The Mahakam Delta which was once among the largest mangrove forests in Indonesia, has been subjected to dramatic changes in structure and function due to massive shifts from mangrove forest to shrimp ponds and oil and gas development. To understand the impacts of mangrove loss to the greenhouse gas (GHG) emissions and its role in climate change mitigation, it is important to know functions related to the carbon dynamics of the three major land covers in the Mahakam Delta i.e. Nypa fruticans stands, broadleaved mangroves (Rhizophora spp., Avicennia spp., Sonneratia spp.) and aquaculture ponds.
The objectives of this study are to quantify (1) total ecosystem carbon stocks and potential emissions of broadleaved mangroves through conversion to abandoned shrimp ponds; (2) the land use carbon footprint of shrimp production in the Mahakam Delta; (3) net primary productivity (NPP) and net ecosystem productivity (NEP) of broadleaved mangroves and abandoned shrimp ponds; and (4) total ecosystem carbon stocks and soil respiration of Nypa fruticans stands.
The mean ecosystem carbon stocks of the broadleaved mangroves, Nypa fruticans stands and abandoned shrimp ponds are 1023 ± 87 Mg C ha-1, 982 ± 51 Mg C ha-1 and 499 ± 56 Mg C ha-1 respectively. We found that the nypa ecosystem C stocks and soil CO2 flux are comparable with the broadleaved mangroves in the Mahakam Delta and tropical mangroves of the Indo-Pacific.
Mangrove conversion to shrimp ponds results in a dramatic decline of carbon loss of about 525 Mg C ha-1. Most of the carbon loss derives from the depletion of soil carbon due to mangrove clearing and draining. The potential emission factor arising from 16 years of mangrove conversion to shrimp ponds was 33 Mg C ha-1 yr-1.
The mean aboveground NPP for broadleaved mangroves (13.5 ± 1.1 Mg C ha-1 yr-1) is significantly higher than those of the abandoned shrimp ponds (0.8 ± 0.4 Mg C ha-1 yr-1). The NEP of broadleaved mangroves (8.8 ± 1.9 Mg C ha-1 yr-1) indicates that carbon sequestration in broadleaved mangroves exceeds the carbon loss. This suggests that broadleaved mangroves function as net carbon sinks. Similarly, Nypa fruticans communities are also significant carbon sinks with comparable soil carbon storage capacity with the broadleaved mangroves. In contrast, abandoned shrimp ponds with NEP of –1.4 ± 0.3 Mg C ha-1 yr-1 indicates that they serve as sources of greenhouse gases to the atmosphere.
Shrimp production in the Mahakam Delta contributes to a significant amount of CO2 emissions. The land use carbon footprint of shrimp production is estimated at 2250 - 4874 kg CO2-e for every kg of shrimp produced in mangroves converted ponds. With the mean shrimp production of only 56 – 121 kg ha-1 yr-1, our results demonstrate that the significant losses generated from mangrove conversion is not comparable with the low shrimp production.
We conclude that mangrove ecosystems have a significant capacity to store substantial amounts of carbon. Once these ecosystems are disturbed, they will become significant sources of GHG emissions. Avoiding disturbance of mangrove ecosystems in the Mahakam Delta will reduce to about 6% of Indonesia’s annual emissions from mangrove deforestation. Promoting better collaborative management with key stakeholders and prioritizing conservation of the remaining mangrove forests are needed to ensure the sustainability of this imperiled ecosystem.
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