80
and an estimate of turbulence caused by the flowing waters interacting with bottom topography
(O'Connor and Dobbins 1958; Bennet and Rathbun 1972; O'Connor 1984; Roberts 1984). However, in deeper waters, currents and bottom topography have little or no influence on gas exchange
with the atmosphere; rather, gas exchange is controlled by turbulence generated through wind shear
on the surface (Jirka and Brutsaert 1984; Marino
and Howarth 1993). Marino and Howarth (1993)
found relatively good agreement among a variety
of methods for estimating gas exchange with the
atmosphere in a variety of lakes, estuaries, and
open-ocean sites. Using that compilation, atmospheric exchange of oxygen can be estimated by
the equation:
F
(5.3)
ALL NATURAL SYSTEMS FIELD DATA
5
A
",'"
4
JII.
.5 2
2
4
6
8
10
12
Wind al10 m
OPEN OCEAN
5 C
4
JII.
3
.5
2
-".
...
".
".
...
-...
...
...
0
0
2
4
6
8
10
12
Wind al10 m
Robert W. Howarth and Anthony F. Michaels
where F is the gas flux per surface area of water,
k is the "piston velocity," C w is the molar concentration of gas in solution, Pais the partial
pressure of gas in the atmosphere, and Kh is the
Henry's Law constant (Fig. 5.1). The term
[C w - (P a X K h )] describes the partial pressure
gradient of the gas. The piston velocity, k, is a
function of wind velocity, with In K increasing
linearly with increasing wind. Thus, for many
lakes, open-ocean systems, estuaries, and relatively deep rivers, the atmospheric exchange of
oxygen can be estimated from data on wind and
the extent of oxygen undersaturation or supersaturation (Marino and Howarth 1993; Howarth
et al. 1992). However, care should be taken to
ensure that this estimation approach will work in
any particular ecosystem of interest. Recently,
Kremer et al. (1999) have demonstrated for one
ESTUARIES
5
B
4
3
...
JII.
'"
.5
-2
...
0
0
2
4
6
8
10
12
Wind al 10 m
LAKES
5
0
... ".'"
,
4
". ...
".
...
...
......
JII.
3
........ "'+.t
.5
.... -....
2
O~~~~~~~T-~~r-~
o
2
4
6
8
10
12
Wind a' 10 m
FIGURE 5.1. The relationship between wind and the "piston velocity" controlling oxygen exchange with the atmosphere for a variety of lakes, estuaries, and open-ocean sites. Upper left panel includes all data from the other three
panels. The dashed lines indicate 95% confidence limits for the regressions. See text for further explanation. (From
Marino and Howarth [1993].)