Limnol.
Oceanogr., 36(3), 199 I, 444-454
0 199 1, by the American
Society of Limnology
and Oceanography,
Inc.
Computations of oxygen fluxes through the sea surface and the
net production of organic matter with application to the
Baltic and adjacent seas
Anders Stigebrandt
Oceanografiska Institutionen, Gijteborgs Universitet, Box 4038, S-400 40 Goteborg, Sweden
Abstract
The transfer F,, of oxygen between the ocean and the atmosphere is computed from the formula
Fo, = UO, - (1 + x)O,s] where V is the transfer velocity, 0, the actual oxygen concentration of
the surface water, 0,s the saturation concentration, and x a factor that takes into account the effect
of gas transfer due to bubbles. Monthly mean oxygen fluxes were calculated from the formula with
historical hydrographical data. By adjusting the value of X, the formula was tuned to obtain a
vanishing net annual oxygen flux through the sea surface in the Baltic proper. This occurs when x
equals 0.025. Bubble-driven gas transfer thus tends to supersaturate surface water.
The gas transfer formula is tested by a comparison between published estimates of the net
production of organic matter in the photic layer and estimates obtained with computed oxygen
fluxes through the sea surface. The test indicates that the formula is reliable. The test further shows
that it is possible to compute the net production of organic matter in the photic zone, using the
formula for oxygen exchange presented here, and salinity, temperature, and oxygen data from the
surface layer.
The flux through the sea surface of a
chemically unreactive gas is thought to be
governed by a transfer velocity and the difference between the partial pressures of the
gas in the surface layer of the sea and in the
atmosphere. The transfer or piston velocity
is controlled by the molecular diffusion of
the gas in a thin viscous water layer or film
at the sea surface. It is not clear whether the
film is stagnant or is renewed on a short
time scale. On natural water bodies the film
thickness, if the film is stagnant, or the film
renewal time, if the film is perpetually renewed, are thought to be functions of the
windspeed and of the molecular viscosity
of water. Both the molecular viscosity and
the molecular diffusivity vary with temperature.
Oxygen concentration in the surface layer
of the sea changes as a result of biological
production and consumption
and gas exchange through the sea surface. Also, entrainment of water underlying the mixed
surface layer, having a different oxygen conAcknowledgments
This work was supported by the Swedish Natural
Science Research Council (NFR). Comments on the
manuscript by Ragnar Elmgren, Lena Lundberg, and
Fredrik Wulff are hereby acknowledged. I owe a debt
of gratitude to all those who have contributed to the
ICES data bank.
centration, may change the oxygen concentration of the surface layer. The oxygen saturation concentration is a function of salinity
and temperature. Changes of the latter two
properties may therefore give rise to oxygen
exchange with the atmosphere. Changes of
atmospheric pressure give rise to proportional changes of the saturation concentration of oxygen in surface water.
It has often been suggested that air bubbles in the surface layer, created by breaking
surface waves, might be important for gas
transfer. Air bubbles increase the area of the
air-water interface. This effect should result
in an increased gas transfer per unit of horizontal area of the sea surface. The pressure
in air bubbles in the surface layer is greater
than atmospheric pressure. This disparity is
due to the hydrostatic pressure of the overlying water column and, in particular for
small bubbles, excess pressure caused by
surface tension. Excess gas pressure in the
bubbles tends to supersaturate surface water. Broecker and Peng (1982) showed that
the surface water in the open ocean typically
is supersaturated with respect to oxygen by
- 3%. They argued that this effect could not
be explained by heating of the water (without accompanying gas exchange). Plant production was estimated to account for supersaturation
of -0.5%. They therefore
444
Oxygen flux and net production
445
Notation
concluded that gas transfer by bubbles probably causes most of the supersaturation.
Air-sea oxygen flux, g 0, m-2 d-l
The piston velocity of gases through the 2,
Biological oxygen production, g O2 m-2 d- ’
Air-sea oxygen flux by bubbles, g 0, m-2
ocean-atmosphere
interface has been de- Fi,,
d-’
termined with, for example, 14Cand Rn (see Fad
Diffusive air-sea oxygen flux, g O2 m-* d-’
Broecker and Peng 1982). A formula for the . r;,diW Diffusive oxygen flux through the base of
piston velocity of unreactive gases as a functhe photic layer, g O2 m-* d-’
tion of the windspeed and the Schmidt
Total air-sea oxygen flux, g 0, m-2 d-’
Fo,
Depth of photic layer, m
H
number, mainly based on laboratory meaRate of change of oxygen storage in the
Int
surements, is given by Liss and Merlivat
photic layer, g 0, m-* d-l
(1986).
Rate of net production, g C m -2d-l
NP
For an almost enclosed sea like the Baltic,
Concentraiton of organic matter, g C me3
oc
Actual oxygen concentration, g 0, mw3
the annual net flux of oxygen through the 02
Saturation oxygen concentration, g 0, m-3
02s
sea surface should be small. If some marine
02mpc Mean oxygen concentration in the surface
organic matter is permanently buried in the
layer during production conditions, %
bottom sediments there should be a correOxygen-to-carbon ratio for production and
sponding net flux of oxygen to the atmodecomposition of organic matter, - 3.5
Salinity, psu
sphere. On the other hand, the oxidation of
Time, s
t
organic matter of nonmarine origin (e.g. imT
Temperature, “C
ported from the surrounding land) should
V
Transfer velocity, m d-’
result in a net flux of oxygen from the atW
Windspeed, m s-l
mosphere to the sea. One expects rather large x
Oxygen supersaturation factor
z
Vertical coordinate, m
fluctuations of the oxygen exchange with the
atmosphere in the course of a year in response to cycles of biological production and
consumption, of heating and cooling of the
concentration (units given in list of notasurface layer, and of entrainment of oxygention). From Eq. 1 it follows that the exdepleted water from the lower layers into
the surface mixed layer during autumn and change is counted positive when the gas flux
is from the sea to the atmosphere. The exwinter. For the Baltic Sea proper I expect
change velocity Scan be computed from a
that the annual net flux of oxygen through
formula given by Liss and Merlivat (1986):
the air-sea interface should be quite small.
If so, one can use the wealth of historical
V = 5.9 SC--%(AW + B).
(2)
hydrographical
data from this area to test
formulae for oxygen exchange with the at- Here W is windspeed. For the interval 3.6
< W < 13 the empirical constants A and
mosphere. Such a test is my subject here.
B are equal to 2.85 and -9.65. Outside this
Positive net production of organic matter
in the photic layer of the sea will result in windspeed interval A and B have other values. The formula for the exchange velocity
corresponding production of oxygen. I will
also takes into account that the Schmidt
try to infer the net production of organic
number SC (the ratio between the kinematic
matter in the photic layer during the proviscosity and the molecular diffusivity
of
duction season (spring to autumn) from
computations of monthly oxygen fluxes to oxygen) varies with temperature. From table 1 of Liss and Merlivat,
showing the
the atmosphere and storage changes of oxSchmidt numbers for oxygen at four differygen in the water column.
ent temperatures, one can derive the followMethods and data sources
ing analytical expression for SC:
Oxygen exchange with the atmosphere F,
SC = 1,450 - 71T + l.lr;!
is usually described by the expression
where T is temperature. This formula for
F0 = V(0, - 0,s).
(1) SC should be applicable in the interval 0 <
Here O2 is the actual oxygen concentration
T < 40. The fact that the power of SC in
of the surface water and 0,s is the saturation
Eq. 2 is equal to - ‘12 implies that the filmoCratio
s
446
Stigebrandt
fOO"E
1S"E
.zo"E
Fig. 1. Map of the Baltic and adjacent seas. The areas from which data came are indicated.
40-m depth; solid contour- 100-m depth.
renewal model should apply (e.g. Broecker
and Peng 1982).
From the ICES data bank covering the
period 1957-l 982 I extracted data from the
areas indicated in Fig. 1. I used data only
when salinity (s), temperature (T), and oxygen (0,) had been measured simultaneously. The concentration of oxygen at saturation was computed from the formula of
Weiss (1970). For windspeed I used the
Dotted
contour-
monthly means for the Baltic Sea given by
Stigebrandt (1985).
Test of the formula for air-sea gas
exchange
For the areas indicated in Fig. 1, I first
computed monthly oxygen fluxes through
the sea surface with Eq. 1 and 2. I excluded
the Bothnian Bay (the northern basin of the
Gulf of Bothnia), however, since there are
Oxygen flux and net production
447
no data from this area for some winter _ good since it seems to work well in labomonths. When summing monthly fluxes I ratory experiments as well as for field exfound that there are net annual fluxes of periments on gas evasion with, for example,
Rn (see Broecker and Peng 1982). There is
oxygen to the atmosphere for all areas. They
are far greater than could be explained by no direct reason to suspect that V is badly
net burial of marine organic matter. For the described. It is, however, believed that bubstrongly stratified areas (Skagerrak and Katbles may contribute to oxygen transfer, as
tegat), where waters below the photic layers
repeatedly suggested in the literature (a wellcaused
are renewed essentially by horizontal ad- known example is supersaturation
vection of water from outside areas, one by bubbles entrapped by water falls).
may, however, expect net oxygen flux to the
To discuss the contribution
of bubbles to
gas exchange, let us imagine a case where
atmosphere. The reason is that organic matter sinking below the photic layer is decomthere is no net transfer of oxygen through
posed with oxygen that comes at least in the sea surface. Thus I assume that there
part from other areas.
are no biological activities and no temperThere should be no large annual net oxature and salinity changes in the water. Due
to the excess pressure in bubbles, however,
ygen flux through the sea surface in the Balthe water can be expected to be supersatutic proper. The decomposition
of humic
substances from land should be ~4.4 g C rated by the amount x, i.e. O2 = O,s(l +
rnd2 yr-’ (Wulff and Stigebrandt 1989). Jonx). A steady state should be achieved where
son et al. (1990) found that - 10 g C m-2 the downward oxygen transport by air bubyr-* are sequestered in the sediments. They
bles Fobuis balanced by an upward diffusive
argued, however, that a major part of the transfer FOd through the air-sea interface
sequestered carbon is fossil and emanates
and we obtain
from shallow marine postglacial sediments
F Od 0= - F Obu
eroded due to uplift of the Baltic region.
= V(0, - 0,s)
Thus probably only 2-3 g C mm2 yr-’ of
(3)
= VOo,s[(l + x) - l]
recently formed marine organic C is sequestered. Oxygen fluxes through the sea
= v 0,s x.
surface due to decomposition of humic subThis condition for equilibrium
between
stances from land and sequestering of redownward
transport
by
bubbles
and
upcently formed marine organic matter thereward transport to the surface by diffusion
fore seem to almost cancel for the Baltic
of the bubble
proper. The exchange time of water in the gives a first approximation
transport
FObu,
which
is
Fob”
= - V 02S x.
Baltic proper is several decades. Using the
If x is positive, which it should be, bubble
water budget for the Baltic proper (see Wulff
transport is from the atmosphere to the sea
and Stigebrandt 1989) and realistic concenand
is independent of actual gas concentratrations, one can easily verify that the exchange of oxygen and organic matter be- tion in the surface water. I account for the
tween the Baltic proper and neighboring seas bubble-driven flux into the water by simply
adding Fobu to the diffusive flux F. deshould be negligible in the present context.
scribed by Eq. 1. The net transport F,, of
It seems well justified to adjust the formula
for oxygen flux through the sea surface in oxygen through the sea surface should then
the Baltic proper (represented by the BY 15 be described by
area, see Fig. I) so that the computed net
F ot o= Fo + Fob,,
(4)
flux during a year vanishes for this area.
= vo,s[(o,/o,s)
- 1 - X].
The apparent shortcoming of the formula
for the air-sea gas exchange may be due to For the steady, purely bubble-driven
case
poor description of the transfer velocity V. discussed above, for which O,/O,s = 1 +
The starting point for the parameterization
X, we obtain F,, = 0, i.e. there is no net flux
of V is that the gas flux through the sea of oxygen through the surface.
surface is controlled by a viscous film at the
I ran the computations
of the monthly
sea surdace. This assumption is probably
mean oxygen fluxes through the surface for
448
Stigebrandt
the BY 15 area, where I expected no annual
net oxygen flux through the sea surface, with
the expression for F,, in Eq. 4 and various
x values. It appears that the annual net flux
of oxygen through the surface is -0 if x =
0.025. Due to the bubble-driven
transport,
the effective saturation concentration of oxygen is not equal to 0,s but 1.025 x 0,s.
A net flux of oxygen to the atmosphere is
expected when the supersaturation is > 2.5%
and a net flux to the water is expected when
the supersaturation is ~2.5%. The following formula for the total (“true”) net oxygen
flux through the sea surface should thus replace Eq. 1
(5)
Fat = V 02s(02/02s - 1.025).
The transfer velocity V, adopted from Liss
and Merlivat (1986), is given by Eq. 2. The
value of the effective supersaturation as determined here can be looked upon as a climatological average for the Baltic proper
(BY 15 area). There should be differences
between summer and winter which have not
been estimated. In my computations I used
the monthly mean windspeed, which may
underestimate
the gas exchange because
there should be several days each month
with windspeeds > 13 m s-l. For such strong
winds gas transfer velocities are enhanced.
Broecker and Siems (1984) showed in a
wind-wave tank that the enhancement starts
at winds > 10 m s-l. In future refinements
of Eq. 4 the supersaturation factor x should
be a function of the concentration and size
distribution
of bubbles in the surface layer
and hence a function of windspeed (cf.
Thorpe 1984; Spitzer and Jenkins 1989).
It is satisfying that the effective supersaturation of oxygen as determined above is
in accordance with what is generally accepted (e.g. Broecker and Peng 1982).
Thorpe (1984) concluded, however, that the
presently available data on supersaturation
of oxygen in the surface ocean cannot be
explained by the flux of gas bubbles. The
generalized formula for gas exchange between the sea and the atmosphere proposed
here (Eq. 4) is equivalent to that proposed
by Smith and Jones (1985) who applied it
to the exchange of CO,. The x values for
CO, they presented, obtained over a very
short period in a surf zone, was, however,
extreme, leading to criticism by Broecker et
al. (1986). In their reply, Smith and Jones
(1986) pointed out that it is not reasonable
to compare results from relatively instantaneous measurements under changing wind
and wave conditions with those calculated
from monthly mean windspeeds.
One might wonder why the bubble effect
apparently does not appear in laboratory
experiments. The reason may be that due
to scale effects laboratory experiments fail
to simulate the bubble-driven flux correctly.
For instance, states of the water surface with
respect to waves at a certain windspeed certainly are not similar in laboratory and natural water bodies. It should be mentioned
that special arrangements have been made
in laboratories to study the dependence of
gas flow on bubbles as well as on the wave
field (e.g. Broecker and Siems 1984; Jghne
et al. 1984). In field determinations
of airsea gas exchange with gases like Rn (which
are practically absent in the atmosphere),
there should be almost no bubble effect of
the kind envisioned here, and for this kind
of gas one expects x M 0.
Computations of the net production of
organic matter in dicfferent
parts of the Baltic Sea area
The ability
to compute oxygen flux
through the sea surface with Eq. 5 can be
tested indirectly by comparing estimates of
the net biological production of oxygen in
the photic layer, essentially based on Eq. 5,
and other independent estimates from the
literature. To this end I use a one-dimensional framework to describe the oxygen
balance in the photic layer of depth H. Horizontal advection and diffusion are neglected. This approximation
seems good at least
for the Baltic proper (e.g. Stigebrandt 1985).
In some of the adjacent seas there may be
effects of horizontal transport of oxygen and
organic matter.
Oxygen is produced by the process of
photosynthesis and consumed by decomposition of organic matter (OC, measured
as carbon). The net production rate of OC
in the photic zone during timespan dt is
dOCldt. If I assume the oxygen-to-carbon
ratio for these processes to be OC&tio( - 3.5
by weight for organic matter composed ac-
Oxygen flux and net production
cording to the Redfield ratios), I obtain for
the rate of change of oxygen in the photic
layer:
H
s0
dO,/dt
dz
H
=
OGatio
dOC/dt dz
s0
- FOG - FOdie
(6)
Here F,, is the rate of oxygen flux to the
atmosphere described by Eq. 5 and FOdir,
the rate of diffusive oxygen flux through the
base of the photic layer at depth H. If we
assume that FodiR is small during the productive period of the year (FodidFot -K I),
we obtain
F Ob
=
NP
OCratio
H
dO,/dt
=
dz + F,,.
s 0
Here I have introduced
oxygen production and
(7)
Fob for biological
H
NP =
s0
dOC/dt
dz
(8)
is the rate of net production in the photic
layer. The method presented here to determine NP in the photic layer from field measurements of S, T, 02, and W, explicitly
expressed by Eq. 5 and 7, seems to be simpler than other methods currently in use (cf.
Platt et al. 1989).
If there are simultaneous
temperature
changes in the photic layer, they will change
the solubility of oxygen 0,s. Some of the
oxygen flux through the sea surface may actually be caused by this effect. Fortunately
I do not need to compensate for this nonbiological component of surface flux in Eq.
7 because the storage of oxygen in the water
column changes by exactly the same amount.
As an example, for heating conditions, the
increase of F,, due to a decreasing 0,s is
exactly compensated by a decrease in oxygen storage.
I have neglected the diffusive flux of oxygen below the photic layer (FOdiK).This flux
may be nonnegligible in some areas. In such
449
a case Eq. 7 will underestimate NP. The NP
of organic matter in the photic zone may
temporarily increase the storage of organic
matter in this zone. The rest is exported to
deeper layers. If I consider the whole production season, however, NP should be
about equal to the net amount of organic C
that sinks out of the photic layer. This conclusion is supported by Platt et al. (1989).
Due to the high content of humic (yellow)
substances, light is strongly absorbed by waters in the Baltic and adjacent seas, whereby
the depth of the photic layer H becomes
rather shallow. For the following computations, I will assume H = 15 m except in
the Bothnian Bay where I put H = 10 m.
In the computational
results (Table l),
some of the primary data-monthly
means
of S, T, O2 at the sea surface-are presented.
It should be remarked that in the Bothnian
Bay there are no data from January, February, and April. This gap is due to the extensive pack ice regularly occurring on this
basin each winter, which hinders hydrographical measurements. The monthly oxygen flux Fat, computed from Eq. 5, is given
in the 7th column and the exchange (piston)
velocity Yin the 8th column of Table 1. In
the 9th column I give the value of Int = J
dO,/dt dz, where the integration limits are
at the top and bottom of the photic layer.
To estimate the value of this integral, I use
interpolated values of 0, valid for the first
day of each month from 0- and 1O-m depth.
In the last column the biologically induced
oxygen flux Fob, defined by Eq. 7, is given.
Positive
Values
Of Fob are interpreted
as due
to positive NP in the photic layer. The
equivalent amount of carbon NP can be obtained through division of Fob by OCratio.
Negative values of Fob occur in autumn and
winter when there is no or low primary production by phytoplankton
and undersaturated water from lower layers is entrained
or mixed into the surface layer. These EOb
values are quite uncertain because the contribution of FOdiEpneglected in Eq. 7, may
be great in these seasons.
The Skagerrak and Kattegat areas
The seasonal development of the positive
NP of organic matter in the photic layer in
Stigebrandt
450
Table 1. Computations of F,,, Int, Fob (g 0, m-2 month-l), and Y (m d-l). Also given are S (psu), T (“C),
O,/O,s (%), and 0,s (g 0, m 3) at the sea surface. (Number of observations--N.)
s
z
T
op/o,s
N
02s
r;b,
V
Int
F--Ob
Skagerrak area
Jan
31.11
3.5
99.60
41
10.64
Feb
29.16
2.5
102.00
39
11.10
Mar
28.58
1.6
107.20
59
11.44
Apr
28.7 1
4.9
106.30
29
10.41
May
27.56
8.2
106.10
46
9.56
Jun
27.38
15.5
109.80
70
7.98
Jul
27.47
16.6
107.30
39
7.78
Aug
29.01
17.2
107.50
59
7.59
SeP
30.67
14.1
104.40
39
8.09
Ott
30.33
10.4
102.70
26
8.87
Nov
32.74
8.3
99.00
12
9.24
Dee
32.60
6.8
97.50
17
9.63
Net oxygen flux’ = 91.37 (=196.10-104.74) (g m-2 yr-‘)
-25.00
-3.76
37.78
24.32
20.40
41.52
27.48
29.29
13.78
1.52
-31.93
-44.06
2.70
2.26
2.34
2.05
1.98
2.38
2.45
2.57
2.99
2.86
3.29
3.05
9.6
9.5
1.5
-8.5
-7.3
-8.5
-7.5
-5.2
0.7
3.9
3.1
8.8
Kattegat area
83
11.61
114
12.14
136
12.28
139
10.72
154
9.54
159
8.53
97
7.92
147
7.86
135
8.30
187
9.08
122
9.87
72
10.43
(g m-2 yr-I)
- 24.48
0.00
52.2 1
25.45
31.24
14.87
22.5 1
24.06
6.14
-0.80
- 22.07
-25.77
2.60
2.20
2.32
2.08
2.10
2.32
2.49
2.62
3.08
2.94
3.24
2.94
11.8
9.2
-5.0
- 14.0
-13.3
- 12.4
-5.7
-1.7
4.6
7.1
7.6
11.7
9.2
47.2
11.5
17.9
2.5
16.8
22.4
10.7
6.3
-36.18
- 30.37
- 18.44
46.40
42.45
37.87
32.72
17.19
5.88
-27.25
-46.96
-57.68
2.66
2.22
2.36
1.99
1.93
2.15
2.38
2.49
3.11
2.92
3.31
3.03
12.5
5.0
4.3
-4.5
-17.1
- 17.0
-11.9
-3.5
4.7
8.0
7.6
12.7
41.9
25.4
20.8
20.8
13.7
10.6
East Gotland (BY 15) area
Jan
7..42
2.5
98.60
122
12.84
-39.52
Feb
7.,54
2.2
98.80
44
12.96
-32.18
Mar
7.51
1.2
99.30
64
13.38
-29.74
Apr
7.36
1.2
103.90
95
13.38
10.48
May
7.51
5.4
113.50
59
11.79
71.05
Jun
7.56
10.0
115.20
162
10.36
79.78
Jul
7.14
15.5
107.40
107
9.02
31.54
Aug
7’.05
16.5
105.40
107
8.82
19.31
SeP
7’.11
15.1
103.20
67
9.12
5.90
Ott
7.22
10.1
101.50
30
10.37
-8.80
Nov
Y.37
7.2
98.30
91
11.19
-45.09
Dee
7.33
5.6
96.80
24
11.73
-59.28
Net oxygen flux = 3.43 (=218.05-214.62) (g m-2 yr-‘)
2.63
2.24
2.32
1.86
1.83
2.02
2.38
2.51
3.08
2.83
3.20
2.96
8.7
4.8
7.3
1.8
-8.5
-26.6
-19.1
-3.1
7.4
10.1
6.3
10.8
12.3
62.6
53.2
12.4
16.2
13.3
1.3
Jan
24.25
2.1
99.80
Feb
20.08
1.5
102.50
Mar
19.56
1.3
108.60
Apr
21.71
5.5
106.30
May
19.16
10.4
107.70
Jun
19.35
14.8
105.00
Jul
22.35
17.1
106.30
Aug
21.01
17.8
106.40
SeP
22.5 1
15.1
103.30
Ott
22.9.9
11.3
102.40
Nov
24.78
7.7
100.20
Dee
25.98
5.4
99.70
Net oxygen flux = 103.36 (= 176.48-73.12)
Jan
Feb
Mar
APr
May
Jun
Jul
Am
SeP
act
Nov
Dee
Net oxygen
8.38
2.9
98.90
8.13
1.9
99.00
8.08
1.8
100.50
7.98
3.6
108.80
7.85
7.5
109.10
7.79
12.2
108.50
7.69
15.5
107.60
7.89
16.2
105.10
7.96
15.4
103.20
7.93
11.2
99.40
8.35
8.5
98.10
8.63
6.5
96.90
flux = -34.37 (-182.51-216.88)
Arkona (BY 1) area
54
12.60
49
13.03
54
13.05
46
12.37
61
11.09
158
9.78
118
8.99
106
8.84
51
9.01
52
10.02
69
10.74
27
11.34
(g m-2 yr-*)
5.7
39.3
15.8
13.L
33.0
20.0
24.1
14.5
5.4
451
Oxygen flux and net production
Table 1. Continued.
s
Jan
5.88
Feb
6.08
Mar
6.08
Apr
5.96
May
5.83
Jun
5.68
JUl
5.46
Aug
5.58
Sep
5.55
Ott
5.52
Nov
5.83
Dee
5.79
Net oxygen flux = -
T
o,/o,s
N
02s
FCJ,
Bothnian Sea area
0.7
97.80
126
13.72
-48.62
-0.1
98.40
11
14.06
-36.53
-0.2
96.80
14
14.08
- 53.96
0.2
102.10
10
13.91
-3.04
3.8
113.00
116
12.47
68.87
7.0
113.70
342
Il.38
71.05
14.2
106.80
180
9.42
27.77
15.0
102.30
183
9.22
-1.33
13.0
101.10
95
9.71
-11.80
6.8
98.00
110
11.48
- 39.96
5.1
96.30
197
12.04
-67.43
4.8
99.00
24
12.12
- 36.83
13 1.82 (-167.69-299.50) (g m-2 yr-l) (uncertain due to
V
Int
2.51
11.2
2.11
3.8
2.24
3.4
1.82
1.6
1.75
-6.1
1.86
-24.1
2.29
-24.5
2.41
-4.0
2.89
10.9
2.58
10.5
3.01
5.1
2.89
12.2
winter ice cover)
F Ob
62.8
47.0
3.3
Bothnian Bay area
Jan
Feb
Mar
Apr
May
Jun
Jul
Aw
Sep
Ott
Nov
Dee
Net oxygen
0”
3.74
-0.2
98.80
20
0
3.54
1.2
97.70
53
3.28
6.1
106.90
200
3.14
13.0
105.80
110
3.24
13.8
102.70
117
3.33
11.7
99.40
62
3.26
6.9
96.50
102
3.49
4.3
96.50
64
3.39
3.1
98.40
11
flux = (cannot be computed due to lack
the Skagerrak and Kattegat areas (Fig. 2)
represents averages over the period 19571982. As can be seen, the spring bloom culminates in March. Thereafter follows a period of lower NP in April and May. In June
there is a big difference between the two
z
14.32
13.73
11.87
9.84
9.66
10.17
11.60
12.50
12.97
of data)
2.24
-
-32.38
28.38
21.51
1.34
-26.38
- 54.07
-66.41
-44.10
1.64
1.81
2.21
2.32
2.79
2.59
2.95
2.76
- 12.0
-13.0
-2.6
5.1
16.4
8.5
curves with rather strong NP in the Skagerrak and little in the Kattegat. The explanation for low production in the Kattegat
in June may be that vertical stratification is
maximal and the winds are minimal so that
the supply of nutrients by entrainment of
water from below is minimal. The great production in the Skagerrak in June is harder
to explain. From July the curves again follow each other amazingly well, with a weak
maximum in August, probably due to increased winds that mix nutrients into the
surface layer. The term FodiE may perhaps
be significant during certain periods in the
shallow Kattegat where at times the water
below the pycnocline (just below the photic
layer) may be substantially depleted in oxygen.
The total annual net production in the
photic layers of the Skagerrak and Kattegat
is here computed to be 49 and 41 g C m-2
yr-‘. The figure for the Kattegat is in accord
with estimates based on oxygen consumption in deep water and is -30% of the total
30
Month
Fig. 2. Estimated distribution in time of positive
net production of organic matter in the Skagerrak and
Kattegat.
-35.60
452
Stigebrandt
Month
Fig. 3. As Fig. 2, but in the Arkona (BY 1) and
East Gotland basins (BY 15).
Month
Fig. 4. As Fig. 2, but in the Gulf of Bothnia (Bothnian Sea-B. Sea; Bothnian Bay-B. Bay).
gross annual primary production (see Rydberg et al. 1990). The computed annual net
oxygen fluxes from the sea to the atmosphere are - 100 g m-” in both the Kattegat
and Skagerrak. This result means that - 30
g C mu2 yr-’ is either permanently buried
in the bottom sediments or oxidized below
the photic layer by water from outside areas
ventilating these seas in subsurface position.
The latter explanation seems more probable.
Total net production in the BY 1 and BY
15 areas is 38 and 49 g C rnT2 yr-‘. The
figure for BY 15 is in accord with the net
production estimated by Stigebrandt and
Wulff (1987) and it is -30% ,of the total
gross annual primary production in the Baltic proper as estimated by Elmgren (1984).
The Baltic proper
The spring bloom starts later in the Arkona Basin (BY 1) and the central parts of
the Baltic proper (BY 15) than in the Skagerrak and Kattegat (cf. Fig. 2) and is maximal in April in the BY 1 area and in May
in the BY 15 area (Fig. 3). The reason for
the early spring bloom in the Skagerrak and
Kattegat is that these seas have shallow haloclines at - 15-m depth. In the Baltic Sea,
the halocline is at - 60 m in the BY 15 area
and at 30-40 m in the BY 1 area. The spring
bloom in these areas therefore cannot start
before thermal stratification
is established
(cf. Stigebrandt and Wulff 1987). According
to present computations, the spring bloom
and the annual net production at BY 15 is
somewhat larger than at BY 1. An explanation for the latter may be that episodes
of coastal upwelling during westerly winds
bring undersaturated water to the surface in
the BY 1 area. The curves for the two areas
are quite similar from about July onward.
The Gurfof Bothnia
The spring bloom occurs in May and June
in the Bothnian Sea and Bothnian Bay (Fig.
4). The curve for the sea is quite similar to
the curve for the BY 15 area until July.
Thereafter NP seems to be quite small in
both the regions of the gulf. The amplitude
of the spring bloom in the bay is quite small.
Total net production in the seti and the bay
is 32 and 7 g C rnd2 yr- l. These amounts
are 30 and 25% of the total gross annual
primary production in these two areas as
estimated by Elmgren (1984). The low total
NP in the bay is in accord with the low P
content of water in this area (cf. Wulff and
Stigebrandt 1989; Elmgren 1989).
The Bothnian Bay in particular, but also
the Bothnian Sea, receives great amounts of
riverborne,
allochthonous
humic substances. WulfF and Stigebrandt (1989) estimated that about 6 x lo5 t of C in humic
substances are destroyed (or permanently
deposited) each year in the bay. If this
amount of humic substances is oxidized,
some 2 x lo6 t of oxygen would be required.
The area of the bay is -3.5 X lo4 km2, SO
a Aux of -60 g O2 m-2 yr-’ is required to
Oxygen J%LXand net production
sustain decomposition.
For the Bothnian
Sea, the annual oxygen flux per unit of area
needed to decompose humic substances
should be about half of this amount. The
computed net annual oxygen flux from the
atmosphere to the Bothnian Sea, - 130 g
m-2 (Table l), is certainly overestimated because I have not accounted for the shielding
effect of the often extensive and long-lasting
ice cover in winter.
The absence of net production after July
in the Gulf of Bothnia may be caused by
intense decomposition of humic substances
in the surface layer, which at this time of
year is warm and probably also rich in organisms. If the figure for the decomposition
of humic substances in the bay is correct,
the importance of allochthonous
organic
substances as C and energy sources for animals and bacteria should almost equal that
of net production of organic matter by photosynthesis in the photic layer. This partitioning is in accord with an estimate by
Elmgren (1984).
Trends in oxygen supersaturation
during the period 1957-l 982
Most of the oxygen supersaturation during plant production is due to net production in the photic layer, and from Table 1
it should be clear that there is a high correlation between the intensity of net production and the magnitude of oxygen supersaturation.
One may then get rough
estimates of trends in net production from
trends in oxygen supersaturation. Thus using least-squares analysis I fitted straight
lines 0,mpc = a + b (yr-1957)
to all observations of O,/O,s > 102.5 (%). 0,mpc
should be read “mean oxygen concentration
in the surface layer during production conditions.” The trend is described by the regression coefficient b. The resulting values
of a and b and their standard errors (a) for
the areas defined in Fig. 1 are:
Skagerrak
0,mpc = 108.9zk 1.0 - (0.091 -t-0.039)
(yr- 1957)
Kattegat
0,mpc = 107.3 11 .O - (0.020+0.04 1)
(yr- 1957)
453
Arkona (BY 1)
0,mpc = 105.9-10.9 + (0.207_+0.038)
(yr- 1957)
Central Baltic (BY 15)
02mw = 108.4a 1.7 + (0.197+0.067)
(yr- 1957)
Bothnian Sea
O,mpc = 109.4+1.0 + (0.139LO.038)
(yr- 1957)
Bothnian Bay
02mw = 106.6kO.8 + (0.025+0.031)
(yr- 1957).
The purpose of this trend analysis is to
demonstrate more examples of the potential
hidden in the historic oxygen data and to
get a first indication of the trend in net production in the various areas.
The result of the analysis above suggests
that net production in the Baltic proper and
the Bothnian Sea has increased dramatically
(roughly doubling) in the 25-yr period considered. This increase agrees with model results for the development of nutrient (P and
N) concentrations in the Baltic proper as
computed by Wulff and Stigebrandt (1989)
and with estimates by Elmgren (1989). Net
production in the photic layer of the Skagerrak, however, seems to have decreased.
This trend appears to agree with observations of plankton which show a decreasing
trend in the northeastern North Sea (cf.
Dickson et al. 1988). For the Kattegat and
Bothnian Bay the computed (small) trends
are not statistically significant.
The amplitudes and patterns of net production in the photic layer in the Baltic and
adjacent seas as computed here seem realistic. Annual net production as determined
here is -30% of the total annual gross primary production as given by Elmgren (1984)
and, for the Kattegat, by Rydberg et al.
(1990). For the Baltic Sea the long time scale
f-ratio <J, (cf. Platt et al. 1989) equals 0.3.
According to Parsons et al. (1977), this value is what one should expect for a thermocline depth of - 15 m, which applies to
the Baltic and adjacent seas. The realistic
prediction of NP is indirect evidence for the
goodness of Eq. 5 for conditions of supersaturation in the surface layer. If there are
enough data, it should be possible to use
this method, as explicitly expressed by Eq.
454
Stigebrandt
5 and 7, to estimate the distribution
of net
production in the photic layer in areas of
the world ocean where production has a seasonal behavior, i.e. in cold and temperate
regions. In areas with good time series it
will also be possible to estimate trends in
net production.
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Submitted: 16 April 1990
Accepted: 10 January 1991
Revised: 6 February 1991