Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 98

where o sf is the saturation concentration of dissolved oxygen in fresh water at 1 atm
[mg l −1 ] and T a is absolute temperature [K].
The following equation can be used to establish the dependence of DO saturation
concentration on salinity. 68

1 3

1 0754 . × 10 2 1407 . × 10

− 2

ln () o = ln ( o ) − S 1 7674 . × 10 − + (4.7)

ss sf 2

T T

a a

where o ss is saturation concentration of dissolved oxygen in salt water at 1 atm
[mg l −1 ] and S is salinity [ppt].
Reaeration is represented by a rate coefficient and depends on average water
velocity, water depth, and wind speed. No universal formula for estimation of the
reaeration coefficient exists. Many investigators have developed formulae for pre-
dicting the reaeration coefficient in different aquatic systems. Water flow and/or
wind speed can affect oxygen transfer rates. In shallow waters (free flowing streams)
the reaeration coefficient depends largely on turbulence generated by bottom shear
stress. In deeper systems (e.g., lakes and estuaries) effects of wind may dominate
the reaeration process. 56,67 For estuaries, there are reaeration equations that combine
the wind- and current-induced reaeration terms. 67,69 Chapra 45 describes the use of
the commonly used formula for reaeration in estuaries.

U
H

12 / 2

0 728 . U − 0 317 . U + 0 0372 . U

0
32

k = 393 . +

w w w

(4.8)

a /

H

where
k a = reaeration rate coefficient [d −1 ]

U o = mean current velocity [m s −1 ]

U w = wind speed measured 10 m above the water surface [m s −1 ]

H = depth [m]

The reaeration rate coefficient can also be affected by temperature and salinity. 56
The temperature dependence is often expressed by an Arrhenius type equation. 69
Stratification of the water column can prevent diffusion of atmospheric oxygen
into bottom waters, and thus oxygen depletion may occur. 28, 70-72 Hypoxic conditions,
in bottom waters can cause mass mortality of benthic organisms and decline in fishery
yields. 72 Similar conditions can also occur in semi-enclosed basins with limited water
exchange, such as lagoons. 15

4.1.4.1.2 Photosynthesis - Respiration
Photosynthesis is the conversion of simple inorganic nutrients into more complex
organic molecules by autotrophic organisms. Via this reaction oxygen is liberated
and CO 2 is consumed. 45,64

106CO 2 + 16NH 4 + + HPO 4 2− + 108H 2 O → C 106 H 263 O 110 N 16 P 1 + 107O 2 + 14H + (4.9)