Professor Isaac R Santos

Mangroves and saltmarshes have elevated groundwater-surface water connectivity due to effective tidal pumping in and out of animal burrows. These blue carbon habitats also have high rates of organic carbon burial due to the anoxic nature of their soils, high sedimentation rates, and high primary productivity rates. Here, we assess whether submarine groundwater discharge and/or porewater exchange may release some of the soil carbon, and whether groundwater-derived alkalinity fluxes may rival carbon sequestration in intertidal wetlands. Seasonal observations were performed in an Australian estuarine tidal creek surrounded by mangrove and saltmarsh vegetation. We used radon to quantify porewater exchange and detailed time series observations to quantify outwelling and outgassing rates of all the key carbon species. Organic carbon burial derived from the 239+240Pu soil dating method were 63±26 and 11±5 g C m-2 yr-1 in the mangrove and saltmarsh, respectively. A radon mass balance model implied tidally-driven porewater exchange rates ranging from 7.3±4.8 in the winter to 20.2±10.7 cm d-1 in the summer. These porewater exchange rates released about 400 g C m-2 yr-1 from soils to surface waters, an order of magnitude greater than soil carbon burial rates. About 65% of the porewater-derived carbon fluxes were DOC, while 24% were alkalinity. The porewater-derived alkalinity fluxes from the soils were greater than the organic carbon burial rates. Therefore, porewater derived alkalinity exports to the coastal ocean may represent an important, but largely unquantified mode of long term carbon sequestration. Much research has already focused on quantifying carbon burial rates in blue carbon habitats. We suggest that groundwater and porewater alkalinity fluxes should also be considered when determining the carbon sequestration potential of mangrove and saltmarsh systems. The ability of these coastal blue carbon ecosystems to produce large amounts of alkalinity associated with porewater flushing may more than double their total carbon sequestration capacity.

A large-scale mangrove dieback occurred in the Gulf of Carpentaria, Northern Australia throughout the second half of 2015. Understanding the fate of mangrove carbon in response to this event is of great importance as the causes may be linked to climatic trends. We undertook field-based measurements of CO2 and CH4 fluxes from soils in dead mangrove areas, and nearby apparently unaffected areas near Karumba, Queensland in August/September 2016. In addition, offshore exports of dissolved inorganic carbon (DIC), and alkalinity (TAlk) were calculated from the same coastlines using radium isotopes to estimate offshore mixing rates. Fluxes of CO2 from soils ranged from 9 mmol/m2/d to 1732 mmol/m2/d with the highest fluxes observed in areas with stressed mangroves (defoliated but not dead). Methane fluxes from soils were generally low, ranging from below-detection to 2 mmol m-2 d-1. Offshore exports from the coastline with dead mangroves of DIC (75.8 mmol/m2/d) and TAlk (84.4 mmol/m2/d) were ~ 5 and 7% lower than from the coastline with living mangroves (79.5 mmol/m2/d DIC and 90.2 mmol/m2/d TAlk). These results suggest that the event has had an impact upon the coastal carbon cycle, but follow-up studies are required to assess the long term effects.

Mangroves are among the most carbon-rich ecosystems on Earth. Most of the carbon is sequestered in sediments and assumed to be stable over long time scales. Here we assess whether century-old buried carbon may be remineralized and exported by measuring Δ14C in the exported dissolved inorganic carbon (DIC) as well as sediment Δ14C profiles in a subtropical mangrove. Porewater exchange released isotopically depleted, old DIC to surface waters. Keeling plots revealed the surface water DIC source had a δ13C-DIC value of -29.4 ±1.9‰ and Δ14C-DIC value of -73±9‰ (~555 years BP). The respired and exported carbon comes from an average depth of ~38 cm, equivalent to ~ 100 years of sediment accumulation. Therefore century-old buried carbon is still susceptible to remineralization and tidal export via porewater exchange from mangroves. We suggest that the time scales over which blue carbon sequestration is assessed should consider carbon losses via porewater exchange.

Mangrove forests are hotspots 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 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 dissolved CO2 between mangroves and the coastal ocean. We found a mean DIC export of 59 mmol/m2/d across the six systems, ranging from import of 97 mmol/m2/d to an export of 85 mmol m2/d. 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/m2/d and an export of 117 mmol/m2/d 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/m2/d), and was equivalent to approximately one third of the air water CO2 flux (33.1 ± 6.3 mmol/m2/d). 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.

An integrated approach to measuring all relevant carbon flux pathways is required to provide a comprehensive evaluation of the net ecosystem carbon budget (NECB). This is especially important in ecosystems that contain an aquatic component where land-atmosphere carbon exchange is not the only carbon pathway. Here, we present findings on the complete annual carbon budget of a subtropical agricultural floodplain, and explore the contribution of aquatic carbon flux to net ecosystem exchange (NEE) with other ecosystems. Carbon fluxes measured included: land-atmosphere CO2 exchange (NEE), aquatic CO2 and CH4 evasion from drainage canals, and export of DOC, POC, and DIC via discharge. The floodplain remained a large atmospheric CO2 sink throughout the study, with an annual NEE of -1057 to -900 gC m-2 yr-1. After accounting for aquatic carbon exports and biomass exchanges, the floodplain had a relatively neutral NECB of 116 gC m-2 yr-1. The largest carbon loss pathway was biomass (1,494 gC m-2 yr-1), which ultimately determined the NECB. Total aquatic carbon fluxes of export and evasion were found to be a very minor component of the NECB, offsetting only ~4% of terrestrial NEE. This is in stark contrast to northern wetlands which report aquatic carbon contributions of 10 to >100%. Despite the small contribution found at this site, the annual aquatic carbon loss of ~40 gC m-2 yr-1 was within the same order of magnitude compared to a range of other ecosystems. Multiple flood events were responsible for 80% of the total aquatic carbon flux, demonstrating the potential for floods to negatively impact NECB’s in ecosystems with smaller annual NEE.

The coupled high-resolution measurement of dissolved inorganic carbon (DIC) and its carbon stable isotope ratio (δ13C-DIC) can provide insights into aquatic carbon cycling (e.g. production/respiration, air-water gas exchange), helping elucidate flows of carbon within and between reservoirs. A new method to autonomously determine concentrations of DIC and δ13C-DIC is presented. The simple method requires no customised design, instead it uses two commercially available instruments currently used in aquatic carbon research. An inorganic carbon analyser utilising non-dispersive infrared detection (AIRICA) is coupled to a Cavity Ring-down Spectrometer (Picarro) to determine DIC and δ13C-DIC based on the liberated CO2 from acidified aliquots of water. Using a small sample volume of 2 ml, the precision and accuracy of the new method was comparable to standard laboratory-based isotope ratio mass spectrometry (IRMS) methods. The system achieved a sampling resolution of 16 mins, with a DIC precision of ± 1.5 to 2 µmol kg-1 and δ13C-DIC precision of ± 0.14 ‰ for concentrations spanning 1000 to 3600 µmol kg-1. Accuracy of 0.1 ± 0.06 ‰ based on DIC concentrations ranging from 2000 µmol kg-1 to 2250 µmol kg-1 was achieved during a laboratory-based algal bloom experiment. The high precision data that can be autonomously obtained by the system should enable complex carbonate system questions to be explored using high temporal resolution observations.