GAS EXCHANGE IN ESTUARIES
M. I. Scranton, State University of New York, Stony
Brook, NY, USA
M. A. de Angelis, Humboldt State University, Arcata,
CA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
Many atmospherically important gases are present in
estuarine waters in excess over levels that would be
predicted from simple equilibrium between the atmosphere and surface waters. Since estuaries are
defined as semi-enclosed coastal bodies of water that
have free connections with the open sea and within
which sea water is measurably diluted with fresh
water derived from land drainage, they tend to be
supplied with much larger amounts of organic matter
and other compounds than other coastal areas. Thus,
production of many gases is enhanced in estuaries
relative to the rest of the ocean. The geometry of
estuaries, which typically have relatively large surface areas compared to their depths, is such that flux
of material (including gases) from the sediments, and
fluxes of gases across the air/water interface, can
have a much greater impact on the water composition than would be the case in the open ocean.
Riverine and tidal currents are often quite marked,
which also can greatly affect concentrations of biogenic gases.
Gas Solubility
The direction and magnitude of the exchange of
gases across an air/water interface are determined by
the difference between the surface-water concentration of a given gas and its equilibrium concentration or gas solubility with respect to the
atmosphere. The concentration of a specific gas in
equilibrium with the atmosphere (C eq ) is given by
Henry’s Law:
C eq ¼ p gas =K H
½1
where p gas is the partial pressure of the gas in the
atmosphere, and K H is the Henry’s Law constant for
the gas. Typically as temperature and salinity increase, gas solubility decreases. (Note that Henry’s
Law and the Henry’s Law constant also may be
commonly expressed in terms of the mole fraction of
the gas in either the gas or liquid phase.) For gases
that make up a large fraction of the atmosphere (O 2 ,
N 2 , Ar), p gas does not vary temporally or spatially.
For trace atmospheric gases (carbon dioxide (CO 2 ),
methane (CH 4 ), hydrogen (H 2 ), nitrous oxide (N 2 O),
and others), p gas may vary considerably geographically or seasonally, and may be affected by anthropogenic activity or local natural sources.
Gas Exchange (Flux) Across the Air/
Water Interface
The rate of gas exchange across the air/water interface for a specific gas is determined by the degree of
disequilibrium between the actual surface concentration of a gas (C surf ) and its equilibrium concentration (C eq ), commonly expressed as R:
R ¼ C surf =C eq
½2
If R ¼ 1, the dissolved gas is in equilibrium with the
atmosphere and no net flux or exchange with the
atmosphere occurs. For gases with Ro1, the dissolved gas is undersaturated with respect to the atmosphere and there is a net flux of the gas from the
atmosphere to the water. For gases with R41, a net
flux of the gas from the water to the atmosphere
occurs.
Models of Gas Exchange
The magnitude of the flux (F) in units of mass of gas
per unit area per unit time across the air/water
interface is a function of the magnitude of the difference between the dissolved gas concentration and
its equilibrium concentration as given by Fick’s First
Law of Diffusion:
F ¼ kðC surf À C eq Þ
½ 3
where k is a first order rate constant, which is a
function of the specific gas and surface water conditions. The rate constant, k, also known as the
transfer coefficient, has units of velocity and is frequently given as
k ¼ D=z
½4
where D is the molecular diffusivity (in units of cm
2
s
À1
), and z is the thickness of the laminar layer at the
475
M. I. Scranton, State University of New York, Stony
Brook, NY, USA
M. A. de Angelis, Humboldt State University, Arcata,
CA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
Many atmospherically important gases are present in
estuarine waters in excess over levels that would be
predicted from simple equilibrium between the atmosphere and surface waters. Since estuaries are
defined as semi-enclosed coastal bodies of water that
have free connections with the open sea and within
which sea water is measurably diluted with fresh
water derived from land drainage, they tend to be
supplied with much larger amounts of organic matter
and other compounds than other coastal areas. Thus,
production of many gases is enhanced in estuaries
relative to the rest of the ocean. The geometry of
estuaries, which typically have relatively large surface areas compared to their depths, is such that flux
of material (including gases) from the sediments, and
fluxes of gases across the air/water interface, can
have a much greater impact on the water composition than would be the case in the open ocean.
Riverine and tidal currents are often quite marked,
which also can greatly affect concentrations of biogenic gases.
Gas Solubility
The direction and magnitude of the exchange of
gases across an air/water interface are determined by
the difference between the surface-water concentration of a given gas and its equilibrium concentration or gas solubility with respect to the
atmosphere. The concentration of a specific gas in
equilibrium with the atmosphere (C eq ) is given by
Henry’s Law:
C eq ¼ p gas =K H
½1
where p gas is the partial pressure of the gas in the
atmosphere, and K H is the Henry’s Law constant for
the gas. Typically as temperature and salinity increase, gas solubility decreases. (Note that Henry’s
Law and the Henry’s Law constant also may be
commonly expressed in terms of the mole fraction of
the gas in either the gas or liquid phase.) For gases
that make up a large fraction of the atmosphere (O 2 ,
N 2 , Ar), p gas does not vary temporally or spatially.
For trace atmospheric gases (carbon dioxide (CO 2 ),
methane (CH 4 ), hydrogen (H 2 ), nitrous oxide (N 2 O),
and others), p gas may vary considerably geographically or seasonally, and may be affected by anthropogenic activity or local natural sources.
Gas Exchange (Flux) Across the Air/
Water Interface
The rate of gas exchange across the air/water interface for a specific gas is determined by the degree of
disequilibrium between the actual surface concentration of a gas (C surf ) and its equilibrium concentration (C eq ), commonly expressed as R:
R ¼ C surf =C eq
½2
If R ¼ 1, the dissolved gas is in equilibrium with the
atmosphere and no net flux or exchange with the
atmosphere occurs. For gases with Ro1, the dissolved gas is undersaturated with respect to the atmosphere and there is a net flux of the gas from the
atmosphere to the water. For gases with R41, a net
flux of the gas from the water to the atmosphere
occurs.
Models of Gas Exchange
The magnitude of the flux (F) in units of mass of gas
per unit area per unit time across the air/water
interface is a function of the magnitude of the difference between the dissolved gas concentration and
its equilibrium concentration as given by Fick’s First
Law of Diffusion:
F ¼ kðC surf À C eq Þ
½ 3
where k is a first order rate constant, which is a
function of the specific gas and surface water conditions. The rate constant, k, also known as the
transfer coefficient, has units of velocity and is frequently given as
k ¼ D=z
½4
where D is the molecular diffusivity (in units of cm
2
s
À1
), and z is the thickness of the laminar layer at the
475