Negligible isotopic fractionation of nitrogen within temperate <i>Zostera</i> spp. meadows

Abstract

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 sewage-derived 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 NH4+ pool, which is most bioavailable. Here, we undertook a study of the 15N∕14N ratios of seagrass tissue, sediment porewater NH4+ 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 δ15N and seagrass δ15N (r2 of 0.92 and mean offset of 0.9 ‰), illustrating a close coupling between the plant and sediment pools. The δ15N of porewater NH4+ was strongly correlated with the δ15N 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 1:1, reflecting an enrichment of the porewater NH4+ δ15N pool relative to seagrass tissue and bulk sediment δ15N at high δ15N values. We suggest that nitrogen fixation is the most likely explanation for the observation that the δ15N of seagrass tissue is lower than porewater NH4+. Conversely, we suggest that the most likely explanation for the enrichment of porewater NH4+ above bulk sediment was through the preferential mineralisation of isotopically enriched algal material (nitrogen derived from sewage sources) within the sediment as δ15N increased in the vicinity of a sewage treatment plant.