Water (H₂O) is unique for being a liquid at room temperature and for its ability to dissolve many substances,
nces, which is vital for life. Its transparency allows
photosynthesis in the ocean and has been essential in the development of life on Earth. Although there is enough water, its uneven distribution and pollution
make access to clean water difficult.
97% of Earth's water is saline, while only 3% is freshwater, and less than 1% of that is accessible for human use. Much of the freshwater is locked in glaciers.
Access to potable water is a global issue, especially impacting rural women, who often have to travel long distances to obtain it.
The water cycle is a continuous process involving evaporation, condensation, precipitation, and infiltration into the ground. This cycle is essential for
maintaining Earth's water balance, circulating water among oceans, rivers, lakes, and the atmosphere, ensuring its availability for living beings.
Water is essential for domestic, industrial, and agricultural uses. In homes, it is primarily used for drinking, cooking, and cleaning. Industries use water for
cooling processes and electricity production, while agriculture relies on it for irrigation. The main sources of freshwater are rivers, lakes, and underground
aquifers.
Groundwater is stored in aquifers and is a significant source for human consumption. Desalination turns seawater into potable water, though it is costly and
produces saline waste. There are two methods of desalination: distillation and reverse osmosis, with the latter being more efficient and commonly used in
energy-rich countries, such as those in the Middle East.
Availability of Safe Drinking Water Around the World
A country's water wealth largely depends on its rainfall, with countries like Brazil and Russia being water-rich and the United Arab Emirates and Kuwait being
water-poor. However, having plenty of water does not always mean that there is enough safe water for everyone. For example, China, with a large population, has
a per capita water availability of 2300 m³ per person per year, whereas Singapore has only 110 m³ per person per year due to its limited water resources. Water
scarcity can be physical (due to low rainfall or high evaporation) or economic (a country has water but lacks the resources to treat and distribute it).
Safe drinking water depends on effective sanitation and water-treatment processes, but these are not universally available. Despite improvements since 2000,
millions still lack access to safe water and sanitation, particularly in less economically developed countries (LEDCs), where fewer people have access to clean water
compared to developed regions. Access also varies within countries, with urban areas generally having better access to clean water and sanitation than rural areas,
due to factors like wealth, population density, and infrastructure costs.
Water scarcity can even lead to conflict, such as the tensions over the use of the River Jordan in the Middle East and the Aral Sea in Central Asia. With growing
populations, more areas may experience such conflicts in the future.
4.6 Multipurpose Dam Projects
Dams are often built across rivers for multiple purposes, including generating hydroelectric power, controlling floods, irrigating farms, supporting tourism,
providing water, creating habitats for wetland species, and enabling boat access to certain areas. However, dams also have disadvantages, such as the need to
relocate people, flooding land, disrupting ecosystems, altering water supplies downstream, reducing soil enrichment from natural flooding, and becoming less
effective over time due to siltation in the reservoir.
Where to Build a Dam
Choosing a dam site requires careful evaluation of multiple factors. The ideal location is in a narrow river valley, which reduces construction costs. A high position
within the valley is also beneficial, as it increases water pressure, enhancing hydroelectric power (HEP) production potential. Additionally, dams should be situated
away from developed areas to minimize pollution risk in the reservoir.
Are Dams Sustainable?
While dams are considered a long-term solution for electricity generation, they face sustainability challenges. Over time, reservoirs can accumulate silt, potentially
reducing efficiency. Dam structures are also subject to deterioration under constant pressure, posing a risk of failure. Environmental impacts, such as disrupting
fish populations, further question their sustainability. However, advocates argue that dams are preferable to fossil fuels since they produce clean energy without
greenhouse gases, placing dams on the more sustainable end of the energy spectrum.
Water-Related DiseasesCholera and Typhoid
These waterborne diseases result from consuming water contaminated by pathogens, often due to poor sanitation. Cholera and typhoid spread through water
polluted by human feces. Key prevention measures include ensuring clean water and effective sanitation systems to prevent sewage from contaminating drinking
water. Hygiene practices, like handwashing and using clean water for food, are also critical.
Malaria:Malaria differs from waterborne diseases as it is transmitted through the Anopheles mosquito, which breeds in stagnant water. This disease mainly affects
Africa, Asia, and the Americas, causing severe symptoms and, in some cases, death. Prevention involves avoiding mosquito bites by staying indoors at night, using
mosquito nets, and applying insect repellent. Governments combat malaria by controlling mosquito populations through insecticide spraying, draining wetlands,
and introducing fish to consume mosquito larvae.
Prevention and Control Measures
For individuals, prevention includes protective clothing, mosquito nets, and repellents. Governments focus on mosquito control by spraying insecticides, draining
wetlands, adding fish to water bodies to eat mosquito larvae, and applying oil to water surfaces to disrupt mosquito breeding. Long-term eradication of malaria
involves not only controlling mosquitoes but also eliminating the malarial parasite entirely.
WHO's Efforts to Eradicate Malaria
The World Health Organization (WHO) aims to eradicate malaria worldwide, with a major challenge being sub-Saharan Africa, where 90% of cases occur. Despite
improvements in diagnosis, treatment, and prevention, which reduced malaria incidence by 25% and deaths by 42% in the early 21st century, eradication is
unlikely using current methods. Resistance to insecticides and drugs is rising, and asymptomatic carriers make it harder to eliminate the disease. Climate change
may further complicate eradication by enabling malaria to spread to previously unaffected regions.
Water Pollution Sources, Impact, and Management
Water pollution results primarily from domestic waste, industrial processes, and agriculture. Pollutants, including untreated sewage, toxic chemicals, and
agricultural runoff, contaminate water sources, harming ecosystems and human health. To manage this, sewage treatment is vital, involving multiple steps to
remove contaminants and reduce biological oxygen demand (BOD), which can otherwise deplete oxygen in rivers and lakes, affecting aquatic life.
Sewage Treatment Stages:Sewage Outfall: Wastewater is transported to treatment plants via sewers.Screening: Large objects are filtered out. Primary
Treatment: Solid waste settles as sludge, which is later treated.Secondary Treatment (Oxidation): Oxygen is added to support bacteria that break down organic
material.Second Settling: Bacteria settle, producing cleaner water as effluent.Effluent Discharge: Treated water is released into rivers or other bodies of
water.Sludge Digestion: Bacteria break down sludge, producing methane for fuel.Sludge Disposal: Treated sludge is used as fertilizer.
Industrial Pollution and Toxic Compounds
Industries release pollutants, including toxic heavy metals like lead, mercury, and cadmium. These metals, through biomagnification, can accumulate in organisms
and ecosystems, causing illness. Industrial pollutants also contribute to acid rain, which damages forests and aquatic ecosystems, a phenomenon recognized since
the 19th century.
Improved Sanitation and Water Treatment
Improved sanitation separates human waste from contact with people, utilizing systems like flush toilets, septic tanks, or pit latrines. Potable water treatment
involves coagulation, filtration, and chlorination to remove contaminants and pathogens. Proper sanitation and water treatment are essential to prevent
waterborne diseases like typhoid, cholera, and malaria, as well as to ensure a safe, sustainable water supply.
Causes of Acid Rain Acid rain is primarily caused by the release of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) from burning fossil fuels, such as coal and oil.
These gases react with water vapor in the atmosphere to form sulfuric acid and nitric acid. When it rains, these acids fall to the ground, significantly lowering the
pH of rainwater, sometimes reaching levels as low as pH 2, similar to battery acid. Acid rain can have severe impacts on ecosystems, including soil, water bodies,
forests, and wildlife. It can leach toxic metals like aluminum from the soil, which then harms aquatic life by clogging fish gills and reducing necessary minerals.
Impact of Acid Rain on Aquatic Ecosystems
Acid rain lowers the pH of rivers and lakes, making the water too acidic for many organisms. Fish reproduction may decline, and young fish may develop
deformities. Additionally, essential minerals like calcium and potassium are leached from the water, reducing food sources for fish. Acidified water can also lead to
heavy metals like aluminum being washed into rivers and lakes, harming aquatic life and affecting the entire ecosystem.
EutrophicationEutrophication is a process in which excessive nutrients, particularly nitrates and phosphates, enter water bodies from sources such as agricultural
runoff, sewage discharge, and detergents. These nutrients cause an overgrowth of algae, known as an algal bloom. When the algae die, bacteria decompose them,
increasing organic matter and consuming oxygen in the process. This depletion of oxygen can result in the death of fish and other aquatic organisms, leading to a
decrease in biodiversity.
Pesticide and Herbicide Pollution Pesticides, including insecticides and herbicides, are chemicals used in agriculture to kill pests and weeds. Although designed to
target specific organisms, pesticides can also affect non-target species, including humans, due to similarities in physiology. Because many pesticides are water-
soluble, they can easily contaminate water bodies and impact aquatic ecosystems and drinking water supplies.
Pollution Control and Legislatio Governments worldwide have implemented legislation to control pollution and limit the environmental impact of industries.
These laws set pollution thresholds and require industries to monitor and minimize their emissions. An example of international pollution control is the Great Lakes
Water Quality Agreement (GLWQA), which helps regulate pollutants entering the Great Lakes and protect water quality across national borders.
Oceans and fishiris The Resource Potential of OceansOceans cover 71% of Earth's surface, serving as a significant source of food, energy, chemicals, and raw
materials. The continental shelf, although making up only about 10% of the ocean, is highly productive and hosts 95% of commercially caught fish. The rich supply
of nutrients and light in these shallow waters supports extensive marine life, making the continental shelves vital for fisheries.
Food Source Oceans provide around 90 million tonnes of fish annually, which is crucial for global food security. Marine food is especially rich in protein, fats, and
oils, benefiting populations that rely on subsistence diets. Despite its vastness, the ocean is still a relatively minor food source compared to land-based agriculture,
which produces over 6 billion tonnes of food.
Chemicals and Minerals The oceans hold numerous dissolved elements, including salt, magnesium, and trace metals like gold, tin, and titanium. Offshore oil
drilling is widespread, with the Gulf of Mexico hosting nearly 4,000 active rigs. Extraction of these resources has significant economic value, but it requires careful
management to avoid environmental degradation.
Wave and Tidal Energy Wave and tidal energy present immense, largely untapped potential. Wave farms like the Islay LIMPET in Scotland and the Aguçadoura
Wave Farm in Portugal have demonstrated the feasibility of harnessing wave power. Tidal energy plants, such as the Sihwa Lake Tidal Power Station in South
Korea, showcase the capacity of tidal power to generate substantial electricity, which could play a critical role in reducing reliance on fossil fuels as technology
advances.
Mining and Construction Materials The ocean floor is a source of diamonds, sand, gravel, and crushed rock, essential for construction. However, ocean mining can
disrupt habitats and create sediment clouds, impacting photosynthesis and introducing heavy metals into the marine food chain. As land-based resources become
scarcer, ocean floor mining may increase, necessitating sustainable practices to protect marine environments. Environmental Concerns and Pollution
Oceans face threats from pollution, including oil spills, mercury, and waste plastic, with over 250,000 tonnes of plastic currently circulating in gyres. These
pollutants affect marine life and ecosystems and highlight the need for better ocean management to preserve these valuable resources for future generations.
Finding the Fish
Fish depend on the ocean’s food web, starting with phytoplankton, microscopic green algae that produce their own food through photosynthesis. Herbivorous fish
feed on phytoplankton, while carnivorous fish eat other fish, forming a chain that begins with these primary producers. Phytoplankton need light, water, and
carbon dioxide for photosynthesis, with light being a limiting factor as it only penetrates the top 200 meters, known as the euphotic zone. Consequently, fish are
concentrated in shallower waters like continental shelves, where light and nutrients support abundant phytoplankton.
In addition to light, phytoplankton need mineral nutrients—nitrogen, sulfur, phosphorus, and magnesium—to thrive. Currents play a crucial role in upwelling
nutrient-rich waters from the seabed, creating areas with high fish populations. One of the largest fisheries, the Peruvian anchovy fishery, relies on nutrient-rich
waters brought to the surface by upwelling off the coast of Peru. However, this upwelling can be disrupted by the El Niño Southern Oscillation (ENSO), leading to
warm, nutrient-poor waters that reduce phytoplankton and fish populations. This natural event, coupled with over-fishing, caused the anchovy fishery to collapse
in the early 1970s.
Exploitation of the Oceans: Impact on Fisheries
The global fish catch expanded from 1950 until the late 1980s, after which it plateaued due to over-fishing. At least 75% of marine fisheries are under threat, with
the North Atlantic herring serving as an example. In the 1950s, herring stocks were abundant, but by the 1970s, over-fishing reduced their population drastically.
Symptoms of over-fishing include smaller fish sizes and a need for increased fishing effort for the same catch.
Bycatch, or the capture of non-target species, is another consequence of over-fishing, particularly in shrimp and tuna fisheries. Advances in fishing technology,
such as large boats, SONAR, and massive nets, contribute to over-fishing. Different types of nets, including trawl nets, drift nets, seine nets, and dredge nets,
capture vast quantities of fish, often damaging habitats and leading to high bycatch rates. Sustainable fishing strategies aim to balance these modern methods with
conservation efforts.
Farming Marine Species
Fishing in the wild is one of the last hunter-gatherer activities humans practice. However, declining wild fish populations have led to an increased focus on fish
farming. Freshwater aquaculture, which has been practiced for centuries, involves the cultivation of fish in ponds and is a traditional method in many regions, such
as Central Europe. Shellfish farming is another form of aquaculture; for example, mussels are often grown on strings hung from rafts, with yields reaching up to 250
tonnes per hectare.
Marine fish farming, or mariculture, is more challenging and less successful than freshwater farming. Many marine fish are carnivores at the top of the food chain,
requiring a diet that varies as they mature. Cobia, a fast-growing species with high-quality meat, is a notable success in mariculture, farmed extensively in the USA,
Mexico, Panama, and parts of Asia. The largest open-water fish farm, located off Panama’s coast, focuses on cobia production, which reached over 43,000 tonnes
globally in 2013.
Salmon farming is the most productive form of mariculture, although salmon spend part of their lives in freshwater. Major salmon farms are located in Chile,
Norway, Scotland, Canada, and the Faroe Islands. By 1996, farmed salmon production surpassed wild-caught salmon. However, salmon farming has environmental
concerns, such as increased susceptibility to diseases and parasites, pollution from fish waste, and potential ecological impact if farmed fish escape.
Strategies for Managing Marine Harvesting
Countries with coastlines have economic exclusion zones (EEZs) extending 200 nautical miles, designated by the United Nations Convention on the Law of the Sea.
Within this zone, nations are responsible for sustainable management of their fisheries. Several strategies help to achieve this:
1. Net Types and Mesh Size: The size and shape of net mesh are crucial in preventing overfishing of juvenile fish. Smaller mesh can catch young fish, reducing the
population’s ability to mature and reproduce. For example, the General Fisheries Commission for the Mediterranean (GFCM) mandates a minimum 40 mm
square mesh or 50 mm diamond mesh for trawling activities. Square mesh panels allow smaller fish to escape, ensuring sustainability.
2. Quotas: Many scientists and policymakers consider quotas essential for sustainable fisheries. Governments or international organizations, like the European
Union, set limits on the number and type of fish that can be caught. These limits are informed by extensive research and data from global networks, aiming to
leave enough fish to repopulate. Studies show that managed fisheries with quotas are less likely to experience collapse.
3. Other Fishing Methods: In some cases, more targeted fishing techniques are necessary. For example, the pole-and-line method in tuna fishing minimizes
bycatch compared to large nets, making it a sustainable alternative. Some tuna fisheries use fish aggregation devices (FADs) to attract tuna but risk catching
non-target species and juvenile tuna. The pole-and-line method addresses these issues by focusing on specific fish without bycatch.
Governments can protect fisheries through closed seasons, which typically occur during breeding periods, and by restricting fishing in certain areas,
especially where target species breed. In addition, conservation laws, such as the Magnuson-Stevens Fishery Conservation and Management Act in the
USA, regulate fisheries and enforce international agreements. Countries have economic exclusion zones of 200 nautical miles, within which they
manage their fisheries. For international waters, agreements like the UN Convention on the Law of the Sea (UNCLOS) are necessary, as seen in the
Mediterranean, where countries rely on bilateral agreements beyond their 12 nautical mile zones.
Enforcement of these laws is crucial for their success. While monitoring is challenging in the vast oceans, countries like Namibia have implemented
effective systems, such as onboard observers, air patrols, and daily catch logs for vessels. Monitoring is more successful in ports, where landings are
closely supervised.
