Tracing species-specific kelp eDNA in marine sediments for blue carbon assessment along the Norwegian Coast

Terrestrial primary production is acknowledged as a major contributor to atmospheric carbon transformation and storage. In contrast, marine primary production, especially the contribution from kelp forests, remains less studied and accounted for. To facilitate studies on marine kelp and organic carbon, we here introduce an environmental DNA-based approach for tracing two common kelp species in Nordic marine sediments, establishing a basis for estimating the organic carbon buried from the targeted species. We developed and tested species-specific quantitative PCR (qPCR) assays for Saccharina latissima and Laminaria hyperborea. Next, we applied the assays on 79 marine surface (0–2 cm) sediment samples along the Norwegian coast (58° N to 72° N), ranging from shallow nearshore (8.1 m) to deep offshore locations (1334 m). Kelp DNA was detected in 87% of the locations, with DNA from S. latissima and L. hyperborea detected in 73% and 72% of the locations, respectively. DNA from both species were detected together in 58% of the locations. Spatial analysis revealed higher kelp DNA concentrations in shallow, coastal areas, particularly for S. latissima while kelp DNA was largely absent at the offshore locations. Both species exhibited a strong correlation with the sediment total organic carbon (TOC) concentration. Interestingly, kelp eDNA concentrations and TOC concentrations were lower in areas where kelp forests were impacted by sea urchin grazing compared to non-impacted areas. Our study presents a novel method for tracing key kelp species and provides a framework to estimate their potential contributions to sediment carbon pools and assessing kelp carbon sequestration. This framework has the potential to improve assessment of blue carbon contributions in sediments and should also be applicable to other organisms beyond kelp.