Development of cavity ring-down spectroscopy
(CRDS) has enabled real-time monitoring of carbon stable
isotope ratios of carbon dioxide and methane in air. Here we
demonstrate that CRDS can be adapted to assess aquatic
carbon cycling processes from microbial to ecosystem scales.
We first measured in situ isotopologue concentrations of
dissolved CO2 (12CO2 and 13CO2) and CH4 (12CH4 and
13CH4) with CRDS via a closed loop gas equilibration device
during a survey along an estuary and during a 40 h time series
in a mangrove creek (ecosystem scale). A similar system was
also connected to an in situ benthic chamber in a seagrass bed
(community scale). Finally, a pulse-chase isotope enrichment
experiment was conducted by measuring real-time release of
13CO2 after addition of 13C enriched phytoplankton to exposed
intertidal sediments (microbial scale). Miller-Tans plots revealed complex transformation pathways and distinct isotopic source
values of CO2 and CH4. Calculations of δ13C-DIC based on CRDS measured δ13C-CO2 and published fractionation factors were
in excellent agreement with measured δ13C-DIC using isotope ratio mass spectroscopy (IRMS). The portable CRDS
instrumentation used here can obtain real-time, high precision, continuous greenhouse gas data in lakes, rivers, estuaries and
marine waters with less effort than conventional laboratory-based techniques