PYTR Class

Learning Objectives

1. Describe the process of eutrophication
2. Identify the primary nutrients responsible for eutrophication.
3. Outline the sequence of ecological impacts caused by eutrophication.
4. [AHL] Describe the impact of eutrophication as harmful algal blooms (HABs)
5. Evaluate how eutrophication affects ecosystem services.
6. Discuss different management approaches to address eutrophication at various levels.

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Part 1: Eutrophication Process

• Occurs when streams, lakes, estuaries, and coastal waters receive excess mineral nutrients (especially nitrates and phosphates).
• Triggers excessive phytoplankton growth, especially if previously limited by low phosphate/nitrate levels.
• Algal blooms result, leading to oxygen depletion and reduced biodiversity in aquatic ecosystems.
• Common human sources include detergents, sewage, and agricultural fertilizers.
• Nutrient enrichment increases algal blooms; as algae die and decompose, more nutrients are released — a positive feedback loop.
• Dense algal and cyanobacteria growth shades submerged plants, blocking sunlight.
• Decomposition of biomass causes anoxia (oxygen starvation), especially in warmer conditions.
• Species composition shifts due to changes in oxygen levels and light availability.
• Eutrophication is a dynamic process influenced by fluctuating nitrate and phosphate concentrations.

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Anthropogenic Eutrophication

• Human activities have doubled nitrogen and phosphorus levels in many rivers; local increases of up to 50x reported.

Phosphorus

• Rare in Earth’s crust; no gaseous atmospheric reservoir.
• In tropical ecosystems, often the limiting nutrient.
• Major source: domestic detergents in sewage.
• Accounts for 20–60% of phosphorus in UK watercourses.
• More phosphorus → more plankton, fewer freshwater plants.

Nitrogen

• 80% of atmosphere is nitrogen; however, in terrestrial ecosystems, it’s often the limiting nutrient due to leaching.
• Increased nitrogen deposition due to air pollution (nitrous oxides from vehicles and power plants).
• Fertilisers contribute via:
  • Leaching through soil by drainage water.
  • Runoff from animal excreta used as fertiliser.
  • Soil erosion or particle movement into drainage systems.
• In regions like Europe and the USA (e.g., Chesapeake Bay), livestock slurry spread on fields adds significant nitrogen and phosphorus to ecosystems.

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Impacts of Eutrophication

• Algal bloom death and decomposition reduce dissolved oxygen (hypoxia/anoxia), harming aquatic life.

Consequences

• Increased turbidity reduces light for submerged plants.
• Higher sediment deposition rates shorten lake lifespan by slowing water flow.
• Elevated net primary productivity due to algal/bacterial blooms.
• Oxygen levels drop as decomposers consume it during biomass breakdown.
• Primary producer diversity initially rises, then falls as cyanobacteria dominate.
• Fish populations decline; oxygen-sensitive species die or migrate, and surface-dwelling coarse fish (e.g., pike, perch) become dominant.
• Submerged macrophytes decline due to light limitation from dense algal blooms, despite nutrient availability.

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Part 2: [AHL] Harmful Algal Blooms (HABs)

• Organisms involved:
  • Cyanobacteria
  • Protoctists such as:
    • Algae
    • Dinoflagellates.
• Only a small number produce potentially fatal toxins.

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Freshwater HABs

• Main toxin producers: Cyanobacteria, which release cyanotoxins.
• Exposure routes:
  • Recreational activities (e.g., swimming)
  • Drinking contaminated water
  • Inhaling aerosolized toxins (contaminated air droplets)
  • Eating contaminated fish or shellfish
• Health symptoms:
  • Skin, eye, throat, lung, and/or nose irritation

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Marine (Coastal) HABs

• Main organisms: Dinoflagellates
  • Many produce potent neurotoxins
• Health risks from contaminated seafood:
  • Paralytic shellfish poisoning (PSP)
  • Neurotoxic shellfish poisoning (NSP)
  • Amnesic shellfish poisoning (ASP)
  • Diarrheal shellfish poisoning (DSP)
• Toxin accumulation:
  • Even when toxic dinoflagellates are in low concentrations, they may cause no immediate impact.
  • At high densities, toxins can bioaccumulate in shellfish, zooplankton, and herbivorous fish.
  • These toxins then biomagnify up the food chain, increasing risk to predators, including humans.

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Other Factors

• Non-toxic algae can still be harmful:
  • They may consume large amounts of oxygen, leading to hypoxia (oxygen depletion) and harming aquatic ecosystems.
• Protists:
  • Not classified as animals, plants, bacteria, or fungi.
  • Examples: Amoeba, brown algae, red algae

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Part 3: Eutrophication on Ecological Services

Effects on Fishing

• Nutrient enrichment (nitrates/phosphorus) can:
  • Increase aquatic productivity.
  • Boost fish stocks in some regions.
• Impact on fish species:
  • Eutrophic waters often favour carp, a less desirable species compared to salmon or trout.
  • Carp disturb bottom sediments, reducing water clarity.
• North American examples:
  • Eutrophic lakes often produce large populations of stunted panfish.
  • Turbidity from algae and sediments limits predator visibility, allowing panfish to proliferate.
• Algae effects:
  • Surface algae block sunlight from reaching deeper water.
  • Oxygen depletion occurs, reducing fish quality and diversity.
• Marine impacts:
  • Open oceans are naturally nutrient-poor; fish stocks are limited by primary productivity.
  • Coastal nutrient runoff can cause algal blooms or red tides (e.g., from dinoflagellates).

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Effects on Recreation

• Dense macrophyte growth can:
  • Obstruct access for fishing, sailing, swimming, and paddle-boarding.
• Aesthetic issues:
  • Scum and odours make waterbodies unpleasant to view and smell.
  • Invasive species (e.g., water hyacinths, Nile cabbage) can:
    • Cover large surface areas.
    • Block light to underwater vegetation.
• Decomposition effects:
  • High levels of dead organic matter lead to:
    • Low oxygen levels.
    • Emissions of methane (CH₄) and hydrogen sulfide (H₂S).
  • Resulting in unpleasant conditions for recreational users.

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Aesthetic Impacts

• Eutrophication reduces the natural beauty of aquatic environments.
• Key aesthetic issues include:
  • Murky or scummy water.
  • Unpleasant smells.
  • Altered biodiversity and habitat degradation.
  • Less appeal for walkers, swimmers, and nature enthusiasts.

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Health Concerns

• Stomach cancer:
  • Linked to high nitrate levels in drinking water.
  • Though debated, studies show elevated cancer rates in areas with nitrates over 90 mg/dm³ (e.g., parts of Nigeria).
• Blue baby syndrome (methemoglobinemia):
  • Caused by low oxygen levels in a baby’s blood, linked to nitrates in water.
  • Risk increases when well water contains over 10 mg/L of nitrate.
  • Most dangerous for infants under six months.

Part 3: Management Strategies

Altering Human Activities

Reducing Emission

Clean-Up Strategies

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Part 4: Management Strategies in USA vs Europe

The management strategies for eutrophication in the USA and Europe have seen varying levels of success, with both regions employing a mix of regulatory, technological, and voluntary approaches.

• Europe tends to have a more cohesive, enforceable framework due to EU-wide policies, but faces difficulty with agricultural resistance and uneven progress.
• The USA has strong regulation of point sources but struggles with nonpoint pollution and decentralised governance.
• In both regions, success depends on cross-sector cooperation, sustained funding, and addressing the legacy of past nutrient loads.

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USA

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Europe