Chapter 5 Review Questions

F (see “Biogeochemical Cycles”, Box 5.1)

F (see “The Hydrological Cycle”, Box 5.8)

F (see “Matter”)

T (see “Biogeochemical Cycles”)

F (see “Matter”)

T (see “Biogeochemical Cycles”)

F (see “Biogeochemical Cycles”)

F (see “Sedimentary Cycles”)

F (see “The Hydrological Cycle”, Table 5.1)

T (see “The Hydrological Cycle”, Box 5.7)

Everything is either matter or energy. However, while the supply of energy is virtually infinite, the supply of matter on Earth is limited to that which we now have. Matter, unlike energy, has mass and takes up space. Matter is what things are made of and is composed of the 92 natural and 17 synthesized chemical elements.

(see “Matter”)

This law tells us that matter can neither be created nor destroyed but merely transformed from one form into another. Thus, matter cannot be consumed so that it no longer exists; it will always exist but in a changed form.

(see “Matter”)

Naturally occurring elements that are a necessary part of living things are known as nutrients, and may be further classified into macronutrients, which are needed in relatively large amounts by all organisms, and micronutrients, required in lesser amounts by most species. (see “Biogeochemical cycles”)

Nutrients are cycled continuously among different components of the ecosphere in characteristic paths known as biogeochemical cycles. Cycles can be classified according to the main source of their matter. Gaseous cycles have most of their matter in the atmosphere (e.g., N). Sedimentary cycles hold most of their matter in the lithosphere (e.g., S, P). Elements in sedimentary cycles tend to cycle more slowly than those in gaseous cycles, so these elements may be locked into geological formations for millions of years.

(see “Biogeochemical cycles”)

Phosphorus is an example of a sedimentary cycle. The main reservoir of phosphorus is the Earth’s crust. Phosphates are made available in the soil water through erosional processes and are taken up by plant roots and passed along the food chain. There is no atmospheric component to the cycle, making it especially vulnerable to disruption. The main human use for phosphorus is as fertilizer. It is a main cause of eutrophication.

Sulphur is also a sedimentary cycle but differs from phosphorus in that it has an atmospheric component. Like phosphorus, it is an essential component for all life. Bacteria enable plants to gain access to elemental sulphur by transforming it to sulphates in the soil. Sulphur is a main component of acid deposition.

(see “Phosphorus (P)”, “Sulphur (S)”)

Both nitrogen and carbon can be considered gaseous in their cycles because the atmosphere is a significant compartment,

Nitrogen is a gaseous cycle. Almost 80 per cent of the atmosphere is composed of nitrogen gas, yet most organisms cannot use it as a source of nitrates. Instead, various bacteria help to transform nitrogen into a form that can be used by plants. As with the other cycles, these nitrates are then passed along the food chain. Nitrates are used as fertilizers and contribute to eutrophication. Various nitrous oxides also contribute to acid deposition and the catalytic destruction of ozone.

Carbon dioxide constitutes only 0.03 per cent of the atmosphere, but it is the main source of carbon—the basis for life—through the process of photosynthesis. Carbon becomes incorporated into the biomass and is passed along the food chain. Respiration by organisms transforms some of this carbon back into carbon dioxide, and the cellular respiration of decomposers helps to return the carbon from dead organisms into the atmosphere. Carbon dioxide emissions from burning fossil fuels are a main contributor to global climatic change. (see “Nitrogen (N)”, “Carbon (C)”)

Water on Earth occurs in a fixed supply that cycles between various reservoirs driven by energy from the sun. By far the largest reservoir, containing more than 97 per cent of the water on Earth, is the ocean. Most of the rest is tied up in the polar ice caps, with only a small amount readily available as the fresh water that sustains terrestrial life. Water travels ceaselessly between these various reservoirs through the main processes of evaporation and precipitation. In the deep ocean, it may take 37,000 years before water is recycled through evaporation into the atmosphere, whereas once in the atmosphere, average residence time is in the order of nine to 12 days. 86 per cent of the water in the atmosphere is evaporated directly from the ocean surface. The remainder comes from evaporation from smaller water bodies, from the leaves of plants (transpiration), or from the soil and plants (evapotranspiration). Once in the atmosphere, the water vapour cools, condenses around tiny particles called condensation nuclei, forms clouds, and is precipitated to the earth as rain, snow, or hail. About 76 per cent of precipitation falls into the ocean. The remainder joins the terrestrial part of the cycle in ice caps, lakes, rivers, groundwater, and transport between these compartments. Gravity moves water down through the soil until it reaches the water table, below which all the spaces between the soil particles are full of water. This water in the saturated zone is Areas containing large volumes of groundwater are called aquifers. Lakes, streams, and other water bodies are known as surface water (generally where the land surface is below the water table). Glaciers accumulate freshwater from precipitation. Water exists in all three phases of matter (solid, liquid, and vapour). Sublimation is the process for direct transfer between the solid and vapour phases of matter, regardless of direction.

(see “The Hydrological Cycle”)

Groundwater is found within spaces between soil and rock particles and in crevices and cracks in the rocks below the surface of the Earth. Above the water table is the unsaturated zone where the spaces contain both water and air. In this zone, water is called soil moisture. Permeable materials allow the passage of water, usually through cracks and spaces between particles. An aquifer is a formation of permeable rocks or loose materials that contains usable sources of groundwater. They vary greatly in size and composition. Porous media aquifers consist of materials such as sand and gravel through which the water moves through the spaces between particles. Fractured aquifers occur where the water moves through joints and cracks in solid rock. If an aquifer lies between layers of impermeable material, it is called a confined aquifer, which may be punctured by an artesian well, releasing the pressurized water to the surface. If the pressure is sufficient to bring water to the surface, the well is known as a flowing artesian well. Areas where water enters aquifers are known as recharge areas; discharge areas are where the water once more appears above ground. These discharge areas can contribute significantly to surface water flow, especially in periods of low precipitation.

(see “The Hydrological Cycle”, Box 5.10)

More than 95 per cent of Canada is snow covered for part of the winter. Spring melt is hence a critical part of the hydrological cycle in Canada as water moves from the solid to liquid phase. This creates a runoff regime for many Canadian rivers, characterized by low late-winter flows and high spring melt flows that slowly diminish over the summer into the winter lows as water becomes stored in the solid phase once more.

(see “The Hydrological Cycle”)

Eutrophication is a natural process of nutrient enrichment of water bodies that leads to greater productivity. The natural progression from an oligotrophic to eutrophic condition through succession may take place over thousands of years, depending upon the geological makeup of the catchment area and the depth of the receiving waters. Catchments with fertile soils will progress more quickly than those with soils lacking in nutrients. Depth is important, because shallower lakes tend to recycle nutrients more efficiently. Phosphorus and nitrogen are often the two main limiting factors for plant growth in aquatic ecosystems. Cultural eutrophication (eutrophication caused by human activity) speeds up the natural eutrophication process by promoting even more change in the same direction through excessive addition of N and P. These additions are from many processes including runoff from cropland, feedlots, and urban areas, and sewage and industrial discharge.

(see “Eutrophication”, “What Causes Eutrophication?”)

PTV is a policy target value set by politicians whereas STV is a scientific target value set by scientists. Many environmental problems occur when PTVs were established that conflicted with STVs.

(see “What Can We Do about It?”)

(see “Eutrophication”, Table 5.3)

Nutrient enrichment promotes increased growth of aquatic plants, particularly favouring the growth of floating phytoplankton over benthic plants rooted in the substrate. Benthic plants become out-competed for light by the phytoplankton, thus they produce less oxygen at depth.

(see “What Are the Effects?”)

When organic wastes are added to a body of water, the oxygen levels fall as the number of bacteria rises to help break down the waste. This is known as oxygen sag and is measured by the biological oxygen demand (BOD); the amount of dissolved oxygen needed by aerobic decomposers to break down the organic material in a given volume of water at a certain temperature over a given period. At the discharge source, oxygen starts to fall, and there is a corresponding rise in the BOD. As distance from the input source increases and the bacteria digest the wastes, then the oxygen content returns to normal, and the BOD falls.

(see “What Are the Effects?”)

Non-point sources are diffuse, such as runoff from urban areas and agricultural land. Such flows really have to be controlled at the source, since they enter the water body, by definition, in so many different locations. Point sources of pollution are single discharge points, such as effluent discharges from sewage plants or industrial processes. These are easier to locate and control.

(see “What Can We Do about It?”)

In 1972, Canada and the US signed the Great Lakes Water Quality Agreement primarily to try to deal with eutrophication in Lake Erie. The signing of the 1985 Great Lakes Charter, in which the two countries agreed to take a co-operative and ecosystem-based approach to the lakes, further strengthened international efforts. Since the 1970s, phosphorus controls, implemented under the Canada Water Act, have led to significant reductions in the phosphorus concentration of the water. Phosphate-based detergents were banned and municipal waste treatment plants upgraded. These measures have led to improvement of water quality, but significant problems still remain. The controls are largely on point-source pollution, discharges that have a readily identifiable source, such as waste treatment plants and industrial complexes; however, much of the remaining nutrient load comes from non-point sources, such as runoff from agricultural fields, lawn fertilizer, and construction sites that are much more difficult to regulate. Phosphorus levels exceed objective levels in the western, eastern, and central basins. The levels of nitrite and nitrate concentrations have also been rising, leading to increased eutrophication as a result of other nutrients.

(see “Lake Erie: An Example of Eutrophication Control”)

Generally, a pH value of 5.6 is given to “clean” rain. Acid rain is defined as deposition that is more acidic than this. Acidic deposition is a more generic term that includes not only rainfall but also snow, fog, and dry deposition from dust. It is caused by human interference in the sulphur and nitrogen cycles, resulting in excessive emissions of SOx and NOx into the atmosphere, which combine with water to form sulphuric and nitric acid. The largest sources are the smelting of sulphur-rich metal ores and the burning of fossil fuels for energy, including transportation.

(see “What is Acid Deposition?”, “What Causes Acid Deposition?”)

Acid deposition causes ecological damage, killing plants, bacteria, fish and aquatic insects through direct contact with acids and by disrupting food chains. Acidification of soil and water has direct effects and also makes metals more soluble, leading to secondary toxicity. Acids in soil cause leaching of nutrients so that they are not available to plants. Decomposition is inhibited as bacteria and fungi die off. Crop growth is inhibited as well, and this together with fish and tree losses and direct damage of buildings, has negative socio-economic impacts. There are also human health effects due to inhalation of acidified airborne particles, which damage lungs, and increased levels of metals in water supplies.

(see “What Are the Effects of Acid Deposition?”)

Acid deposition circulates in the atmosphere, thus is not limited to the areas generating the emissions. International efforts are required to decrease emissions. Any efforts must be based on science to be effective. A reduction in fossil fuel combustion is a major target, so switching to renewable energy sources is key. Technologies to remove SO_X and NO_X from industrial sources and vehicle emissions is also vital.

(see “What Can We Do about It?”)

The pH scale is a range of acidity (concentration of hydrogen ions in a solution) that goes from 0 to 14, with 0 being most acidic and 14 most basic (alkaline). A pH of 7 is neutral, where acidic hydrogen ions (H+) and basic hydroxyl ions (OH-) are in balance. The pH scale is logarithmic. A decrease in value from pH 6 to pH 5 means that the solution has become 10 times more acidic. If the number drops to pH 4 from pH 6, then the solution is 100 times more acidic.

(see “Phosphorus”)