Consequences of short-term water temperature variability to fish: Current and future climate change impacts

Water temperature has a profound influence on aquatic ectotherms by affecting all aspects of biological organization; from chemical and molecular functioning to whole-organism and population-level impacts. Natural processes and anthropogenic activities can create conditions where temperatures vary greatly through space and time. While exposure to temperature changes lasting ≥ 24 hours have been examined in some species, it is unclear how more rapid (\u3c 24 hours; sub-daily) thermal fluctuations affect aquatic ectotherms. I used a combination of field observations, laboratory experiments, and modeling simulations to understand: 1) how habitat quality of aquatic ectotherms is affected in thermally dynamic environments; 2) the role of sub-daily temperature fluctuations on growth, survival, and stress responses in juvenile and adult fish; 3) how fish early life stages are affected by sub-daily temperature fluctuations; and 4) what impact thermally dynamic environments have on a model species at a population-level. A model quantifying habitat quality around power plant thermal discharges indicated that elevated and variable discharge temperatures affected habitat quality over a relatively small spatial area for aquatic ectotherms. Models examining elevated temperatures representing climate warming showed that the effects of industrial discharges and climate warming could have an interactive effect on habitat quality by increasing the spatial area and duration over which industrial thermal effluents impact aquatic ectotherms. Laboratory experiments indicated that closely related fishes can respond differently to the same sub-daily temperature fluctuations. Yellow perch (Perca flavescens) had higher consumption and growth under sub-daily temperature fluctuations but developed skin lesions; an indication of thermal stress. In contrast, these same fluctuations reduced growth in physiologically similar walleye (Sander vitreus) but did not induce additional stress responses. Experiments examining early life stages of fathead minnows (Pimephales promelas) revealed that survival and developmental rates were not affected by sub-daily temperature fluctuations, which may be due to the cyclical gene expression of the heat-shock protein hsp70. These experiments also showed that long-term exposure of juveniles to high-magnitude, sub-daily fluctuations can induce non-directional sex reversal, thereby potentially affecting population sex ratios and population-level reproductive success. Finally, a spatially-explicit, individual-based model of smallmouth bass (Micropterus dolomieu) near a power plant thermal discharge revealed that the combined effects of thermal effluent and impingement mortality caused by power plant operations can have minimal effects on population-level characteristics, including prey consumption rates, growth rates, and population abundance. Observations and climate change projections indicate that the magnitude and frequency of temperature variability in many regions of the world will increase in the future. Therefore, it will become increasingly important to explore in detail how aquatic ectotherms are affected by thermal variation across multiple spatial and temporal scale