IB Environmental systems and societies 4.4E Algal Blooms and Anoxic/hypoxic Waters Notes

Harmful algal blooms turn nutrient-rich, warm water toxic and oxygen-poor

1. Harmful algal blooms (HABs) are the excessive growth of cyanobacteria, protists, algae, or dinoflagellates in aquatic ecosystems.
2. They are driven by nutrient pollution, warming, or water stagnation.
3. Most blooms are non-toxic but still harmful, blocking light and stripping oxygen as they decompose.
4. Some release toxins with neurotoxic, hepatotoxic, or paralytic effects on humans and animals.

Main groups that cause HABs

1. Cyanobacteria (blue-green algae): dominant in freshwater.
2. Dinoflagellates: common in marine environments and estuaries.
3. Protists and other algae: diverse eukaryotes that thrive in nutrient-rich water.
4. Most blooms are non-toxic but still harmful, depleting oxygen and blocking sunlight.

What drives HABs

1. Eutrophication: excess nitrates and phosphates promote algal growth.
2. Rising temperatures: warmer water speeds algal reproduction.
3. Stratification: blocks oxygen mixing and creates stagnant conditions.
4. Sewage and agricultural runoff: major nutrient inputs.
5. Calm, well-lit water: favours photosynthetic growth and bloom persistence.

Freshwater HABs: cyanobacteria and cyanotoxins

1. Common genera are Microcystis, Anabaena, Nostoc, and Cylindrospermopsis.
2. They thrive in nutrient-rich, warm, and stagnant water.
3. Cyanotoxins
   1. Microcystins: damage liver tissue.
   2. Anatoxins: attack the nervous system.
   3. Cylindrospermopsin: causes liver and kidney damage.
4. Exposure pathways
   1. Drinking contaminated water.
   2. Skin contact during recreation such as swimming.
   3. Inhaling aerosolized toxins near blooms.
   4. Eating contaminated fish or shellfish.