Contrasting carbon export dynamics of human impacted and pristine tropical catchments in response to a short‐lived discharge event

Abstract

Abstract:Utilising newly available instrumentation, the carbon balance in two small tropical catchments was measured during two discharge events at high temporal resolution. Catchments share similar climatic conditions, but differ in land use with one draining a pristine rainforest catchment, the other a fully cleared and cultivated catchment. The necessity of high resolution sampling in small catchments was illustrated in each catchment, where significant chemical changes occurred in the space of a few hours or less. Dissolved and particulate carbon transport dominated carbon export from the rainforest catchment during high flow, but was surpassed by degassing of CO 2 less than 4 h after the discharge peak. In contrast, particulate organic carbon dominated export from the cleared catchment, in all flow conditions with CO 2 evasion accounting for 5-23% of total carbon flux. Stable isotopes of dissolved inorganic carbon (DIC) in the ephemeral rainforest catchment decreased quickly from~1.5 % to~À16 % in 5 h from the flood beginning. A two-point mixing model revealed that in the initial pulse, over 90% of the DIC was of rainwater origin, decreasing to below 30% in low flow. In the cultivated catchment, d13 C DIC values varied significantly less (À11.0 to À12.2 %) but revealed a complex interaction between surface runoff and groundwater sources, with groundwater DIC becoming proportionally more important in high flow, due to activation of macropores downstream. This work adds to an increasing body of work that recognises the importance of rapid, short-lived hydrological events in low-order catchments to global carbon dynamics.

Continuous monitoring of stream δ18O and δ2H and stormflow hydrograph separation using laser spectrometry in an agricultural catchment

“…In this study, to analyse the isotopic composition of liquid water, we used a diffusion sampling cell developed by Munksgaard et al (2011) which utilised gas permeable Teflon tubing contained in an air tight, desiccated chamber to extract water vapour from a continuously refreshed water supply (Diffusion Sampling-(DS-) CRDS). This system has successfully been used to delineate carbon sources in storm flow (Bass et al, 2014a), track oceanic water mass boundaries in tropical (Munksgaard et al, 2012b) and polar waters (Bass et al, 2014b) and elucidate precipitation formation processes in a tropical cyclone (Munksgaard et al, 2015).…”

Carbon dioxide and methane emissions from an artificially drained coastal wetland during a flood: Implications for wetland global warming potential

“…Flood pulses often drive the metabolism in wetland-river systems [Junk et al, 1989]. Degassing of CO 2 from water during episodic flood events has proven to account for large proportions of total carbon loss from drainage basins [Bass et al, 2013;Bianchi et al, 2013]. However, gaseous losses of carbon during episodic flood events are not well constrained for wetlands and tributaries globally [Battin et al, 2009;Borges and Abril, 2011].…”

Leaky savannas: the significance of lateral carbon fluxes in the seasonal tropics

“…In the Mitchell River, the seasonal increase in the river DIC load during periods of high rainfall and river discharge are due to seasonal fluctuations in subsurface inflows. A recent study in a separate catchment of the wet tropics in northern Australia presents an example of increased river DIC loads during a rainfall event due to increased groundwater inflow; however, this study was conducted at the event scale (Bass et al , ). In the Mitchell River, during some of the early wet season peak river floods, there may be an inverse relationship between DIC concentrations and discharge at this event scale (data sampled at a higher resolution would be required to confirm this).…”

“…Riverine DIC is therefore an important and dynamic component of the global carbon cycle (e.g. Telmer and Veizer, ; Brunet et al , ; Cartwright, ; Aufdenkampe et al , ; Tobias and Böhlke, ; Bass et al , ), and the changes in river DIC loads are affected by a range of biogeochemical processes (e.g. Bouillon et al , ).…”