Syllabus Edition

First teaching 2024

First exams 2026

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

Revision Note

Alistair Marjot

Written by: Alistair Marjot

Reviewed by: Jacque Cartwright

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

Examiner Tips and Tricks

The prefix hypo- means 'below normal' or 'deficient'. So, hypoxia refers to low oxygen levels in tissues, but not necessarily a complete lack. You might recall hypothermia (low body temperature) to remember that hypo- indicates a deficiency.

The prefix an- means 'without'. Therefore, anoxia means a complete absence of oxygen. You can link this to anaemia, where blood lacks sufficient healthy red blood cells or haemoglobin, leading to insufficient oxygen transport.

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Jacque Cartwright

Author: Jacque Cartwright

Expertise: Geography Content Creator

Jacque graduated from the Open University with a BSc in Environmental Science and Geography before doing her PGCE with the University of St David’s, Swansea. Teaching is her passion and has taught across a wide range of specifications – GCSE/IGCSE and IB but particularly loves teaching the A-level Geography. For the past 5 years Jacque has been teaching online for international schools, and she knows what is needed to get the top scores on those pesky geography exams.