Greenhouse Gas Dynamics in a Salt-Wedge Estuary Revealed by High Resolution Cavity Ring-Down Spectroscopy Observations

Estuaries are an important source
of greenhouse gases to the atmosphere,
but uncertainties remain in the flux rates and production pathways
of greenhouse gases in these dynamic systems. This study performs
simultaneous high resolution measurements of the three major greenhouse
gases (carbon dioxide, methane, and nitrous oxide) as well as carbon
stable isotope ratios of carbon dioxide and methane, above and below
the pycnocline along a salt wedge estuary (Yarra River estuary, Australia).
We identified distinct zones of elevated greenhouse gas concentrations.
At the tip of salt wedge, average CO2 and N2O concentrations were approximately five and three times higher than
in the saline mouth of the estuary. In anaerobic bottom waters, the
natural tracer radon (222Rn) revealed that porewater exchange
was the likely source of the highest methane concentrations (up to
1302 nM). Isotopic analysis of CH4 showed a dominance of
acetoclastic production in fresh surface waters and hydrogenotrophic
production occurring in the saline bottom waters. The atmospheric
flux of methane (in CO2 equivalent units) was a major (35–53%)
contributor of atmospheric radiative forcing from the estuary, while
N2O contributed <2%. We hypothesize that the release
of bottom water gases when stratification episodically breaks down
will release large pulses of greenhouse gases to the atmosphere.