Quantifying indigenous kelp contributions to New Zealand’s deep-sea carbon sinks

Anthropogenic activities have caused an increase in greenhouse gas emissions, leading to the alarming rate of global temperature increase and associated climate change impacts. Due to these challenges, nature-based solutions for capturing greenhouse gases, in the form of carbon (C), are becoming of significant interest. Several marine carbon dioxide removal (mCDR) methods are being advanced, including the utilisation of brown macroalgae detritus, such as kelp, as potentially significant contributors to deep-sea C sinks. Although there are several studies that have utilized models to calculate kelp contribution to C sequestration, there is very little observational data to validate these hypotheses. Despite the many unknowns about the role kelp could have in mCDR, many industries are beginning to farm kelp in the open ocean for carbon credit schemes, with little regulation or understanding of the impact these aquaculture farms may have on the open ocean environment. The aim of the present study is to use a multi-methodological approach to quantify kelp contribution to mCDR in Aotearoa New Zealand’s (NZ) submarine canyons and deep-water environments. Multicore sediment samples and water samples were collected for eDNA analyses, bulk stable isotope analyses of C and nitrogen (N), and ¹⁴C and ²¹⁰Pb sediment dating, and the evaluation of environmental factors at 35 sites throughout central New Zealand, to identify the transport mechanisms of kelp detritus from coastal habitats to deep-sea benthic environments. Five genus-specific droplet digital PCR (ddPCR) eDNA assays were developed and applied to 350 sediment and filtered water samples, to identify if kelp contributions to C sequestration are genus-specific and to quantify how much kelp biomass may be transported and deposited from the coast to deep-sea submarine canyons. Bulk stable isotope signatures of sediment were analysed to identify δ¹³C and δ¹⁵N values that are unique to the five target kelp genera. Results from the ddPCR indicate that the amount of kelp detritus deposited in natural C sinks varies between genera and is dependent on geographical location. Although kelp was present in the sediment at some sites, there does not appear to be a significant contribution to deep-sea C stocks, therefore raising questions about the efficacy of macroalgal deposition as an effective mCDR tool.