Investigations carried out at a coastal station in the Great Barrier Reef lagoon (GBRL) at Townsville, Australia have shown that the cycling of several trace metals (Fe, Mn, Zn, Cu, Ni, Cd, and Pb) was significantly influenced by the presence of Trichodesmium , a blue-green alga, which throughout the year, frequently forms red tide densities along much of the Queensland coral coast. Whilst decomposition of large masses of Trichodesmium significantly affected metal concentrations, metal speciation changes also occurred, and took place during the presence of senescent Trichoedesmium and high concentrations of humic material (10 mg/l).