Dr Victor Shelamoff, Dr Cayne Layton, Dr Jeff Wright, Dr Jeff Ross, Dr Craig Johnson
1Institute for Marine and Antarctic Studies, Battery Point, Australia
Net ecosystem production (NEP) refers to balance between gross primary production (GPP) and community respiration (CR) and is one measure of the potential of kelp forest ecosystems to provide a CO2 mitigation service. We used respirometry chambers on artificial reefs seeded with kelp (Ecklonia radiata) at four densities (zero (0), low (4), natural/medium (8), and high (16 kelp m-2)) to determine the effect of restoring kelp at different densities on daily NEP. While GPP and CR were highest on reefs supporting kelp at the natural density, CR exceeded GPP across all kelp densities, demonstrating net heterotrophy (CO2 source) for these systems. However, NEP of reefs supporting medium and high kelp densities were approximately half that of reefs with no kelp or kelp at low densities, indicating a relative reduction in CO2 in coastal waters resulting from restoring dense kelp stands, compared to allowing a degraded turf-dominated reef state devoid of kelp to establish. Relative CO2 emissions during the export of organic carbon to the deep ocean also needs to be considered to holistically assess any CO2 mitigation benefit. The observed negative NEP values shows that external subsidies lead to consequential increases in CO2 emissions affecting the value of kelp systems as a natural solution for climate change.