06 Aquatic ecosystems, water quality and sustainability
A lake or a river is an ecosystem in which the food chain runs from phytoplankton (producers) to zooplankton (primary consumers), small fish, large fish and top predators. Energy moves one way and is lost at each trophic level as heat, giving the classic 10% rule: only about one tenth of the energy of a level reaches the next.
- Good-quality fresh water: pH 6.5–8.5, DO above 5 mg/L, low BOD, low turbidity and no pathogenic coliforms.
- Eutrophication: excess nitrates and phosphates from fertilisers and detergents cause an algal bloom; the algae die, bacteria decompose them, BOD rises, DO falls and fish die — the water becomes septic and anaerobic.
- Thermal pollution from power stations lowers the solubility of oxygen and speeds metabolism, worsening the oxygen deficit.
- Sustainable use: lining canals and pipes to stop seepage, drip and sprinkler irrigation instead of flooding, treated sewage re-use for agriculture, industrial cooling loops, rainwater harvesting and desalination of seawater for drinking.
- The water–energy–food nexus: pumping, desalinating and treating water all consume energy, so saving water saves energy and cuts carbon emissions at the same time.