High pore-water derived CO2 and CH4 emissions from a macro-tidal mangrove creek in the Amazon region

This paper presents the first aquatic CO2 and CH4 flux estimates from a macro-tidal mangrove creek located in the 0–5° latitude band, where ∼30% of the world’s mangroves occur. High resolution dissolved CO2, CH4 and 222Rn (a natural pore-water tracer) concentrations were measured over a spring-neap tidal cycle from a mangrove tidal creek located in North Brazil (∼0.8°S). Surface water pCO2, CH4 and 222Rn ranged from 592 to 15,361 μatm, 58 to 1469 nM, and 585 to 16,583 dpm m−3 with considerable temporal variability observed semi diurnally (i.e. hourly) and over the spring-neap cycle (i.e. weekly). Tidally-driven pore-water exchange (tidal pumping) drove surface water pCO2 and CH4, leading to high concentrations at low-tide (semi-diurnal variability). Higher pCO2 and CH4 were also observed after the inundation of the upper inter-tidal flat, with peak values coinciding with the “first flush” of aged pore-waters. We hypothesise that additional pore-water exchanges occur during forest inundation in macro-tidal mangrove systems, controlling mangrove creek water pCO2 and CH4 over spring-neap cycles. Estimated CO2 and CH4 water-atmosphere fluxes were 174 ± 129 mmol m−2 d−1 and 855 ± 406 μmol m−2 d−1, respectively. These emissions are amongst the highest reported for mangrove systems worldwide and suggests that the most recent global estimates based mostly on data from higher latitudes may have underestimated the role of mangroves in greenhouse gas emissions.