occur over short time periods on the order of weeks,
corresponding to phytoplankton blooms. The DMS
flux for an estuary in Florida, USA, was estimated to
be on the order of o1 nmoles m
À2 h
À1
. Insufficient
data are available to determine reliable global DMS
air/water exchanges from estuaries.
Hydrogen (H 2 )
Hydrogen is an important intermediate in many
microbial catabolic reactions, and the efficiency of
hydrogen transfer among microbial organisms
within an environment helps determine the pathways
of organic matter decomposition. Hydrogen is generally supersaturated in the surface waters of the few
estuaries that have been analyzed for dissolved
hydrogen with R-values of 1.5–67. Hydrogen flux to
the atmosphere from estuaries has been reported to
be on the order of 0.06–0.27 nmol m
À2 h
À1 . The
contribution of estuaries to the global atmospheric
H 2 flux cannot be determined from the few available
data.
Carbon Monoxide (CO)
Carbon monoxide in surface waters is produced
primarily from the photo-oxidation of dissolved organic matter by UV radiation. Since estuarine waters
are characterized by high dissolved organic carbon
levels, the surface waters of estuaries are highly
supersaturated and are a strong source of CO to the
atmosphere. Reported R-values for CO range from
approximately 10 to 410 000. Because of the highly
variable distributions of dissolved CO within surface
waters (primarily as the result of the highly variable
production of CO), it is impossible to derive a
meaningful value for CO emissions to the atmosphere from estuaries.
Carbonyl Sulfide (OCS)
Carbonyl sulfide makes up approximately 80% of
the total sulfur content of the atmosphere and is the
major source of stratospheric aerosols. Carbonyl
sulfide is produced within surface waters by photolysis of dissolved organosulfur compounds.
Therefore, surface water OCS levels within estuaries
exhibit a strong diel trend. Carbonyl sulfide is also
added to the water column by diffusion from anoxic
sediments, where its production appears to be coupled to microbial sulfate reduction. Diffusion of OCS
from the sediment to the water column accounts for
B75% of the OCS supplied to the water column and
is responsible for the higher OCS concentrations in
estuaries relative to the open ocean. While supersaturations of OCS are observed throughout
estuarine surface waters, no trends with salinity have
been observed. Atmospheric OCS fluxes to the atmosphere from Chesapeake Bay have been reported
to range from 10.4 to 56.2 nmol m
À2 h
À1
. These
areal fluxes are over 50 times greater than those
determined for the open ocean.
Elemental Mercury (Hg
0
)
Elemental mercury is produced in estuarine environments by biologically mediated processes. Both
algae and bacteria are able to convert dissolved
inorganic mercury to volatile forms, which include
organic species (monomethyl- and dimethyl-mercury) and Hg
0 . Under suboxic conditions, elemental
mercury also may be the thermodynamically stable
form of the metal. In the Scheldt River estuary, Hg
0
correlated well with phytoplankton pigments,
suggesting that phytoplankton were the dominant
factors, at least in that system. Factors that may
affect elemental mercury concentrations include the
type of phytoplankton present, photo-catalytic reduction of ionic Hg in surface waters, the extent of
bacterial activity that removes oxygen from the estuary, and removal of mercury by particulate scavenging and sulfide precipitation. Fluxes of elemental
mercury to the atmosphere have been estimated
for the Pettaquamscutt estuary in Rhode Island,
USA, and for the Scheldt, and range from 4.2–29
pmol m
À2 h
À1
, although the values are strongly
dependent on the model used to estimate gas exchange coefficients.
Volatile Organic Compounds (VOCs)
In addition to gases produced naturally in the
environment, estuaries tend to be enriched in byproducts of industry and other human activity. A few
studies have investigated volatile organic pollutants
such as chlorinated hydrocarbons (chloroform, tetrachloromethane, 1,1-dichloroethane, 1,2-dichloroethane, 1,1,1-trichloroethane, trichloroethylene
and tetrachloroethylene) and monocyclic aromatic
hydrocarbons (benzene, toluene, ethylbenzene,
o-xylene and m- and p-xylene). Concentrations
of VOCs are controlled primarily by the location of
the sources, dilution of river water with clean marine
water within the estuary, gas exchange, and in
some cases, adsorption onto suspended or settling
solids. In some cases (for example, chloroform)
there also may be natural biotic sources of the
gas. Volatilization to the atmosphere can be an important ‘cleansing’ mechanism for the estuary
system. Since the only estuaries studied to date are
heavily impacted by human activity (the Elbe and
480 GAS EXCHANGE IN ESTUARIES
corresponding to phytoplankton blooms. The DMS
flux for an estuary in Florida, USA, was estimated to
be on the order of o1 nmoles m
À2 h
À1
. Insufficient
data are available to determine reliable global DMS
air/water exchanges from estuaries.
Hydrogen (H 2 )
Hydrogen is an important intermediate in many
microbial catabolic reactions, and the efficiency of
hydrogen transfer among microbial organisms
within an environment helps determine the pathways
of organic matter decomposition. Hydrogen is generally supersaturated in the surface waters of the few
estuaries that have been analyzed for dissolved
hydrogen with R-values of 1.5–67. Hydrogen flux to
the atmosphere from estuaries has been reported to
be on the order of 0.06–0.27 nmol m
À2 h
À1 . The
contribution of estuaries to the global atmospheric
H 2 flux cannot be determined from the few available
data.
Carbon Monoxide (CO)
Carbon monoxide in surface waters is produced
primarily from the photo-oxidation of dissolved organic matter by UV radiation. Since estuarine waters
are characterized by high dissolved organic carbon
levels, the surface waters of estuaries are highly
supersaturated and are a strong source of CO to the
atmosphere. Reported R-values for CO range from
approximately 10 to 410 000. Because of the highly
variable distributions of dissolved CO within surface
waters (primarily as the result of the highly variable
production of CO), it is impossible to derive a
meaningful value for CO emissions to the atmosphere from estuaries.
Carbonyl Sulfide (OCS)
Carbonyl sulfide makes up approximately 80% of
the total sulfur content of the atmosphere and is the
major source of stratospheric aerosols. Carbonyl
sulfide is produced within surface waters by photolysis of dissolved organosulfur compounds.
Therefore, surface water OCS levels within estuaries
exhibit a strong diel trend. Carbonyl sulfide is also
added to the water column by diffusion from anoxic
sediments, where its production appears to be coupled to microbial sulfate reduction. Diffusion of OCS
from the sediment to the water column accounts for
B75% of the OCS supplied to the water column and
is responsible for the higher OCS concentrations in
estuaries relative to the open ocean. While supersaturations of OCS are observed throughout
estuarine surface waters, no trends with salinity have
been observed. Atmospheric OCS fluxes to the atmosphere from Chesapeake Bay have been reported
to range from 10.4 to 56.2 nmol m
À2 h
À1
. These
areal fluxes are over 50 times greater than those
determined for the open ocean.
Elemental Mercury (Hg
0
)
Elemental mercury is produced in estuarine environments by biologically mediated processes. Both
algae and bacteria are able to convert dissolved
inorganic mercury to volatile forms, which include
organic species (monomethyl- and dimethyl-mercury) and Hg
0 . Under suboxic conditions, elemental
mercury also may be the thermodynamically stable
form of the metal. In the Scheldt River estuary, Hg
0
correlated well with phytoplankton pigments,
suggesting that phytoplankton were the dominant
factors, at least in that system. Factors that may
affect elemental mercury concentrations include the
type of phytoplankton present, photo-catalytic reduction of ionic Hg in surface waters, the extent of
bacterial activity that removes oxygen from the estuary, and removal of mercury by particulate scavenging and sulfide precipitation. Fluxes of elemental
mercury to the atmosphere have been estimated
for the Pettaquamscutt estuary in Rhode Island,
USA, and for the Scheldt, and range from 4.2–29
pmol m
À2 h
À1
, although the values are strongly
dependent on the model used to estimate gas exchange coefficients.
Volatile Organic Compounds (VOCs)
In addition to gases produced naturally in the
environment, estuaries tend to be enriched in byproducts of industry and other human activity. A few
studies have investigated volatile organic pollutants
such as chlorinated hydrocarbons (chloroform, tetrachloromethane, 1,1-dichloroethane, 1,2-dichloroethane, 1,1,1-trichloroethane, trichloroethylene
and tetrachloroethylene) and monocyclic aromatic
hydrocarbons (benzene, toluene, ethylbenzene,
o-xylene and m- and p-xylene). Concentrations
of VOCs are controlled primarily by the location of
the sources, dilution of river water with clean marine
water within the estuary, gas exchange, and in
some cases, adsorption onto suspended or settling
solids. In some cases (for example, chloroform)
there also may be natural biotic sources of the
gas. Volatilization to the atmosphere can be an important ‘cleansing’ mechanism for the estuary
system. Since the only estuaries studied to date are
heavily impacted by human activity (the Elbe and
480 GAS EXCHANGE IN ESTUARIES