CADDIS Volume 2: Sources, Stressors & Responses
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Many human activities can change the temperature or modify the thermal regime of waterbodies and, subsequently, the structure and function of aquatic ecosystems (Poole and Berman 2001, Galli and Dubose 1990). Mechanisms underlying thermal modifications range from simple, direct relationships (e.g., discharge of heated cooling water) to complex, indirect relationships (e.g., bank erosion leading to increased channel width, increased solar radiation, reduced stream flow, and greater conductive heating). Climate change may also affect water temperature via several interrelated causal steps (e.g., changes in air temperature, humidity, cloud cover, precipitation quantity and intensity, vegetative composition and cover). However, a discussion of climate change impacts on aquatic ecosystems is beyond the scope of this module, because of its complexity and broad spatial and temporal scale.
Sources of altered thermal regimes are best evaluated by considering them in context of the atmospheric and hydrologic processes that influence temperature in streams and rivers (Figure 1, on Introduction tab). Dominant sources of heat flux into and out of streams include solar radiation, groundwater input, upstream temperature and flow, atmospheric exchange (via convection and evaporation), streambed conduction, and longwave radiation. Additional heat exchange processes not depicted in Figure 1 may be important in specific streams or situations (e.g., friction generated by rapidly moving water in steep-sloped streams).
Importantly, many naturally occurring environmental conditions can substantially modify the impact of these heat flux processes on stream temperature. For example, riparian cover can greatly reduce the amount of solar radiation that reaches the stream surface, particularly in small streams. Modifying factors associated with the underlying geology include stream flow, hyporheic exchange, channel morphology and complexity, and upland vegetation. Other modifying factors include relative humidity and wind speed.
Some common anthropogenic sources and activities that modify temperature are described below.
Sources and activities that influence temperature can also increase other stressors, which also may need to be included on your list of candidate causes. For example, removal of riparian vegetation and increased impervious surfaces can result in increased sediments, altered flow and increased ionic strength. Increased water temperature reduces the amount of oxygen that water can hold and increases the solubility of many ions; for these reasons, consider including reduced dissolved oxygen and increased ionic strength on your list of candidate causes whenever you include increased temperature.
Except for phase changes of water (freezing or vaporization), alterations in water temperature are not directly observable under typical field settings and observers usually must measure water temperature to detect a change. Site observations indicative of temperature change are therefore indirect and largely restricted to observing the presence of one or more sources of thermal modification. For example, because incident solar radiation often has a strong effect on stream temperature, observations of reduced riparian cover over a stream relative to reference conditions may suggest changes in thermal regime. Congregation of coldwater fish near groundwater inputs to streams during summer may indicate suboptimal temperatures outside these zones, or congregation of fish near industrial discharges during winter may indicate the presence of heated discharges.
The effects of modified thermal regimes can occur at various levels of biological organization and involve numerous endpoints. For example, biochemical changes related to temperature include the rates of enzymatic reactions, metabolic processes, and protein synthesis (e.g., heat shock proteins). At the organism level, modified thermal regimes can affect survival, growth rate, gamete production, swimming speed, disease susceptibility, migratory behavior, timing of metamorphosis, and other traits. At the population, community, and ecosystem levels, modified thermal regimes can alter attributes such as population density, age- or size-class structure, predator/prey dynamics, temporal dynamics of populations and communities, species richness, rates of microbial decomposition, and ecosystem productivity. For reviews of the thermal effects literature, see U.S. EPA (2003), U.S. EPA (2001), Beitinger et al. (2000), Galli and Dubose (1990), Cravens and Harrelson (1987), Ward and Stanford (1982), and Vannote and Sweeney (1980).
Biotic responses to altered stream temperatures are often linked to different spatial or temporal attributes of a stream's thermal regime. For example, the timing of fish migration and spawning or the emergence of benthic insects may be initiated by the gradual warming of stream temperatures during spring or the cooling of temperatures in the fall. However, increases in maximum temperatures during summer or decreases in minimum temperatures during winter may be stressful enough to cause acute lethality. The availability of thermal refugia, or small stream areas with thermally preferred habitat, may ameliorate the effects of otherwise lethal thermal conditions, at least over short time periods. Longer-term increases in stream temperatures may make organisms more susceptible to disease.
Aquatic organisms are believed to be adapted to specific thermal regimes or "thermal niches" to maximize their competitive advantage (Johnson and Kelsch 1998 Beitinger and Fitzpatrick 1979, Magnuson et al. 1979, Hokanson 1977). These thermal niches are reflected when scientists classify organisms in terms of thermal preference (e.g., cold water or warm water). However, because water temperatures often fluctuate extensively over different temporal and spatial scales (e.g., diurnally, seasonally, laterally, vertically, and longitudinally), organisms in aquatic ecosystems have developed behavioral and physiological mechanisms to acclimate to or avoid suboptimal temperatures. This can make it difficult to interpret observations of species’ presence and absence. In addition, prior exposure to fluctuating temperatures [i.e., an organism's "thermal history" (Beitinger et al. 2000)] influences an organism's ability to tolerate suboptimal temperatures. This suggests that an organism's thermal history may influence its response to temperature and makes it to difficult to make generalizations.
Although numerous and diverse, the biological effects of modified thermal regimes generally are not sufficiently specific to diagnose thermal modification as the primary causative agent, or to rule out other candidate causes. There are, however, a number of readily observed biological effects that may suggest modified thermal regimes, including:
- Absence of coldwater taxa (e.g., salmonids, stoneflies) in streams where these taxa are known or expected to occur naturally;
- Presence of warmwater taxa in streams where coldwater taxa are known or expected to occur naturally;
- Changes in the onset of certain reproductive or developmental events cued to temperature (e.g., earlier insect emergence or fish migration in warmer waters); and
- Behavioral changes, such as congregation of fish near cold- or warmwater inputs.
Consider including temperature as a candidate cause when you see changes in aquatic community structure or acute biotic effects as described above. Please note, however, that observations of these effects do not confirm a causal relationship, because they may also be caused by other stressors, or by a combination of factors.
There are no site observations that specifically provide evidence of the absence of thermal regime changes. General reasons for excluding a candidate from the list are described in Step 2 of the Step-by-Step guide and in Tips for Listing Candidate Causes.
We strongly caution against using benchmarks of effects (e.g., water quality criteria) as evidence for excluding temperature from your initial list of candidate causes, because different species have different temperature requirements and different sites have different naturally occurring temperatures.