The tropical Gulf of Carpentaria, Australia, has recently been identified as one of the world's least impacted marine areas, presenting a unique opportunity to understand the nutrient drivers of productivity.The present study examined the nitrogen (N) sources and transformations in this pristine area and the role of N in fuelling primary productivity, principally based on summer data. The N budget estimates on a whole-of-Gulf basis suggest that river N inputs are unlikely to be major contributors to primary productivity. In the deeper waters of the Gulf, beyond the coastal boundary current, the main source ofNis estimated to beNfixation by cyanobacteria, principally the abundant genusTrichodesmium.The present study measured high N fixation rates and depleted d15N-N ratios in the particulate matter in the water column during a summer bloom. During summer, bottom N concentrations increased and d15N-N ratios were depleted, suggesting that benthic mineralisation is occurring. It is therefore likely that detrital material from N-rich Trichodesmium is an important contributor to benthic processes. During winter, wind-driven mixing results in N from the bottom waters reaching the euphotic zone, and fuelling primary productivity. Therefore, Trichodesmium has an important direct and indirect role in contributing to primary productivity in this pristine tropical ecosystem.