QUANTIFYING THE CONCENTRATION AND SOURCES OF DISSOLVED
CARBON FROM FORESTED AND GRASSLAND MOUNTAIN CATCHMENTS,
SOUTHERN ALPS, NEW ZEALAND
Emily E. Diack,1 Sarah M. Mager,1 Robert van Hale2
1
Department of Geography, University of Otago, New Zealand
2
Department of Chemistry, University of Otago, New Zealand
Aims
Estimations of carbon flux are essential for understanding the build-up of carbon dioxide in the
atmosphere and its terrestrial sinks and sources. The transfer of carbon between the atmosphere,
biosphere and lithosphere is typically characterised through the global flux of riverine carbon,
which has become of increasing importance in the context of carbon budgets. Carbon is
constantly moving between the atmosphere, biosphere and hydrosphere with rivers acting as
partial sinks for carbon as well as advecting carbon between the terrestrial and marine
environments. Riverine carbon may be temporarily held within water, as dissolved carbon, or
stored in riverbeds and banks as particulate carbon. Carbon naturally occurs in many forms:
particulate or dissolved, and is derived from either inorganic or organic sources. The partitioning
of carbon between the different forms e.g. particulate organic carbon (POC), or dissolved organic
carbon (DOC) is central to understanding the geochemistry of carbon fluxes in the terrestrial
sphere as well as understanding the structure and functioning of aquatic ecosystems. The aim of
this paper is to assess the concentration of dissolved organic and inorganic carbon within forest
and grassland mountain catchments in the Southern Alps of New Zealand and use the stable
isotopic composition of dissolved inorganic carbon (13CDIC) to determine the sources of dissolved
carbon within the forested and grassland catchments.
Method
Water samples were collected every 3 to 6 months, under base flow conditions, from 64
catchments draining the Southern Alps of New Zealand from January 2012 until Febuary 2015.
Dissolved organic carbon (DOC) samples were collected in 40 mL glass vials with silicon septa
caps and analysed using a Shimadzu Total Carbon Analyser with an analytical precision of 0.2
mg C/L. Dissolved inorganic carbon (DIC) samples were collected in 60 mL LDPE bottles.
Concentrations were measured using total alkalinity titrations and adjusted by pH to determine
bicarbonate concentration (in mg C/L) and cross calculated using charge balance estimations of
dissolved ions with an analytical precision of 5 % of total dissolved solids (in meq/L). 13CDIC
samples were collected in 100 mL glass vials with silicon septa caps and preserved with 10 %
mercuric chloride and then analysed on a Thermo Advantage IRMS with a Gasbench sample
preparation system, with a precision of 0.08‰.
Results
The median concentrations of DOC for each catchment ranges from 0.6 to 3.7 mg C/L, with the
analysis of temporal variability showing no systematic increase or decrease in organic carbon
between summer and winter. Thus, the amount of dissolved organic carbon in the Southern Alps
rivers does not appear to respond to changes in plant uptake with seasons, or temperature. The
median concentration of DIC ranges from 2.0 to 13.6 mg C/L, and similarly does not show
seasonal differences. The higher values of DIC are mostly associated with highly erodible
catchments, so that the inorganic carbon is likely from rock sources. Analysis of the 13CDIC
shows values ranging from -15 ‰ to -1 ‰, and is highly variable across the Southern Alps.
Preliminary analysis suggests that the inorganic carbon is influenced by the portion of forest and
grassland cover in the catchment. Forest dominated catchments produce depleted values of
13CDIC, whilst high grassland cover shows an enrichment in 13CDIC values.