Water Pollution and Aquatic Ecosystems: Causes, Impacts, and Management

Water Resources and Distribution

Global Water Availability

Water is a critical resource for all life on Earth, but its distribution and accessibility are uneven. Understanding the global distribution of water helps explain challenges in water management and pollution control.

• Earth's Surface Water: Over 70% of the Earth's surface is covered by water, but only 2.5% is freshwater.

• Accessible Freshwater: Less than 1% of all water is available for human use, as most freshwater is locked in glaciers or underground.

• Water Scarcity: Approximately 800 million people lack access to improved water sources, with the majority living in rural areas.

• Health Impact: Water-related illnesses account for about half of the world's hospital beds.

Distribution of Earth's Water:

Water Type	Percentage
Oceans	~97%
Glaciers	~70% of freshwater
Underground	~29% of freshwater
Rivers & Lakes	<1% of freshwater

Additional info: Most accessible freshwater is found in rivers, lakes, and shallow groundwater.

Water Pollution: Definitions and Types

Definition of Water Pollution

Water pollution is any contamination of water that reduces its value to humans and other species. It can affect drinking water, aquatic habitats, and ecosystem health.

Broad Types of Water Pollution

• Point Source Pollution: Pollution that originates from a single, identifiable source (e.g., a factory discharge pipe).

• Nonpoint Source Pollution: Pollution that comes from multiple, diffuse sources (e.g., agricultural runoff, urban stormwater).

Types of Pollutants

• Sediment

• Inorganic Pollutants: Nutrients (nitrogen, phosphorus), heavy metals, inorganic gases

• Thermal Pollution

• Pathogens

• Toxic Organic Compounds

• Oxygen-Demanding Organic Wastes

• Pharmaceuticals

Sediment Pollution

Sources and Impacts

• Annual Load: About 0.9 billion tons of sediment enter water bodies annually.

• Aquatic Impacts:

  • Blocks sunlight, reducing photosynthesis

  • Clogs fish gills

  • Damages hydroelectric turbines

  • Makes substrate unavailable for aquatic organisms

  • Destroys breeding grounds for fish

  • Causes reservoir sedimentation

• Terrestrial Impact: Reduces air quality

Prevention of Sediment Pollution

• Conservation Tillage: Reduces soil erosion in agriculture

• Construction Practices: Use of drift fences and erosion mats

• Logging Management: Shelterwoods and proper stream crossings

Contaminants: Excess Nutrients

Nitrogen and Phosphorus

Excess nutrients, especially nitrogen and phosphorus, are major contributors to water pollution and can lead to eutrophication.

• Sources: Fertilizers, sewers, livestock manure

• Impact: Algae blooms, hypoxia (low oxygen), fish kills

Eutrophication

• Definition: Enrichment of water bodies with nutrients, leading to excessive growth of algae and aquatic plants.

• Cultural Eutrophication: Human-induced, occurs over decades

• Natural Eutrophication: Occurs over millennia

Hypoxia and Anoxia

• Hypoxic Water: Dissolved oxygen (DO) < 2 mg/L (or 2 ppm)

• Anoxic Water: DO < 0.2 mg/L (or 0.2 ppm)

Inorganic Contaminants

Gases and Acid Rain

• Nitrogen Oxides (NOx) and Sulfur Dioxide (SO2): Produced by combustion in power plants and vehicles; react in the atmosphere to form acid rain (low pH precipitation).

• Health Impact: Linked to respiratory diseases.

CO2 and Ocean Acidification

• Process: CO2 from the atmosphere dissolves in water, forming carbonic acid and lowering pH.

• Equation:

$CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow 2HCO_3^- $

• Impact: Reduces carbonate ions, impeding calcification in marine organisms.

Heavy Metals

• Common Heavy Metals: Mercury, cadmium, copper, zinc, nickel, lead, chromium, aluminum, cobalt, rare earths

• Impact: Toxic to aquatic life and humans; bioaccumulate in food webs.

Thermal Pollution

Sources and Impacts

• Sources: Discharge of heated water from industrial processes

• Impacts:

  • Interferes with reproduction of aquatic organisms

  • Increases vulnerability to disease

  • Direct mortality

  • Facilitates spread of invasive species

  • Potential for toxic algae blooms

  • Depletes dissolved oxygen (DO)

Pathogens and Disease-Producing Organisms

Human and Animal Pathogens

• Human Pathogens: Parasites, botulism, E. coli, cholera, hepatitis A

• Animal Diseases: Protozoans (e.g., Pfiesteria), bacterial, viral

• Prevention: Chlorination and purification of water supplies

Toxic Organic Compounds

Examples and Impacts

• Examples: PCBs, DDT, dioxin

• Impacts: Poisoning, bioaccumulation in food webs

• Prevention: Tighter manufacturing controls, better waste disposal regulations

Endocrine Disruptors

Definition and Effects

• Definition: Chemicals that interfere with hormone systems in organisms

• Effects: Altered fertility, abnormalities in sex organs, early puberty, nervous system and immune dysfunction, certain cancers, respiratory problems

• Prevention: Reducing use and improving wastewater treatment

Oxygen-Demanding Organic Wastes

Biological Oxygen Demand (BOD)

• Definition: The amount of oxygen required by microorganisms to decompose organic matter in water

• Impact: High BOD can lead to anoxia (absence of oxygen), harming fish and invertebrates

Prevention and Control of Water Pollution

General Strategies

• Ban phosphate soaps

• Apply fertilizers more precisely

• Upgrade water treatment facilities

• Improve livestock management

• Ban grazing in floodplains

• Reduce carbon emissions

• Prevention is more cost-effective than remediation

Legislation and Management

Key U.S. Laws

• Federal Water Pollution Control Act (FWPCA) 1972: Goal to make water fishable and swimmable by 1985; included standards for drinking, swimming, fishing, transportation, and agriculture.

• Safe Drinking Water Act 1974: Established EPA as drinking water regulator; set drinking water standards.

• Clean Water Act (Amendment 1977): Strengthened pollution control measures.

• Amendment 1987: Addressed non-point source pollution.

• Watershed Management: Holistic approach to protect water supplies.

Aquatic Ecosystem Structure and Function

Lake and Stream Zones

• Littoral Zone: Shallow area near shore, high light penetration, supports photosynthesis.

• Aphotic Zone: Deep water, little or no light, no photosynthesis.

• Profundal Zone: Deep, open water below the limnetic zone, low light.

• Benthic Zone: Bottom substrate, often low light, inhabited by decomposers.

Decomposers in Aquatic Ecosystems

• Role: Break down dead organic matter and recycle nutrients, supporting ecosystem productivity.

Riparian Zones

• Definition: Areas adjacent to streams and rivers, often vegetated.

• Functions: Filter pollutants, provide wildlife habitat, stabilize banks, regulate water temperature.

Human Impacts on Aquatic Ecosystems

• Invasive Species: Introduction of non-native species can disrupt native communities and ecosystem function.

Sample Table: Point Source vs Nonpoint Source Pollution

Type	Definition	Example
Point Source	Single, identifiable source	Factory discharge pipe
Nonpoint Source	Multiple, diffuse sources	Agricultural runoff

Key Equations

• Ocean Acidification:

$CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow 2HCO_3^-$

• Hypoxic Water Threshold: $DO < 2 \text{ mg/L}$ or $DO < 2 \text{ ppm}$

Summary Table: Types of Water Pollutants and Their Effects

Pollutant Type	Source	Main Effects
Sediment	Soil erosion, construction	Blocks light, clogs gills, habitat loss
Nutrients (N, P)	Fertilizer, sewage	Eutrophication, algae blooms
Heavy Metals	Industry, mining	Toxicity, bioaccumulation
Pathogens	Sewage, animal waste	Disease outbreaks
Organic Compounds	Pesticides, PCBs	Poisoning, endocrine disruption
Thermal	Power plants	Reduced DO, stress on organisms

Practice Questions

• What is nonpoint source pollution? Pollution from multiple, diffuse sources (e.g., agricultural runoff).

• Which zone of a lake supports photosynthesis? Littoral zone.

• What role do decomposers play? Break down dead organic matter and recycle nutrients.

• What is a significant human impact on lake ecosystems? Introduction of invasive species.

• Which of the following is most likely to lead to eutrophication? Livestock manure run-off.