Greenhouse gas fluxes from a South African tidal salt marsh post-flood event

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#06 2024

Greenhouse gas fluxes from a South African tidal salt marsh post-flood event

By Daniel Buttner (a,b,c) and Lucienne Human (a,b)

Coastal wetland habitats such as mangroves, salt marsh and seagrass meadows have garnered much attention in recent decades from both scientific and public audiences as a logistically viable nature-based solution (NbS) for climate change abatement and intervention.

This interest is spurred by the capacity of these coastal wetlands to sequester and, more importantly, store disproportionate quantities of carbon and other nutrients, such as nitrogen, in their sediments with respect to area coverage. In many cases this carbon pool can be stored for millennia with minimal remineralisation to the atmosphere.

Additionally, as transitionary, convergent ecosystems, coastal wetlands are enriched by autochthonous (self-produced) and allochthonous (external sources) carbon and nitrogen, thus elevating their supportive value as contributors to the net uptake of greenhouse gases such as CO₂. The recognition of these carbon storing services afforded by these ecosystems has highlighted their opportunity as potential linchpin mechanisms under the Paris Agreement in remedying global temperatures via their inclusion as National Determined Contributions offsetting greenhouse gas (GHG) emissions.

Making use of innovative technologies and field portable instrumentation for real-time measurements, Elwandle Node scientists aim to improve an understanding of greenhouse gas fluxes from South African coastal wetlands.

Coastal wetlands and climate mitigation

The growing enthusiasm for the potential of coastal wetlands as contributors to climate mitigation through GHG sequestration and carbon storage has highlighted many uncertainties and gaps in understanding regarding carbon and nitrogen dynamics within these habitats. For example, the very same sedimentary physicochemical conditions (anoxic, frequently waterlogged, reducing) favouring the long-term storage of carbon and nitrogen are conducive to the production of radiatively potent GHGs such as methane (CH₄) and nitrous oxide (N₂O), 28 and 300 times greater warming effect than CO₂. These GHGs are globally concerning as the concentrations are steadily increasing, threatening global temperatures and the thermal balance of the planet.

A major factor regulating carbon and nitrogen mineralisation within salt marsh is the effect of salinity. Salinity is predicted to limit methanogenesis, nitrification and denitrification processes, thereby constraining GHG emissions. Many studies have highlighted the effects of salinity on limiting emissions, leading Poffenbarger et al. (2011) to define a threshold (>18 PSU = no methane production) upon which salinity would significantly constrain GHG emissions. Although their threshold was limited to methane it has successfully been applied to other GHGs such as N₂O as well.

Figure 1: Poffenbarger et al. (2011) salinity inhibitory threshold for methane flux emanating from salt marsh.

Figure 2: Greenhouse gas fluxes of A) CH₄, B) CO₂, and C) N₂O delineated across geomorphological gradient at the Berg estuary, Velddrift, South Africa.

Testing the threshold

Recent floods experienced at the Berg salt marsh on the west coast of South Africa provide an opportunity to test the importance of such an inhibitory threshold. We divide the estuary into three geomorphological types typically delineated based on salinity, with the upper being most fresh, middle estuarine, partially saline, to lower estuarine-to-marine saline environments. The impact of the flood homogenised salinity across the marsh transitioning to freshwater (<2PSU) across the geomorphological gradient of the estuary, the implications of which were not evident until GHG flux measurements were undertaken across salt marsh. Figure 2 clearly demonstrates the variance across the geomorphological gradient, with elevated emissions for all GHGs at upper freshwater-influenced areas except for CO₂. Markedly elevated CH₄ emissions in the upper reaches are nearly equivalent to those observed emanating from wastewater treatment works.

In contrast, the reduction in CO₂ is intriguing as it would suggest that elevated freshwater stimulates productivity, reducing emissions. While the mechanisms behind these observations remain preliminary until further analysis on underlying biogeochemical and microbial processes are assessed – they highlight the impact flooding poses to GHG production in typically saline sediments, transforming them from a major sink of GHGs to a potentially substantial source.

Figure 3: Survey instruments for measuring greenhouse gas fluxes on salt marsh (A and B) and floating chamber (C) for measuring emissions from estuaries and seagrass soon to be deployed at several estuaries across South Africa.

Figure 4: Field sampling for GHG fluxes from salt marsh environments, scientists closely examining the real-time measurements of CH₄, CO₂, and N₂O emissions coming off the marsh.

Making use of innovative technologies and field portable instrumentation for real-time measurements, we aim to improve an understanding of greenhouse gas fluxes from South African coastal wetlands. With the intention of integrating these ecosystems into South Africa’s Nationally Determined Contributions (NDCs) and providing informed policies on the impact exogeneous factors (such as pollution and nutrient enrichment) might have on the stability of these environments to represent a net sink or source of GHGs to the atmosphere steering South Africa’s climate change policy and benchmarks for nature-based solutions and restoration programmes within the coastal environment.

Figure 5: Installing long-term ecological research monitoring platforms for blue carbon accumulation measurements and sedimentation processes. Having fun in the sun at one of our study sites. (Photos: Athi Mfikili)

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