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Lichens do not grow in polluted areas. Therefore, their absence from a locality can indicate the presence of environmental pollution.
Ref: NCERT Class 11 Biology Chapter 2: Biological Classification Viruses Viroids Prions and Lichens
Biochemical oxygen demand, or BOD, measures how much dissolved oxygen aerobic microorganisms consume while decomposing biodegradable organic matter under standardized conditions, commonly over five days at 20°C. A high BOD therefore means that the water contains a substantial load of decomposable material or other substances supporting intense microbial respiration. Sewage, manure, food-processing waste, and organic-rich runoff are common sources. As bacteria oxidize this material, dissolved oxygen can fall, stressing or killing fish and oxygen-sensitive invertebrates and favoring organisms tolerant of hypoxia. High BOD is consequently a widely used indicator of organic pollution rather than evidence of abundant oxygen. Clean water generally has little biodegradable substrate and therefore a low oxygen demand. Temperature affects both oxygen solubility and metabolic rate, so it must be controlled during testing; “cold water” is not what BOD itself measures. Chemical oxygen demand is a related but broader assay that chemically oxidizes susceptible compounds. The ecological link is direct: more microbial decomposition creates greater oxygen demand and signals poorer water quality.
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 2-3
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