Declines in seagrass biomass and growth have been widely reported in response to anthropogenic impacts. In contrast, the distribution and biomass of seagrass in the carbonate sediment around Green Island reef, part of Australia's Great Barrier Reef (GBR), has measurably increased during the past 50 yr, possibly due to increases in the availability of nutrients from local and regional anthropogenic sources. Using historical aerial photography, increases in seagrass distribution at Green Island have been mapped. The growth, morphological and physiological responses of 2 seagrass species (Halodule uninervis and Syringodium isoetifolium) to elevated sediment nitrogen (N; 100x control) and/or phosphorus (P; 10x control) were measured to investigate whether increased nutrients could account for the observed increase in distribution. Increases in the growth rate, amino acid composition and tissue nutrient content of both species occurred in response to elevated sediment N, but not P. Concentrations of the N-rich amino acids asparagine and glutamine increased 3- to 100-fold in seagrass leaves from N treatments. The δ15N values of leaves decreased in response to additions of nitrogen, probably due to increased discrimination against the 15N isotope, because N availability was surplus to demand. The low δ15N value of seagrasses in the Green Island back reef suggests that their primary source of N is either from N2 fixation or fertilisers and that the N from sewage is not a large component of their N budget. This study is the first to demonstrate N, rather than P, as the primary limiting nutrient for growth of seagrass in carbonate sediments and supports the hypothesis that the increase in the seagrass distribution and biomass at Green Island was caused by an increase in nutrient availability. We also hypothesise that seagrass distribution and biomass in many regions of the GBR may be limited by nutrients and that the lack of substantial seagrass meadows in the southern GBR could be due to these reefs receiving less nutrients from the mainland.