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Call, M 2018 'Aquatic carbon cycling in the tropics : insights into pore-water exchange, outgassing and outwelling', PhD thesis, Southern Cross University, Lismore, NSW.

Copyright M Call 2018


Constraining the global flows of carbon has become a research priority due to the anthropogenic-induced alteration of the global carbon cycle. Carbon budgets are now more detailed, with considerable focus on constraining the flows and partitioning of carbon within ecosystems and quantifying exchanges with the major reservoirs of land, ocean and atmosphere. Tropical rivers, mangroves and the coastal ocean interact between all three reservoirs, yet data from systems in this region are lacking in global data sets. This thesis addresses key gaps in the cycling of carbon in the tropics and presents a new sampling methodology to enhance coastal carbon cycling research.

The relevancy of pore-water/groundwater exchanges as a source of CO2 and CH4 to higher orders rivers in the Amazon basin (and rivers globally) is largely unknown and a difficult pathway to constrain. Results from longitudinal surveys from the major tributaries of the central Amazon basin suggests that pore-water discharge enhanced CO2 supersaturation by 18 to 47 % in receding black and clear waters and a source of riverine CH4.

Mangroves are now recognised as significant sources of carbon to the coastal ocean and a source of CO2 and CH4 to the atmosphere. I also present the first CO2 and CH4 fluxes from a macro-tidal mangrove creek located in the 0 o to 10 o latitudinal band. The estimated emissions of CO2 and CH4 from the mangrove creek located in the Amazon region of Brazil are amongst the highest reported for mangrove systems worldwide, with pore-water exchange controlling the temporal variability of mangrove water CO2 and CH4 over the spring-neap cycle.

Few studies have attempted to estimate the relative contribution of individual carbon parameters to the total carbon exported from a single mangrove creek. I present the first carbon exports from two mangroves creeks of differing hydrogeomorphologies located in the wet tropics. Carbon outwelling rates were 4-fold higher from a mangrove creek located within a semi-enclosed bay compared to a creek located along the coast and adjacent to fringing reefs, demonstrating the considerable heterogeneity of carbon outwelling from mangroves creeks.

Finally, I present a new approach to autonomously determine concentrations of dissolved inorganic carbon (DIC) and its carbon stable isotope ratio (δ13C-DIC) at high temporal resolutions. Coupling two commercially available instruments, the new method determined DIC and δ13C-DIC at sampling resolutions of 16 min-1 with the precision and accuracy required to assess long-term anthropogenic-induced changes to oceanic carbon chemistry.