1. Algal Bloom
- The term algal bloom refers to a rapid and excessive increase in the population of algae or blue-green algae (cyanobacteria) in a water body such as a pond, lake or reservoir.
- This overgrowth often discolours the water (it may appear green, blue‑green or scummy on the surface).
- Algal blooms generally occur when the water becomes enriched with nutrients like nitrates and phosphates (a process called eutrophication), often due to fertilizer run‑off and pollution.
- When the dense algal population dies and decomposes, it increases the biological oxygen demand (BOD) of the water, leading to a decrease in dissolved oxygen and causing the death of fishes and other aquatic animals.
2. Red Tides
- Red tides are discolourations of seawater (reddish or brownish) caused by the rapid multiplication or “blooms” of red‑pigmented dinoflagellates, especially species like Gonyaulax.
- Due to their extremely high concentration near the surface, the sea or coastal water appears red in colour, hence the name “red tide”.
- Many red dinoflagellates produce powerful toxins. When they occur in large numbers, these toxins may kill large numbers of fish and other marine organisms and may also affect humans through contaminated seafood.
- Red tides are therefore considered a type of harmful algal bloom in marine environments.
Short Exam-style Answer
An algal bloom is a rapid increase in the population of algae or blue‑green algae in polluted or nutrient‑rich water bodies, causing discolouration of water and depletion of dissolved oxygen, which may lead to the death of aquatic animals. Red tides are the red colouration of seawater caused by the explosive multiplication of red‑pigmented dinoflagellates such as Gonyaulax, which often release toxins that can kill large numbers of fishes and other marine organisms.