Seaweed As A Climate Change Solution In New Zealand

Historically seaweed has fallen outside blue carbon estimates because it grows on rocky shores and, therefore, does not trap carbon rich sediments (Macreadie et al. 2021). Long-term carbon sequestration can only occur if seaweed is exported from the seaweed habitat and incorporated into seafloor sediments or deposited in the deep ocean (> 1,000 m; Baker et al. 2022). This spatial disconnect between the carbon source and the storage locations complicates carbon accounting because movement of carbon is difficult to track (Hurd et al. 2022).

New Zealand has extensive seaweed diversity, with over 900 species (Nelson 2020). Seaweed habitats occur along much of New Zealand's coastline (Shears & Babcock 2007), but unfortunately, reliable national-scale estimates of seaweed habitat extent are not available. Due to the difficulties in carbon accounting described above, long-term carbon sequestration rates for New Zealand seaweed habitats are yet to be published. However, a global study estimates that 11% of carbon is exported and sequestered from kelp forests (Krause-Jensen and Duarte 2016). The global coastline is 1.16 million km, kelp can be found in 25% of this area (Wernberg et al. 2019) or 290,000 km. Global seaweed Net Primary Production is 1.32 Pg C/year (Duarte et al. 2022). Given the New Zealand coastline is completely within the kelp biome and is 15,000km long, a best estimate of NZ coastline seaweed Net Primary Production is 68.28 Tg/C/yr-1 (15,000/290,000x1.32). 11% of this may be exported and will become a long term sink (Duarte and Krause Jensen 2016) or 7.51 Tg/ C, which would suggest carbon sequestration of 27.56 Mt/CO2/yr-1. 

Overfishing of urchin predators has led to the decline of kelp forest in parts of New Zealand as high urchin populations have predated on kelp forests (Shears and Babcock 2003, Udy et al. 2019). In addition, New Zealand has suffered from increased coastal sedimentation (Shears et al. 2022), and recently marine heatwaves (Thomsen et al. 2019), which has led to die off of kelp forests. This loss of wild seaweed forests is likely to have reduced the natural carbon sequestration occurring, however, this change remains unquantified. Like other blue carbon habitats, seaweed restoration offers opportunities to increase the carbon storage of our oceans. Seaweed restoration is also becoming of increasing importance as New Zealand faces additional threats such as heat waves that have led to the die-off of some seaweed species (Thomsen et al. 2019). There are several emerging mechanisms for seaweed restoration (Eger et al. 2022) which may be explored in a New Zealand context, however as discussed it is difficult to attribute increased carbon sequestration to seaweed restoration. 

An alternative mechanism for increasing carbon storage in seaweeds in New Zealand is to grow seaweed in an aquaculture setting. New Zealand has a large aquaculture industry and is actively considering seaweed aquaculture expansion beyond what has been only small scale commercial production to date (Clark et al. 2021). New Zealand’s exposed rocky coastlines with greater currents and turbulence means that generally seaweed farms could export a higher portion of seaweed to be sequestered in the deep sea, than for example, large shallow shelves of South East Asia where it is easy for seaweed to accumulate in sediment below farms. However, New Zealand does have some coastal topography that facilitates sediment accumulation, such as the Marlborough Sounds and Akaroa Harbour. Seaweed carbon sequestration is currently not able to be accurately measured if it is exported beyond the farming site (Ross et al. 2023), which will make quantifying seaweed carbon sequestration illusive on the majority of the more exposed New Zealand coastline.

There are other emerging opportunities for seaweed in New Zealand to gain emission abatement from low carbon seaweed products, for example the displacement of high emitting fertilisers with low emission seaweed fertilisers (Duarte et al. 2021), and feeding livestock methane reducing seaweeds (Duarte et al. 2021, Roque et al. 2019). New Zealand has a large agricultural industry with 50% of the country's emissions coming from agriculture (Stats NZ 2021). As governments and managers plan for a low carbon future, seaweed aquaculture and restoration projects may, in concert, provide climate change mitigation opportunities. New Zealand has – because of its extensive coastline, unusual high macroalgal diversity (Nelson 2020) and many near-shore deep canyons – a unique opportunity to be leading globally on seaweed based aquaculture and climate change mitigation research.