Acute toxicity of heavy metals involves short-term exposure at relatively high concentrations with lethality in terms of median lethal concentration (LC50) or median effective concentration (EC50) as the end point. LC50/EC50 values of heavy metals and metalloids vary widely in different groups of freshwater plants and animals. The biotic ligand model (BLM) aims to explain the toxic effects on the premise that heavy metal ions have to compete with other cations such as calcium to adhere to biotic ligands (BLs) which are biological receptors on the surface of an organism, for example, the gill of fish. Some heavy metals primarily impacted respiratory mechanisms, while some others attacked the ionoregulatory physiology in freshwater animals. Chronic or sublethal toxicity studies use sublethal end points which could comprise morphological, physiological, biochemical, metabolic, and genetic effects on freshwater organisms. Metal chelators such as metallothioneins (MTs) and phytochelatins (PCs) are cysteine-rich polypeptides and have sulfhydryl groups to which metals can bind. Aquatic birds which are either resident or migratory visitors to wetlands are at risk of heavy metal – especially lead – toxicity due to the ingestion of sediment particles while feeding.