5.2 PHYSICAL FACTORS, WITH EMPHASIS ON BIOTA

Temperature / pH, Alkalinity & acidity / Light & shade / Substratum / Flow & hydraulics / Seasonal variations

Oxygen--
Dissolved oxygen (DO) is a basic requirement for a healthy aquatic ecosystem. Most fish and aquatic insects "breathe" oxygen dissolved in the water column. Typically the concentration of dissolved oxygen in surface water is less than 8 mg/L in tropical regions. The DO concentration is subject to diurnal and seasonal fluctuations that are due, in part, to variations in temperature, photosynthetic activity and river discharge. The maximum solubility of oxygen (fully saturated) ranges from approximately 15 mg/L at 0°C to 7.5 mg/L at 30°C (at sea level). Salinity also reduces the solubility of oxygen in water, and must be considered in the analysis of inland brackish and saline waters. Oxygen solubility declines exponentially as salt concentrations increase. Water absorbs oxygen directly from the atmosphere and from plants as a result of photosynthesis. Natural re-aeration of streams can take place in areas of waterfalls and rapids. Water loses oxygen primarily by respiration of aquatic organisms.

The amount of oxygen available for aquatic life depends on the factors that effect solubility. The saturation concentration of DO is quickly achieved at the air-water interface, and in shallow moving water, it will be relatively consistent throughout the water column. In large and deep freshwater systems, oxygenation depends on circulation by winds, currents and inflows to move aerated water away from the surface. In absence of these mixing factors and at depth, the oxygen levels in deep systems such as a reservoir can become stratified forming layers of differing DO concentrations.

Anthropogenic discharges of oxygen-demanding wastes or excessive plant growth caused by nutrient loading, followed by death and decomposition of vegetative material, can reduce oxygen concentrations. These include discharges from forest harvesting, pulp mills, agriculture, sewage treatment plant effluent, industrial effluents, and impoundments (dams).

Temperature--
Temperature is a measurement of the intensity (not amount) of heat stored in a volume of water. Surface water temperatures naturally occurring in fresh water systems range from 0°C under ice cover to 40°C in hot springs. Natural sources of heat include: solar radiation, transfer from air, condensation of water vapour at the water surface, sediments, precipitation, surface runoff and groundwater. Temperature is the primary influencing factor on water density. Water achieves its maximum density at 4ºC, which explains why ice floats on water.

Temperature affects the solubility of many chemical compounds and can therefore influence the effect of pollutants on aquatic life. Increased temperatures elevate the metabolic oxygen demand, which in conjunction with reduced oxygen solubility, impacts many species.
Vertical stratification patterns that naturally occur in lakes affect the distribution of dissolved and suspended compounds.

Anthropogenic sources of pollutants that can impact water temperature include industrial effluents, agriculture, forest harvesting, urban developments, and mining.

pH, Alkalinity, and Acidity--
Alkalinity, acidity and buffering capacity are important characteristics of water that affect its suitability for biota and influence chemical reactions. The acidic or basic (alkaline) nature of water is commonly quantified by the negative logarithm of the hydrogen ion concentration, or pH. A pH value of seven represents a neutral condition; a pH value less than seven is acidic, and more than seven is basic. Many biological processes, such as reproduction, cannot function in acidic or basic waters. Acidic conditions also aggravate toxic contamination problems through increased solubility, leading to the release of toxic chemicals in stream sediments.

The pH of runoff reflects the chemical characteristics of precipitation and chemical composition of the soils it passes through. The dominant ion in most precipitation is bicarbonate (HCO₃^-). The bicarbonate ion is produced in the atmosphere when carbon dioxide reacts with water:

H₂O + CO₂ « H⁺ + HCO₃^-

The reaction produces a hydrogen ion (H⁺), thus increasing the hydrogen ion concentration and the acidity and lowering the pH. For this reason most rainwater is slightly acidic, with a pH of approximately 5.6. The presence of other gasses, associated with air pollution such as sulphur and nitrogen oxides, can further increase the acidity of rainwater. The buffering capacity of water is a measure of how resistant water is to changes in pH. Alkalinity refers to the acid neutralizing capacity of water. The amount of buffering is related to alkalinity and is primarily determined by carbonate and bicarbonate concentrations.

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