#05 2023

Turning blue to remedy greenhouse gas emissions and understand fluxes from coastal vegetated ecosystems

By Daniel Buttner (1,2,3), Dr Lucienne Human (1,2), Prof. Thomas Bornman (1,2) and Prof. Janine Adams (2,3,4)

Climate change, particularly in the context of global warming, is both a research priority and a major socio-economic concern for sustainable resource provisioning and utilisation. This climate challenge has broadly been framed from the perspective of de-escalating cumulative greenhouse gas (GHG) emission. 

As governments strive to achieve carbon neutrality to curtail global temperatures at 1.5 °C above preindustrial levels to avoid significant climate impacts (such as drought severity and storm frequencies), a deeper understanding of GHG sources, sinks and fluxes is crucial to developing carbon sequestration strategies and informing climate policy into mitigating GHG-induced global warming.

Aquatic ecosystems play a critical role in GHG modulation, both as sinks and sources of carbon dioxide and other major GHGs. However, exactly how much of an influence they have over climate change remains uncertain, mainly due to a paucity of data particularly for Africa. Despite being a potential source of GHGs, natural or anthropogenically modified aquatic ecosystems can also represent a significant sink, making these key coastal ecosystems either a major contributing threat to or remedy for emissions.  

Blue carbon

First coined in 2009, “blue carbon” recognised the significance of aquatic ecosystems to carbon atmospheric sequestration and storage (in addition to other nutrients such as nitrogen), both in biomass and sediments. The proficiency of stored sedimental carbon has been most evident in coastal environments, where mangroves, salt marsh and seagrass meadows have been characterised by a disproportionate carbon storage pool relative to their global coverage.

These systems earned the title of “blue carbon ecosystems” due to their relatively small total ocean surface area (>2%) but capturing nearly half of the sequestered total oceanic carbon. This elevates their status as “linchpin ecosystems” for future climate change mitigation and as encouraging sustainable nature-based solutions, thereby endorsing their future conservation and restoration, both locally and internationally, not only for climate adaptation but for many ecological (such as water purification) and socio-economic (such as juvenile fishery nursery sites) provisioned co-benefits as well.

While globally blue carbon ecosystems are described as robust GHG sinks, emerging research, particularly from urbanised coastal watersheds, has documented a disturbing trend of shifting sink-source dynamics within many estuarine and other blue carbon ecosystems. This increases the vulnerability of dependable GHG sinks offsetting current emissions becoming significant additive anthropically modified sources.

Information on South Africa’s blue carbon ecosystems carbon sedimentary stocks is limited to a handful of research sites, with none examining the dynamicity (i.e. fluxes) of carbon or nutrient cycling. These uncertainties in GHG fluxes from South African BCE, and in general from aquatic ecosystems (such as rivers and reservoirs), limit our understanding of the contributive importance these ecosystems present to national-scale GHG emissions, their potential utility in offset policy mechanisms (like the Global Carbon Project) and intergovernmental climate change treaties (such as the Paris Agreement).

Coastal mangrove–salt marsh ecotone, two of South Africa’s “linchpin” ecosystems to combating climate change and offsetting greenhouse gas emissions.

Sedimentary carbon stocks measured using sediment cores to provide an inventory of carbon and nutrients sequestered by and stored in salt marsh sediments at the Swartkops Estuary.

Carbon dioxide is not the only GHG evaded from aquatic ecosystems, as methane (CH4) and nitrous oxide (N2O), two abundant and potent global warming gases, are often simultaneously emitted. Although CH4 has a short atmospheric residency time (~12 years decomposing to radicals), it has 28 times more radiative forcing (retains thermal energy) than CO2, thus accounting for almost 30% of global warming. In contrast, N2O has nearly 300 times more radiatively forcing than CO2 and is an atmospheric persistent (over a century) GHG.

Additionally, nitrous oxide is an ozone depleting GHG, thus posing a considerable threat to not only the climate but directly to human wellbeing. These two potent GHGs are often underestimated and can significantly offset CO2 uptake and carbon sequestration for climate mitigation by BCE, even at low emissions, due to their radiative forcing potential. This makes the quantification and understanding of their fluxes a top research priority.

The intentions of this research are to, for the first time in South Africa, measure the three most radiatively potent GHGs (CO2, CH4 and N2O) emitted from BCE and to develop a net carbon budget for these ecosystems. A portable setup consisting of Optical Feedback – Cavity Enhanced Absorption Spectroscopy (OF-CEAS) gas analysers connected to a non-steady-state static chamber will provide in-field measurements of GHG fluxes.

Innovative field portable OF-CEAS gas analyser paired with static non-steady-state chamber from LI-COR providing valuable GHG flux measurements in real-time.

Urbanisation and its implications for greenhouse gas fluxes in coastal blue carbon ecosystems. One of our study sites is the Swartkops Estuary salt marsh in Gqeberha (formerly Port Elizabeth), a highly urbanised catchment flowing into an estuary surrounded by development.

The findings of this research will be crucial to deriving national-specific emission factors (standardised metrics for quantifying emission impacts of land-use change) with greater certainty and confidence in net predictions of emission estimates. This work will also explore the underlying biogeochemical processes and physical drivers affecting GHG fluxes.

In addition, we will investigate anthropogenic impacts (such as urbanisation) on blue carbon ecosystems, which change the local biogeochemistry, often resulting in water pollution and prompt significant biogenic GHG production – which could potentially shift a blue carbon ecosystem from a GHG sink to a source.

Of the nearly 290 coastal estuaries in South Africa, almost two thirds are eutrophic and while focus has been on regional specific water quality implications, this research will aim to elevate the importance of water management through a broader global level impact perspective of augmented GHG emissions from highly polluted blue carbon ecosystems – thus highlighting the potential losses of derivable climate benefits.

Overall, this research aims to determine the viability of using coastal blue carbon for climate change abatement, and additionally it will fill crucial knowledge and data gaps in South African estuarine research.


1 South African Environmental Observation Network, Elwandle Node, Ocean Science Campus, Nelson Mandela University, Summerstrand, Port Elizabeth, 6031, South Africa.

2 Department of Botany, Nelson Mandela University, Summerstrand, Port Elizabeth, 6031, South Africa.

3 DSI/NRF Research Chair in Shallow Water Ecosystems, Department of Botany, Nelson Mandela University, Summerstrand, 6031, Port Elizabeth, South Africa.

4 Institute for Coastal and Marine Research, Nelson Mandela University, Summerstrand, 6031, Port Elizabeth, South Africa.