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

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 CO 2 [CO 2 *] 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 [CO 2 *] between mangroves and the coastal ocean. We found a mean DIC export of 59 mmol m À2 d À1 across the six systems, ranging from import of 97 mmol m À2 d À1 to an export of 85 mmol m À2 d À1 . 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 À1 , which accounts for approximately one third of the previously unaccounted for mangrove carbon sink. Alkalinity exchange ranged between an import of 1.2 mmol m À2 d À1 and an export of 117 mmol m À2 d À1 with an estimated global export of 4.2 ± 1.3 Tmol yr À1 . A net import of free CO 2 was estimated (À11.4 ± 14.8 mmol m À2 d À1 ) and was equivalent to approximately one third of the air-water CO 2 flux (33.1 ± 6.3 mmol m À2 d À1 ). 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.

Capturing dynamics of marine inorganic carbon fluxes from diurnal to decadal timescales

“…We assume that the two major remineralization processes were aerobic respiration and sulfate reduction, which is in agreement with other studies from coastal regions such as mangroves, fresh and salt water marshes Borges et al, 2003;Alongi et al, 2005;Sippo et al, 2016;.…”

“…for TA and DIC, respectively (Table 3). The alkalinity flux from this study is similar to 4.2 Tmol yr -1 exported from mangroves (Sippo et al, 2016), while the DIC export almost 2 times greater than mangrove DIC export of 3.6…”

“…Slight differences in export may be due to higher organic carbon in mangroves than in salt marshes, which could lead to differences in anaerobic respiration processes or it is also possible that differences are due to methodology. Sippo et al (2016) measured alkalinity and DIC at various sites along the Australian coast, however sampling periods were only for 25 hrs. In contrast, while this study was limited to only one site, the data coverage was multiple days of bottle sampling and high frequency in situ data for intermittent periods over 2.5 years.…”

Characterization of the CO2 System in a Coral Reef, a Seagrass Meadow, and a Mangrove Forest in the Central Red Sea

“…The slope of the regression between TAn and DICn is informative of the dominant metabolic processes in coral reef ecosystems (Andersson & Gledhill, ; Cyronak, Andersson, Langdon, et al, ; Lantz et al, ; Sippo et al, ). In Al‐Fahal reef, the regression between TAn and DICn over the complete measurement period is statistically significant (F‐statistic = 30.4, p < 0.001, R 2 = 0.51, n = 25), with a slope of 0.42 ( p < 0.001; Figure a).…”

Carbon regeneration in the Cariaco Basin, Venezuela

“…In addition, limiting nutrients for the biota are transported by the upwelling waters to the surface layers and help sustain primary productivity (HUTCHINGS et al, 1994;IANSON, 2003;HUTCHINS, 2013;. The transport of biological material to the ocean's interior is the main driver of the non-zero gradient of many dissolved species in seawater and an important regulator of atmospheric CO 2 on millennial timescales (LOUANCHI;NAJJAR, 2000;SIPPO et al;. In the open ocean, the fluxes of carbon through the water column decease exponentially with water depth due to aerobic respiration by microorganisms (PACE et al, 1987;PACKARD et al, 2015).…”