Algal blooms are rapid increases in algae or photosynthetic microorganisms in water. Some blooms are toxic because particular species produce harmful chemicals, while others cause damage indirectly through oxygen depletion. This content is core for SL and HL, but toxin-producing blooms are studied specifically at HL in subtopic 4.4 Water pollution.
Blooms often begin with nutrient enrichment, especially increased nitrate and phosphate inputs from fertilizers, sewage or animal waste. In a process called eutrophication, these nutrients stimulate rapid growth of phytoplankton or cyanobacteria, particularly where water is warm, well-lit and slow-moving.
The dense bloom may block sunlight from submerged plants, reducing photosynthesis and causing plants and algae to die. Decomposer microorganisms then break down this organic matter through aerobic respiration, increasing biochemical oxygen demand (BOD) and decreasing dissolved oxygen. Severe oxygen depletion produces hypoxic or anoxic conditions, which can kill fish and other aquatic organisms.
| Harmful mechanism | Environmental or human impact |
|---|---|
| Toxin production | Some cyanobacteria produce toxins that may damage the liver or nervous system and contaminate drinking water. |
| Bioaccumulation in seafood | Toxins produced by some marine algae can accumulate in shellfish and poison consumers. |
| Oxygen depletion | Decomposition raises BOD, lowers dissolved oxygen and can cause fish kills. |
| Light blockage | Reduced photosynthesis kills submerged vegetation and disrupts food webs. |
A key misconception is that all algal blooms are toxic. They are not: only certain species produce toxins, but non-toxic blooms can still be harmful because their decomposition removes oxygen.
A named example is the 2014 bloom in Lake Erie, when cyanobacterial microcystin contaminated water supplied to Toledo, Ohio. Residents were temporarily advised not to drink the tap water, showing how agricultural nutrient inputs can affect ecosystems, public health and water security.
Exam technique: For an “explain” question, give a causal sequence: nutrient input → bloom → death and decomposition → increased BOD → reduced dissolved oxygen. If toxicity is asked, distinguish direct toxin production from indirect oxygen depletion and support the explanation with a named example.
