Water quality refers to chemical, physical, and biological characteristics determining its suitability for various uses and ecosystem health. Temperature influences these characteristics. Understanding this relationship is important for maintaining healthy aquatic environments, supporting human well-being, and managing water resources sustainably.
Physical Changes in Water
Water temperature alters its physical properties, influencing aquatic environments. Water density decreases as temperature rises, particularly above 4 degrees Celsius. This leads to thermal stratification in lakes and reservoirs, forming distinct layers. Stratification limits mixing and oxygen transfer to deeper layers, as warmer, less dense water stays at the surface while cooler, denser water settles below.
Temperature inversely affects dissolved gas solubility, especially dissolved oxygen (DO). Warmer water holds less dissolved oxygen, essential for aquatic organisms. For example, oxygen solubility at 20°C is 9.09 mg/L, dropping to 7.63 mg/L at 30°C. This oxygen reduction can stress aquatic life. Temperature also influences water’s viscosity, decreasing as temperature increases, affecting organism and particle movement.
Biological Consequences for Aquatic Life
Temperature impacts biological processes and aquatic life survival, an environmental variable. Warmer water increases metabolic rates of fish, invertebrates, and microorganisms, leading to higher dissolved oxygen demand. Low oxygen levels from reduced solubility, combined with increased demand, create physiological stress for aquatic species. Many fish species require specific temperature ranges for growth and survival; deviations impair their health.
Temperature controls reproductive cycles and developmental stages of aquatic life. It influences breeding timing, egg hatching success, and larval and juvenile survival. Small deviations from ideal temperatures disrupt these processes, reducing reproductive success or causing population declines. Species have varying temperature tolerances; changes alter distribution, favoring tolerant species while displacing sensitive ones.
Elevated water temperatures compromise aquatic organisms’ immune systems, increasing susceptibility to diseases and parasites. For example, fish diseases, like columnaris disease, become more prevalent in warmer waters. Warmer temperatures, with increased nutrients, promote harmful algal blooms (HABs). These blooms deplete oxygen, produce toxins, and block sunlight, harming aquatic ecosystems and water quality.
Chemical Reactions and Pollutant Behavior
Temperature influences water’s chemical environment, affecting reaction rates and compound behavior, including pollutants. Increased temperature accelerates most chemical reactions within water bodies. This includes organic matter decomposition, which consumes dissolved oxygen and worsens oxygen depletion in warmer waters. Nutrient cycling speed, such as nitrification and denitrification, is temperature-dependent, affecting element availability for aquatic organisms.
Solubility of chemical compounds, including many pollutants, is affected by temperature. For example, some heavy metals’ solubility increases with rising temperatures, making them more bioavailable and toxic to aquatic life. Pesticide effectiveness and persistence is altered, affecting degradation rates and environmental impact. This increases organism exposure, even if pollutant concentration remains constant.
Temperature affects ammonia toxicity, a common aquatic pollutant. While total ammonia might not change, warmer water shifts equilibrium towards the un-ionized form (NH3), which is more toxic than the ionized form (NH4+). For example, an increase in temperature from 20°C to 25°C doubles toxic un-ionized ammonia at a neutral pH. Temperature also influences water pH by affecting carbon dioxide solubility and reaction rates.
Sources of Temperature Changes
Water temperature changes stem from natural processes and human activities. Natural variations include seasonal changes, where temperatures fluctuate with air temperatures and solar radiation. Daily solar radiation contributes, in shallower waters, causing diurnal temperature swings. In some geological areas, natural heat sources like hot springs elevate water temperatures.
Human activities contribute to thermal changes, resulting in thermal pollution. Industrial discharges, particularly from power plants and manufacturing, are a source. These facilities use large volumes of water for cooling, discharging heated water into rivers, lakes, or coastal areas at higher temperatures. Urban runoff is another source; stormwater flows over impervious surfaces like roads and rooftops, absorbing solar heat and carrying warmed water into nearby bodies.
Deforestation and riparian vegetation removal along riverbanks contribute to increased water temperatures. Loss of shade from trees exposes water directly to solar radiation, leading to warming in smaller streams. Global climate change, driven by greenhouse gas emissions, increases air temperatures, warming surface waters worldwide. This warming trend exacerbates physical, biological, and chemical impacts on water quality.
Managing Thermal Impacts
Addressing temperature change effects on water quality requires regulations, technology, and ecological restoration. Regulatory bodies implement environmental regulations, like the Clean Water Act in the United States, to limit industrial discharge temperatures. These regulations prevent thermal shock and stress to aquatic ecosystems. Industries must obtain permits specifying maximum allowable effluent temperature, requiring cooling before discharge.
Technological solutions reduce industrial wastewater temperature before it re-enters natural systems. Cooling towers, for example, dissipate heat from industrial process water into the atmosphere, rather than into a river or lake. Cooling ponds serve a similar purpose, allowing heated water to cool naturally over a large surface area before discharge or reuse. These technologies are important for industries relying on large volumes of water for cooling, such as power generation.
Restoring riparian zones along waterways is an effective ecological strategy. Planting vegetation, trees and shrubs, along riverbanks provides shade, reducing direct solar heating. This maintains cooler water temperatures, beneficial for temperature-sensitive species like trout and salmon.
Improvements in wastewater treatment indirectly mitigate thermal impacts by reducing organic load in discharged wastewater. Lower organic loads mean less oxygen is consumed during decomposition, maintaining higher dissolved oxygen levels, important in warmer waters where oxygen solubility is reduced. Continuous monitoring and scientific research are important for understanding and predicting thermal impacts, guiding effective management.