Syllabus Edition
First teaching 2024
First exams 2026
Water Quality in Aquatic Ecosystems (HL) (DP IB Environmental Systems & Societies (ESS))
Revision Note
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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