Chemical and Physical Properties of a Water Body

Water bodies, both freshwater and saline water can be generally characterized with a number of attributes. In this section are a number of some of these attributes and brief descriptions of them.

Temperature: Temperature affects the speed of chemical reactions, the rate at which algae and aquatic plants photosynthesize, the metabolic rate of other organisms, as well as how pollutants, parasites, and other pathogens interact with aquatic residents.

Temperature is important in aquatic systems because it can cause mortality and it can influence the solubility of dissolved oxygen (DO) and other materials in the water column (e.g., ammonia). Water temperatures fluctuate naturally both daily and seasonally.

The maximum daily temperature is usually several hours after noon and the minimum is around daybreak. Water temperature varies seasonally with air temperature.

Vertical gradients in temperature can often be measured in deeper systems, especially in lakes where thermal stratification is common.

A warm upper layer, called the epilimnion, often develops during summer months in temperate regions, while a cool bottom layer, the hypolimnion, can be detected below the thermo cline.

Exceptions to this pattern can be found in ice covered systems, where an inverse temperature gradient may be set up and the upper layer of water is cooler than the bottom layer.

Dissolved Oxygen: Oxygen dissolved in the water column, is one of the most important components of aquatic systems. Oxygen is required for the metabolism of aerobic organisms, and it influences inorganic chemical reactions.

Oxygen is often used as an indicator of water quality, such that high concentrations of oxygen usually indicate good water quality. Oxygen enters water through diffusion across the water’s surface, by rapid movement such as waterfalls or ripples in streams (aeration), or as a by-product of photosynthesis.

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The amount of dissolved oxygen gas depends highly on temperature and somewhat on atmospheric pressure. Salinity also influences dissolved oxygen concentrations, such that oxygen is low in highly saline waters and vice versa.

The amount of any gas, including oxygen, dissolved in water is inversely proportional to the temperature of the water; as temperature increases, the amount of dissolved oxygen (gas) decreases.

pH and Alkalinity: In water, a small number of water (H₂O) molecules dissociate and form hydrogen (H⁺) and hydroxyl (OH^–) ions. If the relative proportion of the hydrogen ions is greater than the hydroxyl ions, then the water is defined as being acidic.

If the hydroxyl ions dominate, then the water is defined as being alkaline. The relative proportion of hydrogen and hydroxyl ions is measured on a negative logarithmic scale from 1 (acidic) to 14 (alkaline): 7 being neutral.

Alkalinity, on the other hand, is a related concept that is commonly used to indicate a system’s capacity to buffer against acid impacts. Buffering capacity is the ability of a body of water to resist or dampen changes in pH.

Alkaline compounds in water such as bicarbonates, carbonates, and hydroxides remove H⁺ ions and lower the acidity of the water (i.e., increase pH).

Turbidity and Suspended Solids: Turbidity refers to water clarity. The greater the amount of suspended solids in the water, the murkier it appears, and the higher the measured turbidity.

The major source of turbidity in the open water zone of most lakes is typically phytoplankton, but closer to shore, particulates may also include clays and silts from shoreline erosion, re-suspended bottom sediments, and organic detritus from stream and/or water discharges.

Suspended solids in streams are often the result of sediments carried by the water. The source of these sediments includes natural and anthropogenic (human) activities in the watershed, such as natural or excessive soil erosion from agriculture, forestry or construction, urban runoff, industrial effluents, or excess phytoplankton growth.

Turbidity is often expressed as total suspended solids (TSS). Water transparency and Secchi disk depth are also commonly-used measures of water quality that quantify the depth of light penetration in a body of water. Water bodies that have high transparency values typically have good water quality.

Salinity and Specific Conductance: Salinity is an indication of the concentration of dissolved salts in a body of water. The ions responsible for salinity include the major cations (calcium, Ca²⁺; magnesium, Mg²⁺, sodium, Na⁺; and potassium, K⁺) and the major anions (carbonates, CO₃^– and HCO₃^2-; sulphate, SO₄^2-; and chloride, Cl^–).

The level of salinity in aquatic systems is important to aquatic plants and animals as species can survive only within certain salinity ranges. Although some species are well-adapted to surviving in saline environments, growth and reproduction of many species can be hindered by increases in salinity.

Salinity is measured by comparing the dissolved solids in a water sample with a standardised solution. The dissolved solids can be estimated using total dissolved solids (see: turbidity) or by measuring the specific conductance.

Specific conductance, or conductivity, measures how well the water conducts an electrical current, a property that is proportional to the concentration of ions in solution. Conductivity is often used as a surrogate of salinity measurements and is considerably higher in saline systems than in non-saline systems.

Major Ions: The ionic composition of surface and ground waters is governed by exchanges with the underlying geology of the drainage basin and with atmospheric deposition.

Human activities within the drainage basin also influence the ionic composition, by altering discharge regimes and transport of particulate matter across the landscape, and by changing the chemical composition of surface runoff and atmospheric deposition of solutes through wet and dry precipitation.

Global average concentrations of the four major cations (calcium, magnesium, sodium, and potassium)

and the four major anions (bicarbonate, carbonate, sulphate, and chloride) in surface water tend to approach patterns in which calcium concentrations dominate the cations and bicarbonate and/or carbonate concentrations dominate the anions.

However, as Table 2 shows, there is considerable variability in the patterns for ca tions in rivers on a global scale.

Table 2: Median Composition of Major Cations in Rivers and Lakes around the World

Source: UN GEMS/Water Programme (2006)

The ionic composition of surface waters is usually considered to be relatively stable and insensitive to biological processes occurring within a body of water.

Magnesium, sodium and potassium concentrations tend not to be heavily influenced by metabolic activities of aquatic organisms, whereas calcium can exhibit marked seasonal and spatial dynamics as a result of biological activity.

Similarly, chloride concentrations are not heavily influenced by biological activity, whereas sulphate and inorganic carbon (carbonate and bicarbonate) concentrations can be driven by production and respiration cycles of the aquatic biota.

External forces such as climatic events that govern evaporation and discharge regimes and anthropogenic inputs can also drive patterns in ionic concentrations. Such forces are probably most responsible for long-term changes in the ionic composition of lakes and rivers.

Nutrients: Nutrients are elements essential to life. The major nutrients, or macronutrients, required for metabolism and growth of organisms include carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulphur, magnesium, and calcium.

In aquatic systems, nitrogen and phosphorus are the two nutrients that most commonly limit maximum biomass of algae and aquatic plants (primary producers), which occurs when concentrations in the surrounding environment are below requirements for optimal growth of algae, plants and bacteria.

There are many micronutrients also required for metabolism and growth of organisms, but for the most part, cellular demands for these nutrients do not exceed supply.

For example, elements such as iron (Fe) and manganese (Mn) are essential cellular constituents but are required in relatively low concentrations in relation to their availability in fresh waters.

Metals:Metals occur naturally and become integrated into aquatic organisms through food and water. Trace metals such as mercury, copper, selenium, and zinc are essential metabolic components in low concentrations.

However, metals tend to bioaccumulation in tissues and prolonged exposure or exposure at higher concentrations can lead to illness. Elevated concentrations of trace metals can have negative consequences for both wildlife and humans.

Human activities such as mining and heavy industry can result in higher concentrations than those that would be found naturally.

Metals tend to be strongly associated with sediments in rivers, lakes, and reservoirs and their release to the surrounding water is largely a function of pH, oxidation-reduction state, and organic matter content of the water (and the same is also true for nutrient and organic compounds).

Thus, water quality monitoring for metals should also examine sediment concentrations, so as not to overlook a potential source of metal contamination to surface waters.

Organic Matter: Organic matter is important in the cycling of nutrients, carbon and energy between producers and consumers and back again in aquatic ecosystems.

The decomposition of organic matter by bacteria and fungi in aquatic ecosystems, inefficient grazing by zooplankton and waste excretion by aquatic animals, release stored energy, carbon, and nutrients, thereby making these newly available to primary producers and bacteria for metabolism.

External subsidies of organic matter that enter aquatic ecosystems from a drainage basin through point sources such as effluent outfalls, or non-point sources such as runoff from agricultural areas, can enhance microbial respiration and invertebrate production of aquatic ecosystems.

Organic matter affects the biological availability of minerals and elements, and has important protective effects in many aquatic ecosystems, by influencing the degree of light penetration that can enter.

Biological components:Organisms, populations, and communities composed of different species make up the biological diversity of aquatic ecosystems.

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From single-celled microbes such as viruses, bacteria, protists, and fungi, to multi-cellular organisms such as vascular plants, aquatic invertebrates, fish and wildfowl, the community of organisms that reside within and near aquatic ecosystems simultaneously plays a vital role in regulating biogeochemical fluxes in their surrounding environment and is influenced by these same biogeochemical fluxes.

Aquatic organisms, often considered ‘engineers’ of aquatic ecosystems, not only react to physical and chemical changes in their environment, but also they can drive such changes and have important roles in cleansing and detoxifying their environment.

The entire biological diversity of aquatic environments ensures that ecosystems can continue to function normally: shifts in species composition through species losses or biological invasions can lead to physical and chemical changes in the environment that may have detrimental effects on both the community of organisms residing within the ecosystem and on humans that rely upon the system for water supply and other activities.

The diversity of aquatic ecosystems can also be influenced by physical and chemical changes in the environment.

Biochemical Oxygen Demand and Chemical Oxygen Demand

Many aquatic ecosystems rely heavily on external subsidies of organic matter to sustain production.

However, excess inputs of organic matter from the drainage basin, such as those that may occur downstream of a sewage outfall, can upset the production balance of an aquatic system and lead to excessive bacterial production and consumption of dissolved oxygen that could compromise the integrity of the ecosystem and lead to favourable conditions for growth of less than ideal species.

Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are two common measures of water quality that reflect the degree of organic matter pollution of a water body.

BOD is a measure of the amount of oxygen removed from aquatic environments by aerobic micro-organisms for their metabolic requirements during the breakdown of organic matter, and systems with high BOD tend to have low dissolved oxygen concentrations.

COD is a measure of the oxygen equivalent of the organic matter in a water sample that is susceptible to oxidation by a strong chemical oxidant, such as dichromate.

Although BOD and COD are usually at or near analytical limits of detection in relatively undisturbed systems, water samples taken near points of organic matter pollution often yield very high observations.