Assessing blue carbon stocks in southern Africa’s endangered seagrass Zostera capensis
By Lucienne R.D. Human*, Johan Wasserman** and Janine B. Adams***
By Lucienne R.D. Human*, Johan Wasserman** and Janine B. Adams***
Blue carbon is the term given to the organic carbon (C) captured by the oceans and coastal vegetative habitats. These consist of mangroves, salt marshes and seagrass. Known as some of the most productive ecosystems on earth, salt marshes and seagrasses are also effective C sinks, playing important roles in mitigating climate change despite the salinity and inundation stressors they face.
Blue carbon research, although rapidly expanding, still requires site- and species-specific assessments to inform decision-making and policy formulation. The role of blue carbon ecosystems in climate change mitigation and adaptation has raised both scientific and political interest towards conserving and restoring coastal habitats.
Understanding this variability is a key research gap in blue carbon science that needs to be addressed through quantifying C stocks in underrepresented regions. In this study undertaken by researchers from SAEON’s Elwandle Coastal Node and the Nelson Mandela University, C stocks are assessed in the endangered seagrass Zostera capensis habitats across three estuaries in South Africa’s warm temperate biogeographic zone – the Knysna, Swartkops and Nxaxo estuaries (Figure 1).
The Knysna Estuary hosts the largest, most stable beds of Z. capensis (eelgrass) in the country, covering a total area of 353 ha. In comparison, eelgrass habitats cover far less area at both the Swartkops (44.7 ha) and Nxaxo (0.04 ha) estuaries. This study aims to provide a first C stock assessment in these Z. capensis habitats.
Figure 1. The locations of the Knysna, Swartkops and Nxaxo estuaries along the South African coastline. Zostera capensis habitats are indicated by green polygons and sampling points are indicated by black points.
Researchers from the Elwandle Coastal Node and the Nelson Mandela University are assessing the endangered seagrass Zostera capensis habitats across three estuaries in South Africa’s warm temperate biogeographic zone.
As is the case for most seagrass habitats worldwide, sediments underlying seagrass habitats store markedly more C than the seagrass biomass. The range of organic C recorded for Z. capensis habitats in this study (0.31 – 1.59%) fell within the lower to middle range of reports for other members of the Zostera genus.
The Swartkops Estuary, which had significantly higher sediment C stocks, receives discharge rich in nutrients and organic matter from three wastewater treatment works, two stormwater canals and an urbanised catchment. Similar higher organic C values have been reported in seagrass beds in nutrient-rich environments elsewhere and has been attributed to increases in productivity, and thus detritus.
Figure 2. Zostera capensis biomass and biomass C storage (secondary axis) in the sampled estuaries (mean + SD). Overall, biomass C storage per unit area was similar among estuaries, with the greatest being in the Nxaxo Estuary with a mean of 0.3 ± 0.12 kg C m-2, followed closely by the Knysna (0.26 ± 0.11 kg C m-2) and Swartkops estuaries (0.21 ± 0.12 kg C m-2).
The Knysna Estuary hosts the largest, most stable beds of Zostera capensis (eelgrass) in the country, covering a total area of 353 hectares.
At the Knysna Estuary, allochthonous C originated from the Knysna River, evident by the decrease in organic matter and C stocks from the head to the mouth of the estuary. An estuary-wide spatial trend in C stocks could not be discerned at the Nxaxo Estuary as Z. capensis only grows in a small patch near the estuary mouth.
This study has provided the first comprehensive C stock assessment for Z. capensis and contributes to global blue carbon datasets.
The blue carbon storage was significant in the seagrass and this work has reinforced the importance of protecting the endangered seagrass Zostera capensis, again highlighting the important ecosystem service it provides through climate change mitigation and adaptation.
Figure 3. The relationship between sediment organic C and log transformed sediment organic matter at Zostera capensis habitats sampled in the estuaries. The model predicted that sediment soil C generally increases with log transformed organic matter, although this relationship differed among the sampled estuaries. Although this model had the best fit (AIC = 102.52), it described just 53.91% of the variability in sediment organic C.
Blue carbon research, although rapidly expanding, still requires site- and species-specific assessments to inform decision-making and policy formulation.
Adams, J.B. 2016. Distribution and status of Zostera capensis in South African estuaries – a review. South African Journal of Botany, 107, pp.63–73.
Adams, J.B. 2020. Salt marsh at the tip of Africa: patterns, processes, and changes in response to climate change. Estuar. Coast. Shelf Sci. 237, 106650. https://doi.org/10.1016/j.ecss.2020.106650
Adams, J.B., Pretorius, L. & Snow, G.C. 2019. Deterioration in the water quality of an urbanised estuary with recommendations for improvement. Water SA 45, pp. 86–96.
Human, L.R.D., Els, J., Wasserman J. & Adams, J.B. 2022. Blue carbon and nutrient stocks in salt marsh and seagrass from an urban African estuary. Sci. Total Environ. 842, 156955.
Jiang, Z., Liu, S., Zhang, J., Wu, Y., Zhao, C., Lian, Z. & Huang, X. 2018. Eutrophication indirectly reduced carbon sequestration in a tropical seagrass bed. Plant Soil 426, pp. 135–152.
Nellemann, C., Corcoran, E., Duarte, C., Valdés, L., Fonseca, L., Grimsditch, G. & De Young, C. 2009. Blue Carbon. A Rapid Response Assessment. United Nations Environment Programme.
Sousa, A.I., Santos, D.B., da Silva, E.F., Sousa, L.P., Cleary, D.F.R., Soares, A.M.V.M. & Lillebo, A.I. 2017. Blue carbon’ and nutrient stocks of salt marshes at a temperate coastal lagoon (Ria de Aveiro, Portugal). Sci. Rep. 7, pp. 1–11. https://doi.org/10.1038/srep41225.
Figure 4. Sediment organic matter and organic C content at the sampled estuaries (mean + SD). The sediment C pool was substantially greater, storing 177.65 ± 122.29 Mg C ha-1 in the top metre of sediment. Sediment organic C content, however, displayed significant spatial variability and was substantially higher at Swartkops (1.38 ± 0.08%) than at Nxaxo (0.85 ± 0.14%) and Knysna (0.39 ± 0.04%).
* Department of Botany, Institute for Coastal and Marine Research, Nelson Mandela University, PO Box 77000, Gqeberha, South Africa, 6070
** DSI/NRF Research Chair in Shallow Water Ecosystems, Nelson Mandela University, PO Box 77000, Gqeberha, South Africa, 6070
*** South African Environmental Observation Network, Elwandle Coastal Node, Nelson Mandela University, PO Box 77000, Gqeberha, South Africa, 6070