Negligible isotopic fractionation of nitrogen within temperate Zostera spp. meadows

Seagrass meadows form an ecologically important ecosystem in the coastal zone. The inline-formula¹⁵N∕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 inline-formula¹⁵N∕¹⁴N ratios of seagrass beds, their associated sediments and, of critical importance, the porewater inline-formula $M3inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimgf83a9f1907f38a5589c34b239e10518b bg-15-7225-2018-ie00001.svg24pt15ptbg-15-7225-2018-ie00001.png pool, which is most bioavailable. Here, we undertook a study of the inline-formula¹⁵N∕¹⁴N ratios of seagrass tissue, sediment porewater inline-formula $M5inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg68d940fa21d9c6691de36bd82f3e56d8 bg-15-7225-2018-ie00002.svg24pt15ptbg-15-7225-2018-ie00002.png 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 inline-formulaδ¹⁵N and seagrass inline-formulaδ¹⁵N (inline-formular² of 0.92 and mean offset of 0.9 ‰), illustrating a close coupling between the plant and sediment pools. The inline-formulaδ¹⁵N of porewater inline-formula $M10inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg1828c3f6b34695d5ee1ef7c7b60e0fa3 bg-15-7225-2018-ie00003.svg24pt15ptbg-15-7225-2018-ie00003.png was strongly correlated with the inline-formulaδ¹⁵N 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 inline-formula1:1, reflecting an enrichment of the porewater inline-formula $M13inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg9d8f7ee8bf88d657d75cdcf077dbe3e2 bg-15-7225-2018-ie00004.svg24pt15ptbg-15-7225-2018-ie00004.png inline-formulaδ¹⁵N pool relative to seagrass tissue and bulk sediment inline-formulaδ¹⁵N at high inline-formulaδ¹⁵N values. We suggest that nitrogen fixation is the most likely explanation for the observation that the inline-formulaδ¹⁵N of seagrass tissue is lower than porewater inline-formula $M18inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg7b888eb222795ca6a6dd2ceb535b59a0 bg-15-7225-2018-ie00005.svg24pt15ptbg-15-7225-2018-ie00005.png . Conversely, we suggest that the most likely explanation for the enrichment of porewater inline-formula $M19inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimgb054521cc8a5d2267742c16e315b1d01 bg-15-7225-2018-ie00006.svg24pt15ptbg-15-7225-2018-ie00006.png above bulk sediment was through the preferential mineralisation of isotopically enriched algal material (nitrogen derived from sewage sources) within the sediment as inline-formulaδ¹⁵N increased in the vicinity of a sewage treatment plant.