Thermal stressors

Thermal stressors are caused by alterations to the temperature of a water body. 

Examples of pressures causing thermal stressors include: 

• climate change causing increased water and air temperatures and alterations to the thermal layering of lake ecosystems,
• reductions in ice, glacier and permafrost cover in mountainous and Arctic regions as a result of climate change, causing cold meltwater inputs to water bodies,
• reductions in ice cover on water bodies during cold periods of the year,
• ‘hydropeaking’ flows of water intermittently emitted from hydropower projects often accompanied by cooling or warming of the water body downstream (cold or warm thermopeaking, respectively)
• point emissions of warm water from industrial cooling processes (warm thermopeaking),
• hydrological alterations such as agricultural abstraction causing low water levels which rapidly warm,
• the removal of riparian woodland and vegetation causing a lack of shading along river and lake margins.

Effects on ecological status of aquatic ecosystems

Thermal stressors can cause a number of impacts on the status of aquatic ecosystems – particularly lakes – and the ecosystem services they provide. 

Phytoplankton communities – the microscopic organisms that underpin most aquatic food webs – are particularly sensitive to temperature alterations. Warming shallow lakes may experience a shift from phytoplankton to cyanobacteria dominance, with resulting reductions in water quality, food web structure and ecosystem productivity and diversity.

Fish populations – increased water temperatures are likely to reduce the available habitat and spawning success for cold-water fish species (e.g. brown trout), particularly in lakes where warming causes stratification of thermal layers in the water column. The growth and productivity of warm-water fish species (e.g. carp) is likely to increase in warming water bodies, potentially leading to shifts in ecosystem community composition, sometimes in favour of invasive species better adapted to warmer conditions.

Macroinvertebrates – an abrupt change in temperature caused by releases from hydropower plants is likely to result in behavioural drift with taxon-specific effects. Taxa respond quickly to cold thermopeaking during warm season and more pronounces drifting occurs (Carolli et al. 2012).

Oxygen deficiencies and physiological stresses – warming water temperatures can hold progressively less dissolved oxygen. As a result, warming water can become oxygen deficient (or ‘hypoxic’), which causes organisms to undergo physiological stress as they are forced to respire faster or potentially suffocate, or to migrate to other connected water bodies.

Common combinations as multiple stressors

Thermal stressors are most commonly observed to act in combination with nutrient stressors, most frequently in lake ecosystems. Increased water temperatures can act synergistically with nutrient loadings to cause eutrophication and algal blooms, particularly in shallow lakes. In rivers, thermal stressors are most commonly paired with hydrological stressors. This is likely to be a result of the two stressor types interacting: where river flows and lake levels are reduced and flows slowed, it is likely that water temperatures will rise in response to increasing air temperatures. (Nõges et al. 2015). Furthermore, instream releases from hydropower plants can result in an abrupt change of thermal conditions, either by releasing cold hypolimnetic water (cold thermopeaking) or warm cooling water (warm thermopeaking) (Schuelting et al. 2016).

Examples of ecological effects

Sporadic fish kills in the shallow Lake Võrtsjärv in Estonia sometimes occur in cold winters. Yet in June 2013 an unexpected massive fish-kill occurred and affected mainly the bottom-dwelling ruffe (Gymnocephalus cernuus), which indicates that the fish kill started at the lake bottom. Analysis of environmental monitoring parameters concludes that quick warming of the lake induced a combination of extreme water parameters (high water temperature, alkaline pH, low dissolved oxygen concentration) close to the lake bottom and temporary thermal stratification.  Bottom-dwelling fish such as ruffe do not seek refuges during periods of deoxygenation. This fish kill highlights the impact of unsuitable climate events on fish population and fish community structure (Kangur et al. 2016).

Effects of peak flows resulting from hydropower operation and extreme rainfall are expected to vary with river channelisation and climate warming. Established in Austria, the "Hydromorphological and Temperature Experimental Channels" (HyTEC) is used for testing single and combined effects of river flow, riverbed morphology and temperature on fish. HyTEC consists of two large channels (40 m length, 6 m width) fed with nutrient-poor lake water taken at different depths to vary water temperature. A study by Schuelting et al. 2016 at HyTEC indicates significant differences between macroinvertebrate drifting behaviour in response to hydropeaking, thermopeaking and time of day (Freshwater Blog 2016). 

Freshwater ecosystems in semi-arid Mediterranean climates are projected to be particularly affected by climate-induced droughts in the future.  According to a study by Jeppesen et al. 2015, changes in water levels and salinity can have significant effects on Mediterranean lake ecosystems, nutrient dynamics, nutrient concentrations and water quality (Freshwater Blog 2017).

Further reading

MARS publications:

Jeppesen E., Brucet S., Naselli-Flores L., Papastergiadou E., Stefanidis K., Nõges T., Nõges P., Attayde J.L., Zohary T., Coppens J., Bucak T., Fernandes Menezes R., Sousa Freitas F.R., Kernan M., Søndergaard, M. & M. Beklioğlu (2015). Ecological impacts of global warming and water abstraction on lakes and reservoirs due to changes in water level and related changes in salinity, Hydrobiologia 750: 201-227. DOI: 10.1007/s10750-014-2169-x (Read abstract)

Kangur K., Ginter K., Kangur P., Kangur A., Nõges P. & A. Laas (2016). Changes in water temperature and chemistry preceding a massive kill of bottomdwelling fish: an analysis of high-frequency buoy data of shallow Lake Võrtsjärv (Estonia), Inland Waters, 6(4): 535-542.  DOI: 10.5268/IW-6.4.869 (Download report, 1mb)

Nõges P., Argillier C., Borja Á., Garmendia J.M., Hanganu J., Kodeš V., Pletterbauer F., Sagouis A. & S. Birk (2016). Quantified biotic and abiotic responses to multiple stress in freshwater, marine and ground waters. Science of the Total Environment, 540: 43-52. DOI: 10.1016/j.scitotenv.2015.06.045 (Read abstract)

Schuelting L., Feld C. & W. Graf (2016). Effects of hydro- and thermopeaking on benthic macroinvertebrate drift. Science of The Total Environment, Volume 573: 1472-1480. DOI: 10.1016/j.scitotenv.2016.08.022 (Read abstract)

Reports and publications:

Carolli M., Bruno M.C., Siviglia A. & B. Maiolini (2012). Responses of benthic invertebrates to abrupt changes of temperature in flume simulations, River Res. Appl., 28 (6): 678–691. DOI: 10.1002/rra.1520 (Read abstract)

Selected Freshwaterblogs:

Freshwaterblog (2017). Lake Restoration and Management in a Changing Climate (External website)

Freshwaterblog (2016). Investigating the effects of water releases from hydropower on Alpine stream ecosystems (External website)