Studies carried out on lagoon, coastal and estuarine waters of the Great Barrier Reef region at Townsville have established marked changes in the chemical speciation of cadmium and iron during blooms of the blue-green alga Trichodesmium. Cadmium speciation was monitored using differential pulse anodic stripping voltammetry on unacidified, acidified and ultra-violet irradiated samples. Dissolved and particulate forms of iron were determined by atomic absorption spectrophotometry. It was established that during Trichodesmium blooms, labile forms of cadmium, and dissolved and particulate forms of iron markedly increased, and that these increases occurred during high concentrations of ‘marine humic acid’, associated with the presence of Trichodesmium.