Angie Elwin

University of Reading, Reading, UK

Mangrove forests are highly effective carbon sinks that contribute disproportionately to the global carbon budget. Also known as ‘blue carbon’ ecosystems, these habitatscontain particularly large amounts of organic carbon in their sediments, accumulated over very long timescales (centuries to millennia). They also provide numerous social and environmental benefits to coastal communities. Many of these benefits are crucial for climate adaptation and resilience, such as erosion control, stabilising sediments, and coastal protection against storms, ocean waves and sea level rise. Yet mangrove forests are also among the world’s most threatened ecosystems due to anthropogenic stressors. Since around the 1940s, it is estimated that up to 50% of mangroves have been destroyed globally, representing a major loss of natural CO₂sink capacity.

Shrimp aquaculture is one such land-use change substantially driving global mangrove loss. This problem has been particularly acute in Thailand, where extensive areas of mangrove were replaced with aquaculture ponds during the 1980s-2000s. Mangrove cover was reduced from 370,000 ha in 1961 to 167,500 ha in 1996, around half of which was due to aquaculture. 

Many of the shrimp ponds created in Thailand during the 1980s-2000s have proved unsustainable due to disease outbreaks and up to 70% are now thought to be abandoned. While research documenting mangrove carbon stock losses due to land-use change has been steadily growing over the past half-decade, little attention has been paid to understanding the fate and stability of the remaining carbon pools (previously sequestered and stored carbon) following pond abandonment. 

Figure 1. Mangrove forest on Thailand’s Southern Andaman Sea coast (photo credit: Angie Elwin).

To address this knowledge gap, in a recent open access paper, Elwin et al. (2019) quantifiedecosystem carbon stocks of a mangrove forest and 12 abandoned shrimp ponds, which were formally mangrove forest, on an island situated on Thailand’s southern Andaman Sea coast (Koh Klang; 7.78° N, 99.08° E). The authors assessed ecosystem carbon stocks using biometric and soil coring methods along transects to determine aboveground (tree) and belowground (root + soil) carbon pools. Using a 22-year chrono-sequence approach, the study aimed to assess whether, and at what rate, carbon stocks were recovering after ponds had been abandoned. Abandoned ponds of different ages (10-22 years) were compared with natural reference mangrove sites. In addition, abandoned ponds under Ecological Mangrove Restoration (EMR) projects were sampled in order to examine the impact of rehabilitation of abandoned shrimp ponds on ecosystem carbon stocks. 

The study shows that shrimp aquaculture results in a huge loss of carbon, with up to 65% of soil carbon and up to 70% of ecosystem carbon lost when mangroves are converted for aquaculture. However, this study demonstrates that carbon is preserved in deeper soil layers of some abandoned ponds, and that carbon accumulates in the surface soil layer after pond abandonment. Along the studied chrono-sequence of abandoned pond sites, the authors found the effect of land-use change on soil carbon pools is most substantial in the near surface soil layer (0-15 cm depth). They also found a positive developmental trajectory for carbon pools in the upper soil layer, which they associate with natural mangrove regeneration. The findings indicate that as mangrove trees colonise abandoned ponds, they contribute to the soil carbon building process.

The work demonstrates that the carbon sequestration capacity of mangrove forests may improve in abandoned shrimp ponds over time in areas exposed to tidal flushing as mangroves re-establish. Further, that the carbon stored in the surface soils of ponds may be comparable to natural mangrove forests 22 years after ponds are abandoned.

Reference

Elwin, A. et al. 2019. Preservation and recovery of mangrove ecosystem carbon stocks in abandoned shrimp ponds. Scientific Reports, 9, 18275.

For further information, contact Angie Elwin (angie.elwin@reading.ac.uk)