4.4.9 (HL)—Water pollutants

Classify a pollutant by its harm mechanism because the mechanism determines the useful measurement and treatment.

Organic matter raises microbial oxygen demand; tributyltin disrupts endocrine/reproductive processes; PCBs persist and biomagnify; plastics cause ingestion/entanglement; heat lowers oxygen solubility and exceeds tolerance.

A PCB signal in top predators suggests persistent food-web transfer, while a sewage pulse predicts high BOD and low DO.

Temperature and dissolved oxygen, because the mechanism is heat-driven solubility and stress.

‘Toxic’ is not one pathway; persistent, physical, thermal and oxygen harms require different evidence.