Water Quality in Aquatic Ecosystems (HL) (DP IB Environmental Systems & Societies (ESS)): Revision Note

Water pollutants

Types of pollutants

• A wide variety of pollutants can negatively impact aquatic ecosystems

  • These pollutants harm both water quality and the organisms living in these environments

Organic matter

• Organic pollutants, such as untreated sewage, add large amounts of biological material to water systems

• As bacteria break down the sewage, they consume dissolved oxygen

  • This can lead to oxygen depletion, resulting in the death of fish and other aquatic life

• For example, rivers in rapidly urbanising areas without proper waste treatment, like the Ganges River in India, suffer from sewage contamination

  • This kills fish and poses risks to human health

Dissolved substances

• Tributyltin (TBT) is an example of a harmful dissolved substance

• TBT is an anti-fouling agent once used on ship hulls

• It was found to disrupt the endocrine systems of marine animals, particularly causing reproductive issues in molluscs

  • TBT pollution was responsible for severe declines in oyster populations in some European estuaries before it was banned

Persistent organic pollutants (POPs)

• These include chemicals like polychlorinated biphenyls (PCBs)

• PCBs are resistant to breakdown and can persist in the environment for decades

• These toxins accumulate in organisms (bioaccumulation) and become more concentrated as they move up the food chain (biomagnification)

  • PCBs have accumulated in marine top predators like killer whales and polar bears, leading to immune and reproductive issues

Plastics

• Plastics, especially microplastics, are widespread pollutants in marine environments

• Larger plastics can cause entanglement or ingestion problems

• Ingested microplastics have been found in many species of fish, affecting their health

Heat energy

• Thermal pollution occurs when industrial plants or power stations release warm water into rivers or oceans

• Warmer water holds less dissolved oxygen and can harm species adapted to cooler conditions

  • Some freshwater ecosystems near nuclear power plants experience reduced fish populations due to increased water temperatures

Harmful algal blooms (HABs)

• Harmful algal blooms (HABs) occur when certain types of algae or microorganisms grow rapidly

• They form dense blooms that produce toxins or deplete oxygen in the water

• These blooms can significantly impact water quality

  • This poses threats to aquatic ecosystems, human health, and economic activities like fishing and tourism

• HABs contain a variety of organisms, including:

  • Cyanobacteria

  • Protists

  • Algae

  • Dinoflagellates

• Freshwater examples:

  • In freshwater environments, cyanobacteria (also known as blue-green algae) are the main contributors to HABs

  • These bacteria release cyanotoxins that can:

    • Contaminate drinking water

    • Cause liver damage

    • Affect the nervous system of both animals and humans

• Marine examples:

  • In marine environments, dinoflagellates (a type of protist) are responsible for many HABs

  • They sometimes produce 'red tides' that are toxic

  • The neurotoxins produced by these organisms can cause paralytic shellfish poisoning (PSP) in humans who consume contaminated seafood

• Economic impacts:

  • HABs affect fisheries, aquaculture, and tourism as toxins from blooms can lead to the closure of fisheries and swimming areas

    • This negatively impacts local economies

Anoxic & hypoxic water

• Anoxic waters lack oxygen completely

  • This state is known as anoxia

• Hypoxic waters have very low oxygen levels that are insufficient to support most aquatic life

  • This state is known as hypoxia

• Both conditions can create "dead zones", where marine organisms cannot survive

  • These zone usually occur in coastal regions where nutrient runoff is high

  • For example, the Gulf of Mexico experiences one of the largest dead zones, largely caused by nutrient pollution from agricultural runoff in the Mississippi River

  • The Baltic Sea is another region plagued by dead zones, largely due to nutrient runoff from agriculture and sewage

  • Dead zones can lead to:

    • Severe disruptions in marine food chains

    • Collapse of local fisheries

Causes

• Hypoxia and anoxia are driven by a combination of factors, including:

  • Global warming: Warmer water holds less oxygen, making conditions increasingly hypoxic

  • Thermal stratification: Layering of water by temperature prevents oxygen-rich surface water from mixing with deeper layers

  • Sewage disposal: Decomposing organic matter from sewage lowers oxygen levels

  • Eutrophication: excess nutrients, often from agricultural runoff, fuel algal blooms

    • When the algae die and decompose, oxygen is consumed, creating hypoxic conditions

• With climate change and continued pollution, the occurrence of hypoxic and anoxic waters is expected to rise

  • This will threaten fisheries and coastal ecosystems globally