Seasonal and Spatial Variation in the Growth, Biomass Production, Mortality and Potential Blue Carbon Production of Golden Kelp (Ecklonia radiata) Off Eastern Australia

Kelp removes carbon from the oceans as it grows, with the potential for long-term removal of this ‘blue carbon’ through the sequestration of kelp detritus in deep offshore sediments and waters. However, understanding the magnitude of kelp's blue carbon potential requires more detailed information about kelp growth, biomass production, and detrital production through erosion and mortality. Here, hole punch growth measurements were conducted on tagged kelp plants to quantify these factors at multiple sites and in each season across 7° of latitude in New South Wales (NSW), Australia. Kelp biomass production varied among sites and among seasons, being higher over winter and spring than in summer and autumn and higher at Port Stephens than at Coffs Harbour, Batemans Bay and Eden. However, detrital production per plant, from kelp erosion and mortality, did not vary significantly among sites or seasons, although detrital production per unit area varied among sites with differences in kelp density. Overall, kelp growth and biomass and detrital production in NSW were comparable to those previously recorded in South Australia and Western Australia, although erosion was generally lower and mortality rates were generally higher than in Western Australia. This has important implications for management; the relatively uniform growth, biomass and detrital production across Australia indicates that, to maximise the blue carbon potential of kelp forests across Australia's Great Southern Reef, management should focus on preserving large, high-density kelp forests with high levels of detrital transport into deep offshore waters.