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CADDIS Volume 2: Sources, Stressors & Responses


Sources and activities that suggest listing temperature as a candidate cause

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.

Picture of a cooling water tower on the shore of Lake Michigan.  Source: National Biological Information Infrastructure, http://images.nbii.gov/environments/nbii_t00219.jpg.
Figure 3. A cooling tower lowers the temperature of power plant cooling water to avoid a heated discharge.
Courtesy of National Biological Information Infrastructure.
Discharge of heated water can directly modify downstream temperatures, as well as indirectly affect temperature by altering water volume and velocity (Figure 3).

Picture of a stream with riparian vegetation removed.  Source: Alyson Sappington, Thomas Jefferson Soil and Water Conservation District.
Figure 4. Riparian devegetation may change water temperature.
Courtesy of A. Sappington, Thomas Jefferson Soil and Water Conservation District.
Removal of riparian vegetation (Figure 4) allows increased solar radiation to reach the stream surface, directly increasing water temperature. Removal of trees also decreases evapotranspiration, which reduces evaporative cooling. Vegetation removal also may reduce the insulating capacity of these buffers, due to increased longwave radiation from surface water to atmosphere, decreased longwave radiation from canopy to surface water, or increased wind speed; each of these factors may contribute to lower minimum temperatures. In addition, destabilization of stream banks can lead to increased channel widths, decreased depths, and associated temperature changes (Moore et al. 2005).

Picture of a deforested mountain side.  Source: U.S. Fish & Wildlife Service, Digital Library System, Image No. WO4912; http://images.fws.gov/.
Figure 5. Upland devegetation indirectly increases water temperatures.
Courtesy of U.S. Fish & Wildlife Service.
Removal of upland vegetation (Figure 5) can result in increased surface runoff with elevated temperatures due to increased heat absorption across the landscape. Higher surface soil temperatures, increased sedimentation and channel alteration, and increased or decreased groundwater inputs also may affect stream temperatures (Moore and Wondzell 2005, Johnson and Jones 2000).

Picture of cars on pavement.  Source: Water Quality Protection in Agroecosytems, http://www.iowawaterquality.org/
Figure 6. Impervious surfaces can heat or cool stormwater runoff.
Impervious surfaces (Figure 6) can rapidly delivery of large volumes of warm or cold stormwater runoff, which can increase water temperatures in the summer and decrease them in the winter (Galli and Dubose 1990).

Picture of a channelized stream.  Source: U.S. EPA, Mid Atlantic Integrated Assessment, http://www.epa.gov/maia/html/md-phys3.html.
Figure 7. Channel alterations may include straightening and widening the channel.
Courtesy of U.S. EPA Mid Atlantic Integrated Assessment.
Channel alteration (Figure 7) can change flow regimes, reduce stream-floodplain connectivity, and reduce groundwater discharge. Changes in groundwater discharge affect stream temperature by reducing the capacity of streams to buffer changes in heat loads from other sources (Paul and Meyer 2001, Poole and Berman 2001, LeBlanc et al. 1997, Klein 1979).

Picture showing a small dam on Redland Ck, TN.   Source: U.S. Fish & Wildlife Service, Digital Library System, Image No. WO8369, http://images.fws.gov/.
Figure 8. Temperature changes caused by impoundments vary based on design.
Courtesy of U.S. Fish & Wildlife Service.
Impoundments or dams can directly increase or decrease downstream temperatures, depending on the type (i.e., surface or bottom) and timing of water releases (Figure 8). Upstream of impoundments, associated hydrologic changes may alter temperatures, due to greater stream widths and retention times. In general, discharges from shallow impoundments that do not stratify will increase the maximum summer temperature of the water. Alteration of upstream and downstream sediment loads also may lead to channel alteration and subsequent temperature changes (Webb and Walling 1993, 1996).

Picture showing irrigation of agricultural land.  Source: U.S. Department of Agriculture, http://www.usda.gov/oc/photo/85cs0114.htm.
Figure 9. Water withdrawal for irrigation.
Courtesy of U.S. Department of Agriculture.
Removal of water from surface or groundwaters for agricultural (Figure 9), industrial, or municipal uses can affect stream thermal regimes. For example, surface water withdrawals can reduce stream flow, leading to accelerated heating and higher temperatures; reduced baseflow due to groundwater withdrawals may lead to greater diurnal variation in stream temperatures (Poole and Berman 2001, LeBlanc et al. 1997).

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.

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Site evidence that suggests listing temperature as a candidate cause

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.

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Biological effects that suggest listing temperature as a candidate cause

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.

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Site evidence that supports excluding temperature as a candidate cause

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.

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