Quantifying indigenous kelp contributions to New Zealand’s Blue Carbon sinksEcosystem Services

Mari Deinhart^{1, 2}, Bilewitch Jaret², Robert Hickson³, Roberta D’Archino², Daniel Leduc², Judy Sutherland², Amber Brooks², Lisa Smith², Sarah Bury², Josette Delgado², Ollie Kerr-Hislop², Frontin-Rollet², Clark Alexander⁴, Scott Nodder²

¹Victoria University of Wellington Te Herenga Waka, Aotearoa New Zealand - ²National Institute of Water and Atmospheric Research (NIWA) Taihoro Nukurangi, Aotearoa New Zealand - ³Blue Carbon Services Ltd, Aotearoa New Zealand - ⁴Skidaway Institute of Oceanography, USA

Due to increasing greenhouse gas emissions and the consequential increases in global temperature and climate change, nature-based solutions for capturing carbon (C), are becoming of significant interest. Several Blue Carbon (BC) methods are being developed, including the utilisation of kelp as potentially significant carbon mitigators through sequestration in the deep-sea, however the empirical data to validate these hypotheses is limited. This study aims to use a multi-methodological approach to quantify kelp contribution to BC in Aotearoa New Zealand’s (NZ) submarine canyons and deep-water environments. Multicore sediment samples and water column samples were collected for environmental DNA (eDNA) detection and quantification via droplet digital PCR (ddPCR), bulk stable isotope analyses of C and nitrogen (N), and 14C and 210Pb sediment dating at 35 sites throughout central NZ, to identify the transport mechanisms of kelp detritus from coastal habitats to deep-sea benthic environments. Five kelp-specific ddPCR eDNA assays were applied to 350 sediment and filtered seawater samples, to quantify kelp detritus transport to deep-sea submarine canyons. Bulk stable isotope signatures of sediments were analysed to identify δ13C and δ15N values unique to the five-target kelp, based on a previous experimental study that identified unique sediment profiles that correlated with specific kelp. eDNA detection of all five target species occurred in both sediment and water column samples, with varying abundance. Ecklonia radiata was the most commonly detected species in sediment samples. All five target species displayed heterogeneous eDNA concentrations in specific submarine canyon sites and concentrations were generally higher in water-column samples than sediments. Results from the eDNA analysis indicate that the amount of kelp detritus deposited in deep-sea C sinks varies between genera and location. This study presents a novel approach to begin to understand the role kelp fulfil for BC.