Isotopic and molecular analyses of n-alkanes in a temporal study of coastal sediment contributions to organic carbon degradation induced by algal bloom and terrestrial runoff

The priming effect (PE) refers to the enhanced remineralization of recalcitrant organic carbon (OC) driven by the respiration of labile OC, potentially increasing CO₂ fluxes from aquatic ecosystems. Patterns of PE induced by marine and terrestrial OC inputs can be explored through sedimentary contributions to the degraded OC pool. In this study, coastal sediments (δ¹³C_bulk = -25.26 ± 0.06 ‰, 1.63 ± 0.07 % OC) were spiked with isotopically distinct marine and terrestrial OC sources (Nannochloropis phytoplankton, δ¹³C = -43.18 ± 0.31 ‰; and C₄ corn leaves, δ¹³C = -13.90 ± 0.09 ‰). Source contributions to respired OC were investigated using n-alkane concentration profiles and stable carbon isotopes (C15-C30) across 30 microcosms. Elevated concentrations of ¹³C-enriched high molecular weight n-alkanes (e.g., δ¹³C_C29 = -26.3 ± 0.5 ‰) were observed in corn leaf amendments, whereas the phytoplankton spike exhibited a higher abundance of ¹³C-depleted low molecular weight n-alkanes (e.g., δ¹³C_C17 = -46.8 ± 0.4 ‰). Mixing models indicate the sedimentary OC contribution to the degraded biomarkers, for which an increasing trend suggests a PE. Phytoplankton-amended microcosms showed a sediment OC contribution of 10.3 ± 1.5 % to the degradation of the C17 n-alkane. The corn leaf spike resulted in consistently higher contributions of 30.4 ± 3.6 % for the lost C29 n-alkane, documenting the effect of carbohydrate rich organic matter on sedimentary OC remineralization. A synergistic interaction emerged when sediments received a mix of marine and terrestrial OC, exhibiting contributions to n-alkane loss of 48.3 ± 5.3 % for C17, and 35.2 ± 5.2 % for C29. Following biochemical fractionation that leads to the selective breakdown of certain biochemical structures, our data indicate greater sedimentary degradation during induced terrestrial runoff compared to an algal bloom, providing a quantitative measure of OC remineralization.