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Greenhouse gas dynamics in a salt-wedge estuary revealed by high resolution cavity ring down spectroscopy observations Douglas R. Tait, Damien Troy Maher, Wei Wen Wong, Isaac R. Santos, Mahmood Sadat-noori, Ceylena Holloway, and Perran Louis Miall Cook Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04627 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017
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Greenhouse gas dynamics in a salt-wedge estuary revealed by high resolution cavity ring down spectroscopy observations
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Authors: Douglas R. Tait*,p,t, Damien Maherp,t, WeiWen Wong^, Isaac R. Santosp, Mahmood Sadat-Noorip, Ceylena Hollowayp, Perran L.M Cook^.
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National Marine Science Centre, Southern Cross University, PO Box 4321, Coffs Harbour, 2450 NSW, Australia Southern Cross Geoscience, Southern Cross University, Lismore, New South Wales 2480, Australia Water Studies Centre, School of Chemistry, Monash University, Clayton, Victoria, Australia Water Research Laboratory, School of Civil and Environmental Engineering, University of NSW, Sydney
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*Corresponding Author: Douglas Tait, Email
[email protected] 35 36
Key words: Methane, carbon dioxide, nitrous oxide, submarine groundwater discharge, porewater exchange
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Table of contents art
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ABSTRACT
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Estuaries are an important source of greenhouse gases to the atmosphere, but uncertainties
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remain in the flux rates and production pathways of greenhouse gases in these dynamic
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systems. This study performs simultaneous high resolution measurements of the three major
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greenhouse gases (carbon dioxide, methane, and nitrous oxide) as well as carbon stable
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isotope ratios of carbon dioxide and methane, above and below the pycnocline along a salt
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wedge estuary (Yarra River estuary, Australia). We identified distinct zones of elevated
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greenhouse gas concentrations. At the tip of salt wedge, average CO2 and N2O concentrations
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were approximately five and three times higher than in the saline mouth of the estuary. In
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anaerobic bottom waters, the natural tracer radon (222Rn) revealed that porewater exchange
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was the likely source of the highest methane concentrations (up to 1302 nM). Isotopic
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analysis of CH4 showed a dominance of acetoclastic production in fresh surface waters and
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hydrogenotrophic production occurring in the saline bottom waters. The atmospheric flux of
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methane (in CO2 equivalent units) was a major (35-53%) contributor of atmospheric radiative
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forcing from the estuary, while N2O contributed only 20 times greater than the area of
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global estuaries 2. The fluxes of the other major greenhouse gases methane (CH4) and nitrous
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oxide (N2O) from rivers and estuaries may also be important 3. However, large uncertainties
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remain regarding the drivers of greenhouse gas dynamics in estuaries. For example,
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hydrological features such as salt wedges in estuaries may be hotspots of greenhouse gas
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production that eventually escape to the atmosphere. Salt-wedge estuaries occur when surface freshwater inflow overlies deeper saline
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water in spite of tide and wind induced mixing 4. The bottom layer of the wedge can often be
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anoxic due to the high organic matter loading, microbial activity, and long residence times 5,
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porewater exchange which releases dissolved inorganic carbon (DIC) and ammonium from
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sediments 7. This can lead to increased greenhouse gas production where nutrient and carbon
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rich freshwater inputs meet the anaerobic bottom waters of the salt wedge.
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. The upstream propagation of the salt wedge can also drive convection-driven advective
Greenhouse gas investigations in estuaries often focus on the surface layer that
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directly interacts with the atmosphere 8-10. However, the release of greenhouse gases to the
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atmosphere may be related to the accumulation of gases in the bottom layer of stratified ACS Paragon Plus Environment
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estuaries, that are eventually transported to the surface during periods of high turbulence 11, 12.
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This study reports simultaneous, high resolution analysis of the three major greenhouse gases
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(CH4, CO2 and N2O) as well as CO2 and CH4 stable carbon isotopes using cavity ring-down
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spectroscopy in a salt wedge estuary (Yarra River estuary, Australia). We also use the natural
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porewater tracer radon to determine the influence of porewater exchange on greenhouse gas
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dynamics. We build on the estuarine greenhouse gas literature by (1) focusing on the sources
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of greenhouse gases to the bottom layer, that eventually will mix with the surface layer, (2)
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relying on detailed, high spatio-temporal resolution observations that reveal hotspots of
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greenhouse gases, and (3) comparing the relative importance of the three major greenhouse
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gases, which has rarely been done in estuaries.
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EXPERIMENTAL SECTION The study was conducted in the Yarra River, located in southern Victoria, Australia in
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November 2015. The Yarra River catchment covers an area of 5,640 km2 with the river
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stretching 242 km from its relatively pristine northern sections to the major metropolitan area
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of Melbourne (population >4 million) before discharging to Port Phillip Bay. The saline
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estuary waters reach ~22 km from the mouth before a weir and a series of rapids stop any
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further seawater penetration upstream. Depths in the estuary vary from 1 to 13 m with the
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lower portion extensively modified through dredging, industrial and urban development.
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Rainfall in the catchment is variable with periods of high rainfall occurring in any season.
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Annual freshwater flows average 2.1 m3 s-1 and is generally higher in winter and spring when
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this study took place 13. The estuary has a semi diurnal tidal regime which averages ~0.5 m
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and ranges from 0.3 to 0.9 m. Water residence times in the estuary range between 1 day to
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several weeks depending on upstream freshwater inputs 14. Estuary sediments range from
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muddy sand and gravels in the upper estuary to fine depositional sediments at the mouth 13.
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A high resolution spatial survey of the water column was conducted using a boat travelling at an average of ~2 km h-1. Surveying started at the mouth of the estuary at high ACS Paragon Plus Environment
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tide in the morning and moved upstream until the water column was entirely fresh water (~18
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km from the mouth). Measurements were taken at two different depths; ~30 cm below the
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surface and near the estuary bottom (2 to 4 m deep). Water was pumped from each depth
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using a submersible bilge pump (Rule 600 G.P.H.) into separate shower head equilibrators at
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~3 L min-1 which was then measured via cavity ring down spectroscopy for CO2, CH4 and
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N2O concentrations as well as δ13C-CO2 and δ13C-CH4 15,16. The use of separate exchangers
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and gas measurement loops for each sampling depth allowed for reduced equilibration times
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as the gas in the headspace did not need to equilibrate between large differences in gas
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concentrations at the different depths. Equilibration times between measurements at the
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different depths were further reduced due to the small cavity space in the CRDS and the
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tubing (~60 ml) as opposed to ~1 L the headspace of the exchanger (Figure S1). The air from
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the gas equilibrated headspaces was pumped into a distribution manifold (Picarro A0311)
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before the gas line was split and air was pumped to two Cavity Ring-Down Spectrometers
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(CRDS); one for measuring CH4 and CO2 concentrations and isotopic ratios (Picarro G2301)
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and one for measuring N2O concentrations (Picarro G2308). Both instruments measured at 1
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Hz sampling rates. The outlet gas was then pumped to another distribution manifold before
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being returned to the respective shower head equilibrators. Both the inlet and outlet
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distribution manifolds were programed to switch between the shallow and deep shower head
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equilibrators at five minute intervals. CO2 and N2O data were adjusted by 10 minutes and
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CH4 data by 30 minutes to allow for equilibration time in the exchangers 16. We highlight that
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our equilibrator approach somewhat smooths natural trends and cannot identify spikes
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occurring over small spatial scales. However, with equilibrator response times (~5 min,
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Figure S1) and the slow average boat speed (~2 km h-1), we were able to obtain spatial
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resolution of ~167 m. The last minute of data from each of the 5 minute measurement periods
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was averaged for that data point (an example of this is shown in Figure S1). Precisions given
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by the manufacturer were 210 ppb + 0.05%, 60 ppb + 0.05%, and 10 ppb + 0.05 % of reading
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for CO2, CH4, and N2O respectively and with no calibration drifts observed over the ACS Paragon Plus Environment
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deployment. Samples for NH4+ and NOx- were collected in 10 ml vials, frozen shortly after
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collection and concentrations determined via flow injection analysis (Lachat Quickchem
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8000).
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Water column physicochemical parameters (dissolved oxygen, salinity and
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temperature) were collected at one meter depth intervals every ~1 km along the estuary using
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a calibrated YSI Pro Plus. Radon (222Rn, a porewater exchange tracer) was measured using an
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automated radon-in-air monitor (Rad7 Durridge Co.) coupled to a separate equilibrator for
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each depth 17. Porewater 222Rn and greenhouse gas concentrations were measured along the
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length of the estuary with shallow bores dug above the high tide line to a depth of
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approximately 1 m below the water table. The bores were purged dry three times before
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samples were collected. To determine 222Rn concentrations, gas-tight 6-L sample bottles were
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connected in a closed loop to 222Rn gas analysers and run for ≥2 h 18. Dissolved CO2 and CH4
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were prepared using a headspace technique 19 with samples diluted (10 to 1) with atmospheric
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air before analysis via CRDS (Picarro G2301 and G2308). In-situ gas concentrations were
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calculated using Henry's law in conjunction with the measured gas concentrations,
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temperature and salinity of the sample water, and atmospheric pressure. Concentrations were
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derived from the headspace fugacity of the gases according to Pierrot et al. 20. Solubility
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coefficients for CO2, CH4 and N2O were derived from Weiss 21, Wiesenburg and Guinasso 22
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and Weiss and Price 23, respectively. Area-weighted fluxes of each greenhouse gas were
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calculated as explained elsewhere 24, with the calculations for the gas transfer velocity at the
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water-air interface (k) of Raymond and Cole 25 used.
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RESULTS AND DISCUSSION
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Estuary zones.
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High resolution sampling of greenhouse gas concentrations and isotopes and the
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porewater tracer radon in this study showed a clear salt-wedge formation and distinct zones
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of greenhouse gas production and uptake in the estuary (Figure 1). ACS Paragon Plus Environment
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Figure 1. High resolution observations of the three main greenhouse gases (CO2, CH4 and
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N2O), radon (222Rn) and other physiochemical parameters in the salt wedge Yarra River
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estuary. Salinity is represented by the grey scale background, and contour lines at 5 unit
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intervals are presented. The solid black polygon at the bottom of the plots represents the
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topography along the bottom of the estuary. The color scales were chosen by simple linear
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intervals (8 intervals for each range of data).
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Saline mouth. The water column was well mixed and had salinities >28 the first 5 km from
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the mouth of the estuary. This area had high dissolved oxygen (DO saturation >85%) and low
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partial pressures of CO2 (
