ICOS Science Conference, Session 10

1. Greenhouse gas
concentrations
& fluxes from
seven UK
estuaries
1
Amy Pickard, Andy Rees,
Alison Brown & the LOCATE
team: Ian Brown, Vas Kitidis,
Jenny Williamson, Annette
Burden, Nathan Callaghan,
Chris Evans, Justyna
Olszewska, Bryan Spears &
Dianekke van Wijk

2. GHG cycling in estuaries
Photolysis
Respiratio
n
EstuaryOrganic + inorganic matter
Import
Pelagicresp.
Export
Photolysis
PelagicPP
PelagicPP
CO2 CO2 CO2
CO2
CO2
CO2
BenthicPP
Groundwater
Flocculatio
n
Sedimentation
Benthic
resp.
CO2 &
CH4
Benthic
resp.
Benthic
resp.
Sedimentat
ion
Burial
CO2 & CH4 exchange
CO2 &
CH4
CO2 &
CH4
Atmosphere
Water
Sediment
Water
River Estuary Ocean
Lake/wetland
Bur
ial
Terrestrial
7 Tg CH4 yr-1
(Borges et al. 2016)

3. 3
LOCATE estuary
surveys
Aimed to understand how dissolved
greenhouse gases changed along a salinity
gradient – seven estuaries surveyed on a
quarterly basis from July 2017 to April 2018 Tay
ForthClyde
Conwy
Clywd
TamarDart
7.5 % of
total UK
estuary
area
Figure adapted from
Nedwell et al., 2002

4. 4
Conwy
Clwyd ClydeTamar
Dart
Tay
Forth
Methane concentrations highly variable both
between estuaries and along transects (N.B. y axes vary)
Salinity transect
data

5. Models
CH4
Salinity
a
b
c
e
d
a: conservative mixing
b: sharp decrease followed
by conservative mixing
c: sharp increase followed
by conservative mixing
d: non-conservative mixing;
net CH4 decrease
e: net CH4 increase

6. What’s driving this
variation?
Estuary morphology
Nutrients
Tide & river flow
NO3 NH4 NO2 PO4 R2
Clyde Positive Positive NS NS 86.6
Forth Positive NS NS Negative 64.7
Tay NS NS NS NS 26.9
Conway NS NS NS NS 30.0
Clwyd NS NS NS NS 92.8
Tamar NS NS NS NS 17.0
Dart Negative Negative NS Positive 95.7
Stratified estuaries
•High Freshwater flow (e.g Tay)
•Restricted Mixing (e.g Clyde, Dart)
Fully Mixed Estuaries
•Strong tidal signal
•Locations of maximum varies
•Influences by low flow/ high temperature

7. 7
Methane in the Clyde
Potential influences: anthropogenic inputs, estuary morphology, legacy pollution?

8. 8
Urban influence & channel morphology
Source: SEPA

9. PhD: methane from
source to sea in the
Clyde
Hypothesis: low river flow and
neap tides favour methane
release from estuary sediments
and eventual emission
Alison Brown, IAPETUS PhD studentship

10. 10
Tamar
Catchment
Dart
Catchment
CH4 in the Tamar and Dart Catchments
Dart: Responsive
catchment; upper two
thirds draining moorland.
Land use is low-grade
agriculture and woodland.
Estuary 8.6 km2
Tamar: Fairly responsive
rural catchment of
moderate relief. Land use
is a range of agriculture,
grazing and forestry.
Estuary 39.6 km2

11. 11
0
200
400
600
800
1000
1200
0 20 40
CH4Flux(µmolm-2d-
1)
Salinity
Apr-17
Jul-17
Jan-18
Apr-18
0
1000
2000
3000
4000
5000
6000
0 20 40
CH4Flux(µmolm-2d-
1)
Salinity
Apr-17
Jul-17
Jan-18
Apr-18
Tamar Estuary 2017-18 Dart Estuary 2017-18
Air-Water flux parameterisation sensitive to ∆CH4 and wind speed
• Tamar April 2018: Low CH4 & higher wind speeds result in maximum flux to atmosphere
• Dart July 2017: Max CH4 coincident with higher winds results in maximum observed flux
• Mean Flux from Tamar: 3.6 x 107 g yr-1
• Mean flux from Dart: 1.8 x 107 g yr-1
2.39 kt = 1% of UK total emission for 2015
= chemical industry sector
Estuary
CH4 fluxes

12. 12
Monthly sampling
at 4 freshwater
sites, bi-monthly
sampling at 7
positions along
the estuarine axis.
North
Tamerton
Druxton Bridge
Horsebridge
Gunnislake
Heading back upstream

13. 13
CH4concn(nmolL-1)CH4concn(nmolL-1)2019-20 survey of Tamar
Freshwater:
• High CH4 in north of catchment (N.
Tamerton) dominated by agriculture
including cattle.
• Loss to the atmosphere provides source
of CH4 to atmosphere for whole river
system.
• Gunnislake is just above tidal limit of
estuary; zero salinity point on estuary
plot.
Estuary:
• Production of CH4 in upper reaches of
estuary. Sediment resuspension or water
column production?
• Strong seasonal signal, though February
2020 anomalous.
• Compared to ocean (~2.5 nmol L-1)
significant source to the atmosphere.

14. 14
Coastal methane dynamics
0
50
100
150
200
250
CH4(%Saturation)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mean CH4 saturation
± 1 SD
100 % saturation
• Station L4 = ocean station (salinity 34) on previous slides
• Surface ocean usually in equilibrium with atmosphere (~100% saturation)
• Moderate peaks in CH4 post spring bloom (May/June) and mid-winter (Dec/Jan)
provide potential source of CH4 to atmosphere, related to sediment methanogens.

15. • Estuarine CH4 concentrations are highly variable, across estuaries,
salinity gradients and seasons
• Sources and sinks can be described according to (non-)
conservative mixing model but are seasonally variable
• Conditions of low flow & neap tides favour release from estuarine
sediments
• Microbial oxidation, dilution and flux to the atmosphere mean that
there is little export of dissolved CH4 to coastal waters
• Variability in coastal waters associated with benthic
remineralisation of organic material
• UK estuarine flux of CH4 to the atmosphere (estimated at 2.39 kt
= 1% of UK total emission for 2015 = chemical industry sector)
15
Conclusions