Comment: Blue carbon is not substantial in Australian kelp forests

Blue carbon refers to the greenhouse gas mitigation services, regarded as primarily supplied by seagrass, mangrove, and saltmarsh relative to a likely alternative ecosystem1. However, a recent study calculates that kelp forests across the Australian Great Southern Reef (GSR) represent globally significant blue carbon stocks and sequestration services, suggesting that these ecosystems should be explicitly included within the blue carbon conceptual model. The article brings together data on the ostensibly Ecklonia radiata assemblage of the GSR in a worthy attempt to quantify the magnitude and importance of these systems as carbon sinks. Their sequestration calculations are based on the fraction of Ecklonia’s net primary production (NPP) that is either buried in adjacent sediments or exported away to the deeper parts of the ocean. The article also makes a carbon stock comparison between kelp forests and seagrass, mangrove, and saltmarsh ecosystems, which is based on the remnant standing biomass of these systems in Australia.

Substantial blue carbon in overlooked Australian kelp forests

Recognition of the potential for vegetated coastal ecosystems to store and sequester carbon has led to their increasing inclusion into global carbon budgets and carbon offset schemes. However, kelp forests have been overlooked in evaluations of this ‘blue carbon’, which have been limited to tidal marshes, mangrove forests, and seagrass beds. We determined the continental-scale contribution to blue carbon from kelp forests in Australia using areal extent, biomass, and productivity measures from across the entire Great Southern Reef. We reveal that these kelp forests represent 10.3–22.7 Tg C and contribute 1.3–2.8 Tg C year−1 in sequestered production, amounting to more than 30% of total blue carbon stored and sequestered around the Australian continent, and ~ 3% of the total global blue carbon. We conclude that the omission of kelp forests from blue carbon assessments significantly underestimates the carbon storage and sequestration potential from vegetated coastal ecosystems globally.

Coral bleaching in the southern inshore Great Barrier Reef: a case study from the Keppel Islands

Spatially explicit coral bleaching data can be used to improve our understanding of the causes and consequences of coral bleaching and help identify resilient reefs. In 2016, the Great Barrier Reef (GBR) experienced the most severe coral bleaching in recorded history, yet, as in previous 1998 and 2002 events, the severity and spatial extent of coral bleaching were variable. Cyclonic activity mitigated warming effects in the southern GBR, meaning corals in this region were predicted to bleach less; as a result, southern reef areas received little attention. Herein we report the effects of the 2016 warming event on southern inshore reefs around the Keppel Islands, an area of high conservation importance, with a history of environmental disturbance. Surveys of 14 reefs revealed paling of coral colonies at every site. A total of 21% of living coral, primarily Pocillopora and branching Acropora, was affected. Findings suggest that southern reefs were affected by warming, although significantly less than in the north. Records of milder bleaching help delineate variability in bleaching severity and extent across the GBR, and add to the historical record of bleaching history in the Keppel Islands, essential to understanding the complexity of exposure and recovery dynamics of the Keppel reefs.

The 'Great Southern Reef': social, ecological and economic value of Australia's neglected kelp forests

Kelp forests define >8000km of temperate coastline across southern Australia, where ~70% of Australians live, work and recreate. Despite this, public and political awareness of the scale and significance of this marine ecosystem is low, and research investment miniscule (<10%), relative to comparable ecosystems. The absence of an identity for Australia’s temperate reefs as an entity has probably contributed to the current lack of appreciation of this system, which is at odds with its profound ecological, social and economic importance. We define the ‘Great Southern Reef’ (GSR) as Australia’s spatially connected temperate reef system. The GSR covers ~71000km2 and represents a global biodiversity hotspot across at least nine phyla. GSR-related fishing and tourism generates at least AU$10 billion year–1, and in this context the GSR is a significant natural asset for Australia and globally. Maintaining the health and ecological functioning of the GSR is critical to the continued sustainability of human livelihoods and wellbeing derived from it. By recognising the GSR as an entity we seek to boost awareness, and take steps towards negotiating the difficult challenges the GSR faces in a future of unprecedented coastal population growth and global change.