Tradeoffs of warm adaptation in aquatic ectotherms: Live fast, die young?

In the face of a changing climate, questions regarding sub-lethal effects of elevated habitat temperature on the physiology of ectotherms remain unanswered. In particular, long-term responses of ectotherms to the warming trend in tropical regions are unknown, and understudied due to the difficulties in specimen and community traceability. In freshwater lakes employed as cooling reservoirs for power plants, increased physiological stress from high water temperature can potentially alter the community structure of fishes. We employ this highly tractable system to assess how thermal regimes can alter the physiology and ecology of aquatic species. We documented a significantly reduced lifespan, growth performance, and a shift in the age structure towards younger individuals in the thermally- impacted Coffeen Lake population of bluegill (Lepomis macrochirus), compared to a non-impacted control group (Lake Mattoon). Average age calculated for the Lake Mattoon population was 2.42 years, whereas the average age of bluegill from Coffeen Lake was only 0.96 years, and average specimen mass in Lake Mattoon was more than six times that of Coffeen Lake. During laboratory cross-acclimation studies of bluegill from Lake Mattoon at 17.5 and 35.0°C, citrate synthase (CS) activity obtained from white muscle was regulated through acclimation, whereas cold-acclimated specimens exhibited twice the activity at 25°C, if compared to CS activity values from warm-acclimated specimens. This study raises the questions about the causal relationships between physiological performance and habitat temperature, in particular how thresholds in an organism's physiology may modulate their community structure, and consequently their ecological success.