Temperature has long been recognized as an important environmental factor in both terrestrial and aquatic ecosystems in regard to its pivotal role over biological activity (development, growth and reproduction). Seasonal temperature differences strongly influence the biological activity of aquatic organisms and establish cyclical patterns that often mediate the scope of each activity. Over the long-term, changes to the thermal regime of the surrounding environment can effect the evolutionary, physiological and behavioural responses of individual organisms (Begon et al. 1990).
Thermal regimes can be expected to vary with latitude but, in general, vary daily and on a seasonal basis; the cyclical pattern of diel variation in temperate regimes demonstrates an early morning low and a mid-afternoon high with mean daily temperatures reaching a maximum during summer. Moreover, the amplitude of diel variation (i.e. delta T degrees C) is also greatest during summer and may be affected by the degree of riparian development (vegetation removal that promotes channel widening or reduces channel shading). To this end, local adaptation to thermal regimes by aquatic organisms can also be expected to vary over a species geographical distribution (i.e. latitude) and as a function of environmental condition (i.e. the basin's development history).
The causes of temperature increases in aquatic ecosystems due to human activities in British Columbia, then, are associated with either active or passive heating of the receiving environment. The response of aquatic biota to physical changes in light or photoperiod, nutrients, temperature and flow can often lead to profound changes in individual development and survival or community structure across each trophic level (Barnes and Minshall 1983; Resh and Rosenberg 1984; Stevenson, Bothwell and Lowe 1996). The response of marine and freshwater algae to temperature change have been summarized by DeNicola (1996) and include a variety of effects that have been studied at the cellular, population and community level. Individual responses are highly dependent upon the variability in the physicochemical environment and spatio-temporal pattern in species distribution. Physiological responses to temperature include changes in concentrations of photosynthetic and respiratory enzymes, changes in cell quota and nutrient uptake, as well as alterations in fatty acids and proteins. Individual populations have been shown to exhibit minimum, maximum and optimal temperatures for growth that contribute to species composition and diversity and eventually lead to seasonal succession. Several broad generalizations concerning periphyton responses to temperature have been summarized by DeNicola as follows: 1) as temperature increases, there is a shift in the dominance of algal classes from diatoms (<20 degrees C) to green algae (15-30 degrees C) to blue-green algae (>30 degrees C). 2) species diversity increases from approximately 0 to 25 degrees C and decreases at temperatures > 30 degrees C. 3) the degree to which community composition changes with thermal input depends on the initial ambient temperature. Increases in temperature in environments near 25-30 degrees C usually cause greater changes in community structure than in environments <25 degrees C. 4) community structure usually recovers rapidly (< 1 yr) when temperature stress is discontinued. 5) biomass increases with temperature from approximately 0-30 degrees C, and decreases at 30-40 degrees C. 6) in many natural communities, temperature does not usually limit biomass and primary productivity, but it does set an upper limit for production when other factors are optimal. Maximum areal productivity of lotic periphyton increases exponentially with temperature for temperatures <30 degrees C. 7) the degree to which primary productivity is limited by factors such as light, nutrients, and grazing depends on temperature.
With respect to aquatic insect community response, temperature fluctuations beyond threshold levels can have a dramatic effect on diapause induction (i.e., as a function of endocrine processes; Vannote and Sweeney 1980), hatching success (i.e., decreases at low or high extremes; Elliott and Humpesch 1980), larval growth, adult size and fecundity (i.e., both temperature and nutrition influence on the rate of feeding, assimilation and respiration, food conversion efficiencies and enzymatic kinetics; Anderson and Cummins 1979; Vannote and Sweeney 1980; Sweeney and Vannote 1981), voltinism (i.e., number of generations per year based on larval growth rate; Newell and Minshall 1978) and timing of adult emergence (i.e., premature or delayed depending on temperature increase or decrease; Sweeney and Vannote 1981).
In regard to salmonid community response, increases in surface water temperature beyond diurnal or seasonal averages, have the potential to accelerate embryo development, alter the timing of emergence, growth and downstream migration of juveniles, reduce metabolic efficiencies of food conversion into growth (i.e., due to thermal stress and oxygen deficiency), alter adult spawning migration and spawning timing, increase susceptibility to disease and shift the competitive advantage of salmonids over non-salmonid species (Hicks et al. 1991; De Staso III and Rahel 1994; Flebbe 1994; Dickerson and Vinyard 1999). Sub-lethal temperature effects are also related to metabolic inefficiencies, susceptibility to disease and toxic effects of pollutants, behavioural patterns, intra- and inter-specific competition, predator-prey relationships, community composition and parasite-host relationships.
The negative effects of thermal regime modification are not restricted to temperature increases alone. Temperature reductions to stream environments in winter may result from the loss of insulation of the forest canopy under clear-cut conditions in combination with increased radiant cooling; accelerated freezing may lead to anchor ice formation and minimize interstitial habitat for juvenile fish (Hicks et al. 1991).
The physiological effects of extremely high as well as extremely low temperatures on aquatic organisms are reviewed in CCME (1999). From that document, "...potential effects of extremely low water temperatures on aquatic organisms include insufficient integration of nervous and metabolic processes, insufficient rates of energy liberation, changes in water and mineral balance, increase in osmoconcentration resulting from extracellular freezing followed by the dehydration of cells, liquefaction of cortical protoplasm and gelation of the cell interior...the effects of extremely high temperatures include insufficient supply of oxygen, failures in process integration, desiccation (inter-tidal organisms), enzyme inactivation, change in lipid state, increase in protoplasmic viscosity, increase in cell membrane permeability, protein denaturation, and release of toxic substances from damaged cells...Death can result from exposure to either extremely high or extremely low water temperatures..."
The apparent benefits of increased light and temperature in affected streams that contribute to improved embryo development (shorter incubation time, earlier fry emergence) and juvenile growth rates (longer growing season, greater food availability) in salmonids may have negative long-term implications on later life history stages (Hartman and Scrivener 1990). For example, anadromous species such as chum salmon that migrate to the ocean following an earlier spring emergence, may be at a disadvantage if the timing of food availability in the marine/estuarine environment is lagging. Similarly, early emergence of coho fry that leads to a longer period of summer growth may increase under-yearling size and improve over-winter survival, but may induce a higher emigration of 1+ smolts the following spring that exhibit a lower ocean survival. A lower survival for stream resident trout may also occur if the period of tributary rearing is reduced by accelerated growth, yet the habitat requirements of smaller-sized fish are unavailable in mainstem channels. The greater dilemma for stream resident species may be related to summer temperatures near their upper tolerance limits that force individuals to seek cooler refuges in headwater areas that exhibit a lower capability for food and shelter when compared to the more productive downstream reaches (Vannote et al. 1980).
A comprehensive review of the biological effects of water temperature on marine and brackish water organisms has been summarized in the CCME-Canadian Water Quality Guidelines for the Protection of Aquatic Life, Temperature (CCME; 1999). Sub-lethal temperature effects are similarly related to metabolic inefficiencies, susceptibility to disease and toxic effects of pollutants, behavioural patterns, intra- and inter-specific competition, predator-prey relationships, community composition and parasite-host relationships.