Seagrass meadows form an ecologically important ecosystem in the coastal zone. The 15N=14N ratio of seagrass is commonly used to assess the extent to which sewagederived nitrogen may be influencing seagrass beds. There have, however, been few studies comparing the 15N=14N ratios of seagrass beds, their associated sediments and, of critical importance, the porewater NHC 4 pool, which is most bioavailable. Here, we undertook a study of the 15N=14N ratios of seagrass tissue, sediment porewater NHC 4 pool and the bulk sediment to elucidate the extent of any fractionating processes taking place during organic matter mineralisation and nitrogen assimilation. The study was undertaken within two coastal embayments known to receive nitrogen from a range of sources including marine, urban and sewage sources. There was close agreement between the bulk sediment 15N and seagrass 15N (r2 of 0.92 and mean offset of 0.9), illustrating a close coupling between the plant and sediment pools. The 15N of porewater NHC 4 was strongly correlated with the 15N of both the sediment and the seagrass tissue. For both of these relationships, however, the intercept of the line was not significantly different from 0 and the slopes were not 1 V 1, reflecting an enrichment of the porewater NHC 4 15N pool relative to seagrass tissue and bulk sediment 15N at high 15N values. We suggest that nitrogen fixation is the most likely explanation for the observation that the 15N of seagrass tissue is lower than porewater NHC 4 . Conversely, we suggest that the most likely explanation for the enrichment of porewater NHC 4 above bulk sediment was through the preferential mineralisation of isotopically enriched algal material (nitrogen derived from sewage sources) within the sediment as 15N increased in the vicinity of a sewage treatment plant.