4.4E Algal blooms and anoxic/hypoxic waters (HL) Notes

1. Harmful algal blooms (HABs) occur when cyanobacteria, protists, algae, or dinoflagellates proliferate excessively in aquatic ecosystems, often due to nutrient pollution (eutrophication), climate change, or water stagnation.
2. Some HABs release toxins harmful to humans, fish, and other animals, leading to neurotoxic, hepatotoxic, or paralytic effects.

Organisms Involved in HABs

1. Cyanobacteria (blue-green algae): dominant in freshwater.
2. Dinoflagellates: common in marine environments and estuaries.
3. Protists and other algae: diverse eukaryotic organisms that thrive in nutrient-rich water.

Causes of Harmful Algal Blooms

1. Eutrophication: excess nitrates and phosphates promote algal growth.
2. Rising temperatures: warmer waters accelerate algae reproduction.
3. Stratification: prevents oxygen mixing, creating stagnant conditions.
4. Sewage and agricultural runoff: major nutrient inputs.
5. Light and calm waters: favour photosynthetic growth and bloom persistence.

Freshwater Example - Cyanobacterial Blooms

Cyanobacteria and Cyanotoxins

1. Common genera: Microcystis, Anabaena, Nostoc, Cylindrospermopsis.
2. Thrive in nutrient-rich, warm, and stagnant waters.
3. Produce cyanotoxins such as:
   1. Microcystins: damage liver tissue.
   2. Anatoxins: affect nervous system.
   3. Cylindrospermopsin: causes liver and kidney damage.
4. Exposure pathways:
   1. Drinking contaminated water.
   2. Skin contact during recreation (swimming).
   3. Inhalation of aerosolized toxins near blooms.
   4. Consumption of contaminated fish or shellfish.
5. Effects on humans:
   1. Skin irritation, eye inflammation, nausea, vomiting.
   2. Chronic exposure may lead to liver disease or neurological symptoms.
6. Effects on animals:
   1. Domestic animals and livestock can die after drinking contaminated water.
   2. Fish die-offs due to oxygen depletion and toxin ingestion.

Marine Example - Dinoflagellates

1. Organisms: Karenia brevis, Alexandrium spp., and Gambierdiscus toxicus.
2. Toxin Type: Neurotoxins, paralytic, amnesic, and diarrhetic shellfish poisons.
3. Visual Appearance: “Red tides”, discolored water (red, brown, or orange) caused by dense dinoflagellate blooms.
4. Transmission:
   1. Bioaccumulation in shellfish (e.g., mussels, clams, oysters).
   2. Humans affected by eating contaminated seafood.
5. Human Illnesses:
   1. Paralytic Shellfish Poisoning (PSP): muscle paralysis due to saxitoxins.
   2. Neurotoxic Shellfish Poisoning (NSP): nausea, dizziness, and neurological effects.
   3. Amnesic Shellfish Poisoning (ASP): short-term memory loss due to domoic acid.
   4. Diarrhetic Shellfish Poisoning (DSP): gastrointestinal symptoms.
6. Ecological Impacts:
   1. Fish kills due to oxygen depletion or gill damage.
   2. Decline in biodiversity and alteration of marine food webs.
   3. Aerosolized toxins causing respiratory irritation in coastal populations.

Environmental and Economic Impacts of HABs

1. Ecological Impacts

1. Oxygen depletion (leading to hypoxia).
2. Fish kills and biodiversity loss.
3. Food web disruption from toxin biomagnification.

2. Human Health Impacts

1. Shellfish poisoning syndromes (PSP, NSP, ASP, DSP).
2. Respiratory and skin irritation from aerosols.
3. Long-term neurological or hepatic effects.

3. Economic Impacts

1. Closure of fisheries and aquaculture operations.
2. Loss of tourism revenue due to beach and waterway contamination.
3. Healthcare costs associated with poisoning cases.

Relationship Between HABs and Anoxic/Hypoxic Waters

1. Toxin-producing algae are not the only threat.
2. Dense blooms also block light and deplete oxygen
   1. Decomposition of dead algae by bacteria consumes dissolved oxygen.
   2. Leads to hypoxia (low oxygen) or anoxia (no oxygen).
   3. Creates dead zones where aquatic life cannot survive.
   4. Hypoxia frequency is expected to rise due to eutrophication and climate change.

Process of Dead Zone Formation

1. Nutrients enter the water body (fertilizers, sewage).
2. Algae bloom and block sunlight.
3. Algae die, sink, and decompose.
4. Bacteria consume oxygen during decomposition.
5. Oxygen levels drop, creating hypoxia or anoxia.
6. Aquatic life dies, forming a dead zone.

Consequences of Hypoxic/Anoxic Conditions

1. Mass fish deaths due to oxygen deprivation.
2. Loss of benthic organisms (bottom dwellers like clams, crabs, worms).
3. Collapse of local fisheries and food chains.
4. Release of toxic gases (H₂S, CH₄) from anaerobic decomposition.
5. Formation of permanent dead zones in coastal and estuarine systems.