The distribution and speciation of arsenic within an open marine seagrass ecosystem in Lake Macquarie, NSW, Australia is described. Twenty-six estuarine species were collected from five trophic groups (autotrophs, suspension-feeders, herbivores, detritivores and omnivores, and carnivores). Sediment, detritus, epibiota and microinvertebrates were also collected and were classified as arsenic source samples. There were no significant differences in arsenic concentrations between trophic groups and between pelagic and benthic feeders. Benthic-dwelling species generally contained higher arsenic concentrations than pelagic-dwelling species. Sediments, seagrass blades and detritus contained mostly inorganic arsenic (50-90 %) and arsenoribosides (10-26 %), with some methylarsonate (9.4-14.6 %) and dimethyarsinate (7.9-9.7 %) in seagrass blades and detritus. Macroalgae contained mostly arsenoribosides (40-100 %). Epibiota and other animals contained predominately arsenobetaine (63-100 %) and varying amounts of dimethyarsinate (0-26 %), monomethyarsonate (0-14.6 %), inorganic arsenic (0-2 %), trimethylarsenic oxide (0-6.6 %), arsenocholine (0-12 %) and tetramethylarsonium ion (0-4.5 %). It was concluded that arsenic concentrations and species within the organisms of the Lake Macquarie ecosystem are species-specific and determined by a variety of factors including exposure, diet and the physiology of the organisms.