Beyond the mean: The importance of variability in predicting ecological impacts of stream thermal regimes

Background/Question/Methods

Water temperature is a key regulator of aquatic communities. Human activities, such as dams, land-use, and climate change, impact the variability and complexity of water temperature regimes; yet, biological responses to these altered thermal regimes are just beginning to be understood.  Alterations in thermal regimes may include not only increases or decreases to means, minimums, and maximums but reductions or increases in variability at multiple temporal scales. Changes in variance are often overlooked or poorly studied because science and statistics have focused on mean effects and lethal effects.  Yet, changes in variance have the potential for dramatic sub-lethal effects on food web dynamics, species phenology, and community composition. Understanding these impacts is essential for effective management of aquatic systems; it will require attention to the complexities of natural thermal regimes, a better understanding of how humans impact thermal regimes, and research on the biological and physiological implications of thermal fluctuations.

Results/Conclusions

As background to our experiment, we present data from a relatively undisturbed floodplain on the Sauk River, Washington.  We summarize 7 years of thermal data on multiple channels within that floodplain to examine natural temporal and spatial variation in water temperature regimes. We also review research on how dams and climate change can reduce variability at often-overlooked temporal scales. We then report results from a laboratory experiment measuring the impact of altered thermal regimes on the egg to fry lifestages of Chinook salmon.  We developed eight thermal regimes that differed is strategic ways.  For example, there was a thermal regime at a relatively stable 5C and another at a relatively stable 10C which can be compared with regimes that fluctuated from 5C to 10C every day and that fluctuated from 5C to 10C twice daily.  We exposed eggs from eight genetic lineages to these thermal regimes and observed weight, length, and maturity at emergence as well as emergence timing.  Results suggest that while there are minor differences in length and weight across thermal regimes, emergence timing can be altered substantially not only by total accumulation of degree days but also by the pattern in which those temperature units are delivered.  Furthermore, there are indications of genetic variation in sensitivity to thermal patterns.