Greenhouse gas dynamics in a salt-wedge estuary revealed by high

kWater Research Laboratory, School of Civil and Environmental Engineering, University ... *Corresponding Author: Douglas Tait, Email douglas.tait@scu...
0 downloads 0 Views 1MB Size
Subscriber access provided by READING UNIV

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22

Environmental Science & Technology

24 25

Greenhouse gas dynamics in a salt-wedge estuary revealed by high resolution cavity ring down spectroscopy observations

26 27

Authors: Douglas R. Tait*,p,t, Damien Maherp,t, WeiWen Wong^, Isaac R. Santosp, Mahmood Sadat-Noorip, Ceylena Hollowayp, Perran L.M Cook^.

28 29

p

30

t

31

^

32

k

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

33 34

*Corresponding Author: Douglas Tait, Email [email protected]

35 36

Key words: Methane, carbon dioxide, nitrous oxide, submarine groundwater discharge, porewater exchange

37 38

Table of contents art

39 40 41

ABSTRACT

42

Estuaries are an important source of greenhouse gases to the atmosphere, but uncertainties

43

remain in the flux rates and production pathways of greenhouse gases in these dynamic

44

systems. This study performs simultaneous high resolution measurements of the three major

45

greenhouse gases (carbon dioxide, methane, and nitrous oxide) as well as carbon stable

46

isotope ratios of carbon dioxide and methane, above and below the pycnocline along a salt

47

wedge estuary (Yarra River estuary, Australia). We identified distinct zones of elevated

48

greenhouse gas concentrations. At the tip of salt wedge, average CO2 and N2O concentrations

49

were approximately five and three times higher than in the saline mouth of the estuary. In

50

anaerobic bottom waters, the natural tracer radon (222Rn) revealed that porewater exchange

51

was the likely source of the highest methane concentrations (up to 1302 nM). Isotopic

ACS Paragon Plus Environment

Environmental Science & Technology

52

analysis of CH4 showed a dominance of acetoclastic production in fresh surface waters and

53

hydrogenotrophic production occurring in the saline bottom waters. The atmospheric flux of

54

methane (in CO2 equivalent units) was a major (35-53%) contributor of atmospheric radiative

55

forcing from the estuary, while N2O contributed only 20 times greater than the area of

64

global estuaries 2. The fluxes of the other major greenhouse gases methane (CH4) and nitrous

65

oxide (N2O) from rivers and estuaries may also be important 3. However, large uncertainties

66

remain regarding the drivers of greenhouse gas dynamics in estuaries. For example,

67

hydrological features such as salt wedges in estuaries may be hotspots of greenhouse gas

68

production that eventually escape to the atmosphere. Salt-wedge estuaries occur when surface freshwater inflow overlies deeper saline

69 70

water in spite of tide and wind induced mixing 4. The bottom layer of the wedge can often be

71

anoxic due to the high organic matter loading, microbial activity, and long residence times 5,

72

6

73

porewater exchange which releases dissolved inorganic carbon (DIC) and ammonium from

74

sediments 7. This can lead to increased greenhouse gas production where nutrient and carbon

75

rich freshwater inputs meet the anaerobic bottom waters of the salt wedge.

76

. 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

77

directly interacts with the atmosphere 8-10. However, the release of greenhouse gases to the

78

atmosphere may be related to the accumulation of gases in the bottom layer of stratified ACS Paragon Plus Environment

Page 2 of 22

Page 3 of 22

Environmental Science & Technology

79

estuaries, that are eventually transported to the surface during periods of high turbulence 11, 12.

80

This study reports simultaneous, high resolution analysis of the three major greenhouse gases

81

(CH4, CO2 and N2O) as well as CO2 and CH4 stable carbon isotopes using cavity ring-down

82

spectroscopy in a salt wedge estuary (Yarra River estuary, Australia). We also use the natural

83

porewater tracer radon to determine the influence of porewater exchange on greenhouse gas

84

dynamics. We build on the estuarine greenhouse gas literature by (1) focusing on the sources

85

of greenhouse gases to the bottom layer, that eventually will mix with the surface layer, (2)

86

relying on detailed, high spatio-temporal resolution observations that reveal hotspots of

87

greenhouse gases, and (3) comparing the relative importance of the three major greenhouse

88

gases, which has rarely been done in estuaries.

89 90 91

EXPERIMENTAL SECTION The study was conducted in the Yarra River, located in southern Victoria, Australia in

92

November 2015. The Yarra River catchment covers an area of 5,640 km2 with the river

93

stretching 242 km from its relatively pristine northern sections to the major metropolitan area

94

of Melbourne (population >4 million) before discharging to Port Phillip Bay. The saline

95

estuary waters reach ~22 km from the mouth before a weir and a series of rapids stop any

96

further seawater penetration upstream. Depths in the estuary vary from 1 to 13 m with the

97

lower portion extensively modified through dredging, industrial and urban development.

98

Rainfall in the catchment is variable with periods of high rainfall occurring in any season.

99

Annual freshwater flows average 2.1 m3 s-1 and is generally higher in winter and spring when

100

this study took place 13. The estuary has a semi diurnal tidal regime which averages ~0.5 m

101

and ranges from 0.3 to 0.9 m. Water residence times in the estuary range between 1 day to

102

several weeks depending on upstream freshwater inputs 14. Estuary sediments range from

103

muddy sand and gravels in the upper estuary to fine depositional sediments at the mouth 13.

104 105

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

Environmental Science & Technology

106

tide in the morning and moved upstream until the water column was entirely fresh water (~18

107

km from the mouth). Measurements were taken at two different depths; ~30 cm below the

108

surface and near the estuary bottom (2 to 4 m deep). Water was pumped from each depth

109

using a submersible bilge pump (Rule 600 G.P.H.) into separate shower head equilibrators at

110

~3 L min-1 which was then measured via cavity ring down spectroscopy for CO2, CH4 and

111

N2O concentrations as well as δ13C-CO2 and δ13C-CH4 15,16. The use of separate exchangers

112

and gas measurement loops for each sampling depth allowed for reduced equilibration times

113

as the gas in the headspace did not need to equilibrate between large differences in gas

114

concentrations at the different depths. Equilibration times between measurements at the

115

different depths were further reduced due to the small cavity space in the CRDS and the

116

tubing (~60 ml) as opposed to ~1 L the headspace of the exchanger (Figure S1). The air from

117

the gas equilibrated headspaces was pumped into a distribution manifold (Picarro A0311)

118

before the gas line was split and air was pumped to two Cavity Ring-Down Spectrometers

119

(CRDS); one for measuring CH4 and CO2 concentrations and isotopic ratios (Picarro G2301)

120

and one for measuring N2O concentrations (Picarro G2308). Both instruments measured at 1

121

Hz sampling rates. The outlet gas was then pumped to another distribution manifold before

122

being returned to the respective shower head equilibrators. Both the inlet and outlet

123

distribution manifolds were programed to switch between the shallow and deep shower head

124

equilibrators at five minute intervals. CO2 and N2O data were adjusted by 10 minutes and

125

CH4 data by 30 minutes to allow for equilibration time in the exchangers 16. We highlight that

126

our equilibrator approach somewhat smooths natural trends and cannot identify spikes

127

occurring over small spatial scales. However, with equilibrator response times (~5 min,

128

Figure S1) and the slow average boat speed (~2 km h-1), we were able to obtain spatial

129

resolution of ~167 m. The last minute of data from each of the 5 minute measurement periods

130

was averaged for that data point (an example of this is shown in Figure S1). Precisions given

131

by the manufacturer were 210 ppb + 0.05%, 60 ppb + 0.05%, and 10 ppb + 0.05 % of reading

132

for CO2, CH4, and N2O respectively and with no calibration drifts observed over the ACS Paragon Plus Environment

Page 4 of 22

Page 5 of 22

Environmental Science & Technology

133

deployment. Samples for NH4+ and NOx- were collected in 10 ml vials, frozen shortly after

134

collection and concentrations determined via flow injection analysis (Lachat Quickchem

135

8000).

136

Water column physicochemical parameters (dissolved oxygen, salinity and

137

temperature) were collected at one meter depth intervals every ~1 km along the estuary using

138

a calibrated YSI Pro Plus. Radon (222Rn, a porewater exchange tracer) was measured using an

139

automated radon-in-air monitor (Rad7 Durridge Co.) coupled to a separate equilibrator for

140

each depth 17. Porewater 222Rn and greenhouse gas concentrations were measured along the

141

length of the estuary with shallow bores dug above the high tide line to a depth of

142

approximately 1 m below the water table. The bores were purged dry three times before

143

samples were collected. To determine 222Rn concentrations, gas-tight 6-L sample bottles were

144

connected in a closed loop to 222Rn gas analysers and run for ≥2 h 18. Dissolved CO2 and CH4

145

were prepared using a headspace technique 19 with samples diluted (10 to 1) with atmospheric

146

air before analysis via CRDS (Picarro G2301 and G2308). In-situ gas concentrations were

147

calculated using Henry's law in conjunction with the measured gas concentrations,

148

temperature and salinity of the sample water, and atmospheric pressure. Concentrations were

149

derived from the headspace fugacity of the gases according to Pierrot et al. 20. Solubility

150

coefficients for CO2, CH4 and N2O were derived from Weiss 21, Wiesenburg and Guinasso 22

151

and Weiss and Price 23, respectively. Area-weighted fluxes of each greenhouse gas were

152

calculated as explained elsewhere 24, with the calculations for the gas transfer velocity at the

153

water-air interface (k) of Raymond and Cole 25 used.

154 155

RESULTS AND DISCUSSION

156

Estuary zones.

157

High resolution sampling of greenhouse gas concentrations and isotopes and the

158

porewater tracer radon in this study showed a clear salt-wedge formation and distinct zones

159

of greenhouse gas production and uptake in the estuary (Figure 1). ACS Paragon Plus Environment

Environmental Science & Technology

160 161

Figure 1. High resolution observations of the three main greenhouse gases (CO2, CH4 and

162

N2O), radon (222Rn) and other physiochemical parameters in the salt wedge Yarra River

163

estuary. Salinity is represented by the grey scale background, and contour lines at 5 unit

164

intervals are presented. The solid black polygon at the bottom of the plots represents the

165

topography along the bottom of the estuary. The color scales were chosen by simple linear

166

intervals (8 intervals for each range of data).

167

ACS Paragon Plus Environment

Page 6 of 22

Page 7 of 22

Environmental Science & Technology

168

Saline mouth. The water column was well mixed and had salinities >28 the first 5 km from

169

the mouth of the estuary. This area had high dissolved oxygen (DO saturation >85%) and low

170

partial pressures of CO2 (