Are mangroves drivers of buffers of coastal acidification? Insights from alkalinity and dissolved inorganic carbon export estimates across a latitudinal transect

Study Number

92

Author

James Z. Sippo, Damien T. Maher, Douglas R. Tait, Ceylena Holloway, Isaac R. Santos

Abstract

Mangrove forests are hot spots in the global carbon cycle, yet the fate for a majority of mangrove
net primary production remains unaccounted for. The relative proportions of alkalinity and dissolved CO2 [CO2*] within the dissolved inorganic carbon (DIC) exported from mangroves is unknown, and therefore, the effect of mangrove DIC exports on coastal acidification remains unconstrained. Here we measured dissolved inorganic carbon parameters over complete tidal and diel cycles in six pristine mangrove tidal creeks covering a 26° latitudinal gradient in Australia and calculated the exchange of DIC, alkalinity, and [CO2*] between mangroves and the coastal ocean. We found a mean DIC export of 59 mmol/m2d across the six systems, ranging from import of 97 mmol/m2d to an export of 85 mmol/m2d. If the Australian transect
is representative of global mangroves, upscaling our estimates would result in global DIC exports of 3.6 ± 1.1 Tmol C/yr, which accounts for approximately one third of the previously unaccounted for mangrove carbon sink. Alkalinity exchange ranged between an import of 1.2 mmol/m2d and an export of 117 mmol/m2d with an estimated global export of 4.2 ± 1.3 Tmol/yr. A net import of free CO2 was estimated (11.4 ± 14.8 mmol/m2d) and was equivalent to approximately one third of the air-water CO2 flux (33.1 ± 6.3 mmol/m2d). Overall, the effect of DIC and alkalinity exports created a measurable localized increase in coastal ocean pH. Therefore, mangroves may partially counteract coastal acidification in adjacent tropical waters.

Main Results and Conclusions

• An overload of carbon in the world’s oceans has resulted in “ocean acidification,” a measured drop in pH, which is likely to affect all marine species. (753)
• Mangrove forests provide some of the largest stores of carbon in the coastal zone. (753)
  • “Pore waters of mangrove forests have been extensively documented as having higher pCO2, dissolved inorganic carbon (DIC), and alkalinity than in surrounding waters, due to the high metabolic activity in mangrove sediments.” (753-752)
• The hypothesis of the study is as follows:
  • “…  mangroves export more alkalinity to the coastal ocean, as a result of anaerobic organic matter mineralization processes in sediment.  The net result of high alkalinity exports would be a buffering of coastal acidification.” (753)
• This study was initiated with six mangrove tidal creeks on the north, east, and south Australian coasts: Darwin, Hinchinbrook Island, Seventeen Seventy, Jacobs Well, Newcastle, Barwon Heads. (755)
• The authors found that in fact mangroves can buffer coastal acidification by releasing alkalinity to nearby oceans.
  • “…we have identified that much of the [CO2*] produced is outgassed within the mangrove creeks themselves and mangrove forests can release alkalinity to the nearby coastal ocean.” (764)
  • “The ratio of DIC and alkalinity inputs to coastal waters (1:1.2) would lead to an overall increase in pH and thus a buffering effect.” (764)
  • “The magnitude and extent of this effect on the pH of coastal waters will be dependent on residence times and on other sources and sinks of DIC and alkalinity and will therefore be site specific.” (764)
  • “pH change is likely to be localized and have a greater influence in areas with large mangrove coverage.” (764)
  • “Since the majority of mangroves are located within the tropics [Giri et al., 2011], the effect of alkalinity exports on coastal buffering is likely to be more significant in tropical regions where systems sensitive to ocean acidification (i.e., coral reefs) also occur.” (764