The diverse and productive blue carbon ecosystems (mangrove, seagrass, salt marsh, and macroalgae) provide many ecosystem services and play an important role in climate change mitigation and adaptation. However, less is known about how different biogeochemical processes within blue carbon ecosystems can influence seawater carbonate dynamics especially on the partial pressure of carbon dioxide (pCO2) and pH. Through underway pCO2 measurements and discrete water samples, we examined the spatial and temporal distributions of the carbonate system in two tropical mangrove estuaries and one benthic-macroalgae-vegetated lagoon. In early summer, the mangrove estuaries behaved as weak CO2 sources of 2.0–4.0 mmol m−2 d−1, whereas the tropical lagoon became a CO2 sink of −6.4 mmol m−2 d−1. The chemical stoichiometry, stable carbon isotope, and other geochemical tracers reveal that carbonate dissolution and sulfate reduction followed by aerobic respiration are dominant controls on carbonate dynamics in mangrove estuaries. Without alkalinity enhancement driven by carbonate dissolution and sulfate reduction mainly from mangrove sediments, CO2 emission would be 23 times larger than current observation, and pH would decrease by 0.39 units on an estuary-wide scale. However, in macroalgae-inhabited lagoon primary production followed by carbonate formation could draw pCO2 down to 70 μatm and raise pH to 8.8, which are significantly distinct from the normal estuarine water. This study demonstrates that lateral export of alkalinity from mangrove sediments and the in situ metabolism of macroalgae can significantly influence the estuarine air-sea CO2 flux and acidification status, and emphasizes the importance of protecting and restoring the blue carbon ecosystems.
Plain Language Summary
Blue carbon ecosystems provide vital services and help mitigate climate change. However, the effect of their internal biogeochemical processes on the coastal carbonate system is poorly understood. Our study shows that in early summer, waters in two tropical mangrove estuaries released CO2 into the air, whereas water in the macroalgae-vegetated lagoon absorbed CO2. In mangrove estuaries, carbonate dissolution and sulfate reduction (followed by aerobic respiration) primarily control the carbonate dynamics. If there is no alkalinity enhancement driven by these mangrove sediment processes, CO2 emission would be 23 times higher and the estuary-wide pH would drop by 0.39 units. In contrast, benthic macroalgae in the lagoon took up CO2 and raised pH through photosynthesis and carbonate formation. This demonstrates that alkalinity export from mangrove sediments and the in situ metabolism of macroalgae significantly influence the estuarine air-sea CO2 exchange and acidification. Protecting and restoring these blue carbon ecosystems are therefore essential.
Key Points
- Continuous underway measurements reveal that tidal movement significantly affects the spatial distributions of water properties
- Strong photosynthesis of benthic macroalgae turns the air-sea CO2 status from a source into a sink in a eutrophic tropical lagoon
- Lateral export of alkalinity from mangrove sediments decreases the partial pressure of CO2 and increases pH in the adjacent coastal sea
Su J., Xiao S., Tan C., Huang Z., Xu M. N., Tan E., Guo L., Wang G., Cai W.-J. & Kao S. J., 2025. Blue carbon ecosystems modulate the air‐sea CO2 exchange and coastal acidification in tropical estuaries. Journal of Geophysical Research: Oceans 130(9): e2025JC023089. doi: 10.1029/2025JC023089. Article (subscription required).
