth rates (Menden-Deuer et al., 2023). This is particularly evident in freshwater ecosystems, where fish are less studied compared to their marine counterparts, but the negative impacts on physiology and health are consistent across different species and habitats (Huang et al., 2021). 2.1.3 Shifts in species range and distribution As water temperatures rise, many fish species are shifting their geographic ranges to maintain their preferred environmental conditions. This often involves moving poleward or to higher elevations in search of cooler waters. Such shifts can lead to changes in species distribution and abundance, with warm-adapted species expanding their ranges and cold-adapted species experiencing range contractions (Scheffers et al., 2016; Pecl et al., 2017). These distributional changes can disrupt existing ecological communities and lead to the formation of novel biotic interactions. 2.2 Alterations in hydrological cycles Climate change is also modifying hydrological cycles, affecting the availability and distribution of freshwater resources. Changes in precipitation patterns, streamflow, and water levels are altering the hydrological regimes of aquatic ecosystems. For example, subarctic freshwater ecosystems are experiencing increased nutrient availability and shortened ice cover periods, which impact the ecological responses of freshwater fishes. These alterations can lead to changes in spawning and recruitment dynamics, ultimately affecting species abundance and distribution (Rolls et al., 2017). 2.3 Ocean acidification and its consequences Ocean acidification, driven by increased atmospheric CO2 levels, is another significant impact of climate change on aquatic ecosystems. Acidification affects the physiology and behavior of marine organisms, particularly those with calcium carbonate structures, such as corals and shellfish. The combined effects of warming, acidification, and deoxygenation are intensifying the impacts of harmful algal blooms (HABs) in marine and freshwater ecosystems, further stressing aquatic organisms (Figure 1) (Griffith and Gobler, 2020). These changes can have cascading effects on food webs and ecosystem services. Harmful algal blooms not only release toxins that are harmful to humans and other organisms but also lead to a reduction in dissolved oxygen in the water, creating low-oxygen or even hypoxic zones, which can be fatal for aquatic organisms that rely on oxygen for survival. As climate change intensifies, the frequency and severity of these harmful algal blooms are likely to increase further, particularly in water bodies already affected by pollution and eutrophication, such as coastal areas and freshwater lakes. Research has shown that in environments with high carbon dioxide concentrations, the toxicity of some algae may increase, leading to more widespread biological
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