An assessment of Blue Carbon sources and potential in the Nisqually Estuary using biomarkers and compound-specific isotopes of…

The potential of vegetated coastal ecosystems to sequester significant organic carbon (Blue Carbon) has gained much attention as a climate mitigation strategy. Organic carbon buried in coastal sediment is produced both in situ and externally, and knowledge about sources is essential for Blue Carbon assessments and for ecosystem restoration and management. The goals of this study are to develop and utilize a suite of organic carbon source signatures for Salish Sea Blue Carbon ecosystems. Source signatures integrate information from traditional stable carbon and nitrogen isotope ratios (d13CTOC, d15NTOC), traditional biomarkers (n-alkanes, sterols), and biomarker-specific stable isotopes (CSIA: d13Cn-alk, d13Csterol) in a range of marsh and marine plants and sediment. Quantification of eelgrass carbon contributions to sediment pools using traditional tools is confounded by marine algal carbon with similar n-alkane peaks and d13CTOC ranges. CSIA differentiated eelgrass carbon from other marine sources (median d13Cn-alk of -19‰ versus -27‰). In comparison, terrestrial freshwater, high-marsh, and low-marsh plants had median d13Cn-alk -34‰, and median d13Csterol -39‰. Organic matter source signatures in eelgrass surface and subsurface (sub-root zone) sediment in the Nisqually River Delta six years after restoration of tidal flows to diked wetlands revealed a predominantly terrestrial carbon source. Eelgrass carbon was only important in the upper 4 cm of sediment in which it was rooted. The average chain length (ACL) of n-alkanes increased with depth (one-way ANOVA, p=0.002), indicative of earlier burial of terrestrial carbon from grasses, cattails, and rushes. Biomarkers and CSIA indicated that organic carbon exported from restoring wetlands was trapped in nearby seagrass beds and that contributions of in situ-produced carbon to long-term sedimentary pools may be relatively small. Organic matter source signatures can be used to derive Blue Carbon estimates for the Nisqually Estuary and other Salish Sea ecosystems.