observed to cause elevated methane concentrations
in adjacent coastal oceans. In addition to riverine
input, sources of methane to estuaries include intertidal flats and marshes, ground-water input, runoff
from agricultural and pasture land, petroleum pollution, lateral input from exposed bank soils, wastewater discharge and emission from organic-rich
anaerobic sediments, either diffusively or via ebullition (transport of gas from sediments as bubbles)
and subsequent dissolution within the water column.
Anthropogenically impacted estuaries or estuaries
supplied from impacted rivers tend to be characterized by higher water-column methane concentrations
relative to pristine estuarine systems. Seasonally,
methane levels in estuaries are higher in summer
compared with winter, primarily due to increased
bacterial methane production (methanogenesis) in
estuarine and riverine sediments.
Methane can be removed from estuarine waters by
microbial methane oxidation and emission to the
atmosphere. Methane oxidation within estuaries can
be quite rapid, with methane turnover times of o2 h
to several days. Methane oxidation appears to be
most rapid at salinities of less than about 6 (on the
practical salinity units scale) and is strongly
dependent on temperature, with highest oxidation
rates occurring during the summer, when water
temperatures are highest. Methane oxidation rates
decrease rapidly with higher salinities.
Methane diffusive fluxes to the atmosphere reported for estuaries (Table 1) fall within a narrow
range of 3.6–41.7 mmol m
À2 h
À1 (2–16 mg CH 4 m
À2
day
À1 ). Using a global surface area for estuaries of
1.4 Â 10
6 km
2 yields an annual emission of methane
to the atmosphere from estuaries of 1–8 Tg y
À1
. Because the higher flux estimates given in Table 1
generally were obtained close to the freshwater endmember of the estuary, the global methane estuarine
emission is most likely within the range of 1–
3 Tg y
À1 , corresponding to approximately 10% of
the total global oceanic methane flux to the atmosphere, despite the much smaller global surface area
of estuaries relative to the open ocean.
Methane is also released to the atmosphere directly from anaerobic estuarine sediments via bubble
formation and injection into the water column. Although small amounts of methane from bubbles may
dissolve within the water column, the relatively
shallow nature of the estuarine environment results
in the majority of methane in bubbles reaching the
atmosphere. The quantitative release of methane via
this mechanism is difficult to evaluate due to the irregular and sporadic spatial and temporal extent of
ebullition. Where ebullition occurs, the flux of methane to the atmosphere is considerably higher than
diffusive flux (Table 1), but the areal extent of bubbling is relatively smaller than that of diffusive
flux and, except in organic-rich stagnant areas such
as tidal marshes, probably does not contribute significantly to estuarine methane emissions to the atmosphere. Methane emission via ebullition has been
observed to be at least partially controlled by tidal
changes in hydrostatic pressure, with release of methane occurring at or near low tide when hydrostatic
pressure is at a minimum.
Nitrous Oxide (N 2 O)
Nitrous oxide is another important greenhouse gas
that is present in elevated concentrations in estuarine
environments. At present, N 2 O is responsible for
about 5–6% of the anthropogenic greenhouse effect
and is increasing in the atmosphere at a rate of about
0.25% per year. However, the role of estuaries in the
global budget of the gas has only been addressed
recently.
Nitrous oxide is produced primarily as an intermediate during both nitrification (the oxidation of
ammonium to nitrate) and denitrification (the reduction of nitrate, via nitrite and N 2 O, to nitrogen
gas), although production by dissimilatory nitrate
reduction to ammonium is also possible. In estuaries,
nitrification and denitrification are both thought to
be important sources. Factors such as the oxygen
level in the estuary and the nitrate and ammonium
concentrations of the water can influence which
pathway is dominant, with denitrification dominating at very low, but non-zero, oxygen concentrations. Nitrous oxide concentrations are typically
highest in the portions of the estuary closest to the
rivers, and decrease with distance downstream. A
number of workers have reported nitrous oxide
maxima in estuarine waters at low salinities (o5–10
on the PSU scale), but this is not always the case. The
turbidity maximum has been reported to be the site
of maximum nitrification (presumably because of
increased residence time for bacteria attached to
suspended particulate matter, combined with elevated substrate (oxygen and ammonium)).
Table 2 presents a summary of the data published
for degree of saturation and air–estuary flux of nitrous oxide from a variety of estuaries, all of which
are located in Europe and North America. Concentrations are commonly above that predicted from
air–sea equilibrium, and estimates of fluxes range
from 0.01 mmol m
À2 h
À1 to 5 mmol m
À2 h
À1
. Ebullition is not important for nitrous oxide because it is
much more soluble than methane. Researchers have
estimated the size of the global estuarine source for
N 2 O based on fluxes from individual estuaries
478 GAS EXCHANGE IN ESTUARIES
in adjacent coastal oceans. In addition to riverine
input, sources of methane to estuaries include intertidal flats and marshes, ground-water input, runoff
from agricultural and pasture land, petroleum pollution, lateral input from exposed bank soils, wastewater discharge and emission from organic-rich
anaerobic sediments, either diffusively or via ebullition (transport of gas from sediments as bubbles)
and subsequent dissolution within the water column.
Anthropogenically impacted estuaries or estuaries
supplied from impacted rivers tend to be characterized by higher water-column methane concentrations
relative to pristine estuarine systems. Seasonally,
methane levels in estuaries are higher in summer
compared with winter, primarily due to increased
bacterial methane production (methanogenesis) in
estuarine and riverine sediments.
Methane can be removed from estuarine waters by
microbial methane oxidation and emission to the
atmosphere. Methane oxidation within estuaries can
be quite rapid, with methane turnover times of o2 h
to several days. Methane oxidation appears to be
most rapid at salinities of less than about 6 (on the
practical salinity units scale) and is strongly
dependent on temperature, with highest oxidation
rates occurring during the summer, when water
temperatures are highest. Methane oxidation rates
decrease rapidly with higher salinities.
Methane diffusive fluxes to the atmosphere reported for estuaries (Table 1) fall within a narrow
range of 3.6–41.7 mmol m
À2 h
À1 (2–16 mg CH 4 m
À2
day
À1 ). Using a global surface area for estuaries of
1.4 Â 10
6 km
2 yields an annual emission of methane
to the atmosphere from estuaries of 1–8 Tg y
À1
. Because the higher flux estimates given in Table 1
generally were obtained close to the freshwater endmember of the estuary, the global methane estuarine
emission is most likely within the range of 1–
3 Tg y
À1 , corresponding to approximately 10% of
the total global oceanic methane flux to the atmosphere, despite the much smaller global surface area
of estuaries relative to the open ocean.
Methane is also released to the atmosphere directly from anaerobic estuarine sediments via bubble
formation and injection into the water column. Although small amounts of methane from bubbles may
dissolve within the water column, the relatively
shallow nature of the estuarine environment results
in the majority of methane in bubbles reaching the
atmosphere. The quantitative release of methane via
this mechanism is difficult to evaluate due to the irregular and sporadic spatial and temporal extent of
ebullition. Where ebullition occurs, the flux of methane to the atmosphere is considerably higher than
diffusive flux (Table 1), but the areal extent of bubbling is relatively smaller than that of diffusive
flux and, except in organic-rich stagnant areas such
as tidal marshes, probably does not contribute significantly to estuarine methane emissions to the atmosphere. Methane emission via ebullition has been
observed to be at least partially controlled by tidal
changes in hydrostatic pressure, with release of methane occurring at or near low tide when hydrostatic
pressure is at a minimum.
Nitrous Oxide (N 2 O)
Nitrous oxide is another important greenhouse gas
that is present in elevated concentrations in estuarine
environments. At present, N 2 O is responsible for
about 5–6% of the anthropogenic greenhouse effect
and is increasing in the atmosphere at a rate of about
0.25% per year. However, the role of estuaries in the
global budget of the gas has only been addressed
recently.
Nitrous oxide is produced primarily as an intermediate during both nitrification (the oxidation of
ammonium to nitrate) and denitrification (the reduction of nitrate, via nitrite and N 2 O, to nitrogen
gas), although production by dissimilatory nitrate
reduction to ammonium is also possible. In estuaries,
nitrification and denitrification are both thought to
be important sources. Factors such as the oxygen
level in the estuary and the nitrate and ammonium
concentrations of the water can influence which
pathway is dominant, with denitrification dominating at very low, but non-zero, oxygen concentrations. Nitrous oxide concentrations are typically
highest in the portions of the estuary closest to the
rivers, and decrease with distance downstream. A
number of workers have reported nitrous oxide
maxima in estuarine waters at low salinities (o5–10
on the PSU scale), but this is not always the case. The
turbidity maximum has been reported to be the site
of maximum nitrification (presumably because of
increased residence time for bacteria attached to
suspended particulate matter, combined with elevated substrate (oxygen and ammonium)).
Table 2 presents a summary of the data published
for degree of saturation and air–estuary flux of nitrous oxide from a variety of estuaries, all of which
are located in Europe and North America. Concentrations are commonly above that predicted from
air–sea equilibrium, and estimates of fluxes range
from 0.01 mmol m
À2 h
À1 to 5 mmol m
À2 h
À1
. Ebullition is not important for nitrous oxide because it is
much more soluble than methane. Researchers have
estimated the size of the global estuarine source for
N 2 O based on fluxes from individual estuaries
478 GAS EXCHANGE IN ESTUARIES