Blue carbon coastal ecosystems: A joint SAEON and Nelson Mandela University initiative

By Lucienne Human, Jacqueline Raw, Jessica Els, Sinegugu Mbense, Janine Adams and Thomas Bornman Tweet

Sediment core from beneath the salt marsh in the Swartkops Estuary Seagrass at Swartkops Estuary Sectioning of the sediment core in process Dr Tommy Bornman (right) and Pumlile Cotiyane from SAEON’s Elwandle Node install the rods for a surface elevation (RSET) benchmark in the mangroves of the Nahoon Estuary Dr Jackie Raw (right) and Jaime Johnson, an MSc student, from the Nelson Mandela University Department of Botany extract a sediment core from the mangroves at the Nxaxo Estuary

Carbon sinks are the areas in the natural environment that are able to absorb carbon dioxide from the atmosphere and store it.

The term "blue carbon" refers to the carbon sequestered and stored in vegetated coastal and marine ecosystems (mangroves, salt marshes, seagrasses).

These carbon sinks are important for more than just their ability to capture and store carbon dioxide - they also reduce sediment suspension in the water column, reduce the impact of waves and result in clearer water in general, while acting as crucial habitats for juvenile fish and other invertebrates.

Research in this area has rapidly escalated over the past decade following a 2009 Rapid Response Assessment by the United Nations Environment Programme (UNEP), which revealed the extremely high efficiency of these habitats to act as carbon sinks.

Prior to this, vegetated coastal ecosystems were not explicitly included in estimates of marine or terrestrial carbon sinks, and this created a gap in global extrapolations. Following this, the International Union for Conservation of Nature (IUCN) issued a landmark report on the Management of Natural Coastal Carbon Sinks, which highlighted that national carbon accounting efforts showed a substantial 'blue carbon gap' between knowledge of these carbon sinks and the actions being taken by governments to safeguard their futures.

Loss of blue carbon ecosystems globally reduces the capacity of natural carbon sinks, and results in large amounts of previously stored carbon being released back into the atmosphere in the form of CO₂ emissions. The emissions released through ecosystem conversion are recognised by the Intergovernmental Panel on Climate Change (IPCC) and the United Nations Framework Convention on Climate Change (UNFCC) as significant sources of greenhouse gases. Global CO₂ emissions from degradation and disturbance of blue carbon ecosystems have been estimated at 45 billion metric tonnes annually.

Blue Carbon Initiative

In response to this, the Blue Carbon Initiative was formed. This a global, coordinated programme focused on mitigating climate change through the conservation and restoration of coastal and marine ecosystems. Blue carbon ecosystems are found on every continent, except for Antarctica.

Mangrove forests cover an estimated area between 13.8 and 15.2 Mha, followed by seagrass between 17.7 and 60 Mha and salt marsh between 2.2 and 40 Mha. In large, well-established blue carbon ecosystems, carbon storage can be extremely high. For example, studies on seagrasses in Spain and mangroves in Belize were reported having carbon-rich deposits greater than 10 m thick and were over 6 000 years old.

In South Africa, blue carbon ecosystems are restricted to occurring in estuaries and cover relatively small areas in comparison to other regions. However, these ecosystems are ecologically valuable as they are resilient and provide several ecosystem services. These are defined as the benefits which people obtain from natural systems, including food production (fisheries), water purification, coastal protection and carbon storage.

Current research which is being carried out in a collaboration between SAEON and Nelson Mandela University is focused on a few key aspects of these ecosystems. For example, this research will provide the first comprehensive assessment of carbon storage in mangrove, salt marsh and seagrass habitats for South Africa. Changes in occurrence and area cover for these habitats are also being assessed at a national level.

The Knysna Estuary has some of the largest seagrass beds in the country. A study is currently being conducted on the carbon stored in the seagrass biomass and sediment, through the extraction of sediment cores from various sites within the estuary.

Further research is being done in the Swartkops Estuary to determine the amount of carbon stored in the salt marsh as well as seagrass. Overall, the goal is to determine the total C stored in these ecosystems and verify if they truly are valuable sinks in South Africa.

Climate change responses

Another key research area which is being investigated through this collaboration is related to climate change responses of South African blue carbon ecosystems. Mangroves and salt marshes are generally considered to be ecologically stable as they have persisted through extreme environmental variability during prehistoric periods. In certain regions around the world these ecosystems have been able to persist for millennia through changes in global sea level. There is however a concern that current sea-level rise may be occurring too rapidly for these ecosystems to be able to persist.

To assess these trends, the responses of several mangroves and salt marsh ecosystems are being monitored by measuring surface elevation change. This is a process which occurs through sediment trapping and allows the elevation of the habitat to keep pace with the relative sea-level rise.

Three salt marsh estuaries (Swartkops, Kromme, Knysna) and two mangrove estuaries (Nahoon, Nxaxo) are currently being monitored in this joint research programme. There has been a global effort to measure the responses of blue carbon ecosystems to sea-level rise and this will be the first project to apply these methods to mangrove habitats in South Africa.