ICES Journal of Marine Science, 53: 589–595. 1996
Assessment of eutrophication in Skagerrak coastal waters using
oxygen consumption in fjordic basins
Jan Aure, Didrik Danielssen, and Roald Sætre
Aure, J., Danielssen, D., and Sætre, R. 1996. Assessment of eutrophication in
Skagerrak coastal waters using oxygen consumption in fjordic basins. – ICES Journal
of Marine Science, 53: 589–595.
The mean rates of oxygen consumption have been estimated for 9 different fjordic sill
basins in 1990 on the Norwegian Skagerrak coast. The mean oxygen consumption rate
was about 50% higher than in similar basins on the west coast of Norway. The
observations are in accordance with an empirical model from the Norwegian west
coast, where mean oxygen consumption rate in sill basins is derived as a function of sill
depth and mean basin depth. Historical oxygen observations suggest that increased
oxygen consumption occurred in the first part of the 1980s. Due to intensive water
exchange above sill level and insignificant influence of local inputs of anthropogenic
nutrients, increased oxygen consumption in the sill basins is most likely to be related
to increased large scale eutrophication of the Skagerrak coastal waters. The natural
low oxygen concentrations in many sill basins along the Skagerrak coast make them
particular sensitive to increased oxygen consumption.
? 1996 International Council for the Exploration of the Sea
Key words: coastal eutrophication, fjordic sill basins, oxygen.
Received 22 December 1994; accepted 27 July 1995.
J. Aure and R. Sætre: Institute of Marine Research, PO Box 1870 N-5024 Bergen,
Norway. D. Danielssen: Institute of Marine Research, Flødevigen Marine Research
Station, N-4817 His, Norway.
Introduction
Input of anthropogenic nutrients and eutrophication of
coastal areas were identified as issues of concern in the
Quality Status Report for the North Sea presented at the
ministerial conference in 1987 (Anon., 1987). Several
authors have later contributed to the discussion on these
matters and more recently e.g. Skjoldal (1993). There
is growing concern about an apparent increase in
eutrophication in the eastern part of the Skagerrak and
in the Kattegat. During the last 15–20 years, reduced
growth depth of macroalgae and an increasing biomass
and change in species composition have been reported in
benthic communities (Anon., 1993). Reduced oxygen
concentrations and increased oxygen consumption in the
basin water of some Swedish fjords (Rosenberg, 1990)
and in the Kattegat deep water (Andersson and
Rydberg, 1988, 1993) have been observed during the
same period. Increased organic load in the upper layer
due to eutrophication, and thereby an increase in the
vertical flux of organic matter, has been suggested as the
main driving mechanism for these changes (Anon.,
1993). The role of input from local sources as compared
to the long-distant transport of nutrients and organic
1054–3139/96/030589+07 $18.00/0
matter from the southern North Sea and Kattegat/Baltic
Sea is not clear.
In order to assess the effects of the increase in the
riverine nutrient load on the North Sea, there has been a
tendency to focus on nutrient concentrations in the sea.
The historical nutrient data have been examined in
search of a temporal trend. This search has focused on
winter data to avoid the effect of biological production.
For monitoring purposes we should be looking for a
more direct connection between nutrient input and
eutrophication effects. If the increased input of nutrient
to the North Sea results in increased new production,
there should also be an increase in sedimentation of
biogenic material in deposition sites. Such sediment
accumulation sites for the North Sea are found, among
others, in the Skagerrak. Consequently, it may be
appropriate to look for evidence of an increased supply
of organic matter to these areas.
There are several methods to determine large scale
eutrophication, but in this work we will try to use
coastal, near-fjordic sill basins as ‘‘sediment traps’’ and
use oxygen budgets to calculate the of supply of organic
matter other than that permanently buried and the
exported fractions. One great advantage of using oxygen
? 1996 International Council for the Exploration of the Sea
590
J. Aure et al.
budgets is that oxygen is measured using a standard
method with great accuracy and at low cost. In addition,
there are much more useful historical data for the
application of oxygen methods than for the application
of other methods. Thus, historical data usually permit
the establishment of better averages with higher spatial
resolution. In many areas, trends for the last several
decades may be established.
We assume that in fjords with extensive water
exchange above the sill levels the concentrations of
organic matter at the sill level will be approximately the
same as in coastal water (Aure and Stigebrandt, 1989).
The combination of low sinking velocity for particulate
organic matter, as compared to advection and short
residence time for water above sill level, creates small
effects in sill basins of local primary production. Most
particles will probably be transported out of the fjord
before having sunk to the sill level.
Sill basin water, with its content of different substances, is exchanged both by advective and by diffusive
transport processes. Advective exchange occurs when
sufficiently dense water appears outside and above the
sill. If dense water is present in this position for a
sufficiently long period, water exchange may be quite
extensive and much of the old basin water can be flushed
out of the basin. The inflowing ‘‘new’’ water has a high
content of dissolved oxygen. However, exchange of
basin water may also involve smaller volumes and
partial exchanges. Due to turbulence, the water in a sill
basin is mixed with less dense water from higher levels.
Diffusive vertical exchange is supposed to be a continuous process and implies a rate of decrease in the density
of the basin water which, in turn, is a prerequisite for
future advective water exchange. During stagnant periods, i.e. periods when no advective water exchange takes
place in the sill basin, the biochemical decomposition of
organic matter is usually the main process changing
oxygen and nutrient concentrations of basin water.
However, vertical diffusive fluxes may also contribute.
In this study, we present estimates of volume mean
rates of oxygen consumption of nine selected fjordic sill
basins on the Norwegian Skagerrak coast during a stagnation period in 1990. From this we compute the associated supply of organic matter to the basins. The results
will be compared to a similar investigation on the
Norwegian west coast in 1986 (Aure and Stigebrandt,
1989). Historical oxygen data from some of the sill
basins will be examined to look at long term oxygen
trends and, finally, the effect of changed oxygen consumption on the oxygen minimum level will be discussed.
Materials, method, and area description
From April to December in 1990, temperature, salinity,
oxygen, and inorganic nutrients were observed from
surface to maximum depth by monthly cruises in nine
selected fjordic sill basins along the Norwegian
Skagerrak coast (Fig. 1). Profiles of oxygen, temperature, and salinity have been observed in Østerfjord (Stn
R2) in late autumn since the late 1920s. In the coastal sill
basin, Stn Æ1, oxygen, profiles of oxygen, temperature,
and salinity were regularly recorded during 1975–1979
and after 1990. From 1987, temperature and salinity
were observed by a CTD-sonde (Neill Brown) and from
1975–1987 by high precision thermometer and salinometer (Guildline instrument). Before 1975, salinity was
analysed by titration and temperature by high precision
thermometer. Accuracy of both temperature and salinity
was about 0.01 units. All oxygen values since 1925 have
been determined by the Winkler method.
The investigated basins had sill depths between
12–30 m, the mean basin depths varied between
10–57 m, and surface area between 1.5–20 km2 (Table
1). The sills were located 0–15 km from the coast. The
freshwater supply was greatest in fjord S1–2, with 2.3 m3
sec "1, km "2 and smallest in fjord T3, with 0.2 m3 sec "1
km "2. The input of anthropogenic nutrients to the
fjords was small and was not considered to have any
local effects (Baalsrud et al., 1991). The coastal basin,
Stn Æ1, had a sill depth of 50–60 m and a mean basin
depth of about 30 m (Fig. 1).
Results and discussion
The mean rate of oxygen consumption in 1990
Organic matter, produced locally in the euphotic zone
and imported from the coast, sinks down into the basin
water (Fig. 2). Some is broken down in the water column
and excess organic matter settles on the bottom. There
it is consumed by the benthic community, respired by
micro-organisms or buried permanently in the sediments. For stationary conditions, the volume mean rate
of oxygen consumption in the basin water, including
oxygen consumption in the water column and sediment,
should correspond to the mean rate of supply of organic
matter, minus the buried and exported fractions. Export
is, for example, by fish which feed but do not decompose
in the basin water and re-suspended organic matter
advected out of the basin during inflow periods. During
stagnation periods in the basin, the overall oxygen
budget for the basin is
Where O2 =O2 (z,t) is the oxygen concentration at level z
and time t. The first term on the left side of Equation (1)
is the rate of depletion of oxygen stored in the basin
water, where dO2/dt is the local rate of decrease of
dissolved oxygen. A(z) is the horizontal surface area of
the basin at level z and zs (b) is the vertical coordinate
Eutrophication assessment of coastal waters
N
R3
R4
591
R2
R5
S2 RISOR
0
5
10 Km
R1
S1
58°40'
L1
TVEDESTRAND
T3
T2
2°
T4
64°
Norwegian
Sea
30'
A1
T1
More
and
Romsdal
62°
ARENDAL
Torungen
60°
SW
ST
AD
12°
5°
IM
GR
8°40'
50'
9°
North
Sea
10'
EN
58°
ED
Æ1
Skagerrak Kattegat
9°20'
58°20' N
W
Figure 1. Location of sampling stations at the Skagerrak coast and the Møre and Romsdal region.
Table 1. Sill depth (Ht), mean basin depth (Hb), surface area
(Af), and fresh water supply (Qf) of the sill basin fjords (Fig. 1
for location).
Sill
basin
Ht
(m)
Hb
(m)
Af
(km2)
Qf
(m3 s "1 km "2)
R2-3
R5
R4
S1-2
L1
A1
T4
T2
T3
28
20
27
30
24
22
12
30
15
57
12
21
15
16
10
13
12
18
20.0
1.3
6.6
4.4
2.1
2.0
1.8
3.3
2.4
1.0
0.1
0.1
2.3
0.4
0.4
0.4
0.2
The terms to the left of the equality sign in Equation
(1) are evaluated for stagnant conditions and there
should be no advective exchange of basin water in the
selected period of oxygen observations. Utilizing the
hydrographic and oxygen data it would seem that there
were well-defined stagnation periods in the observed sill
basins during summer and autumn 1990. The results
may be written Vb · DEPL where DEPL is the average
(volume mean) rate of oxygen depletion. Conditions just
below sill level usually change a great deal due to
occasional inflows. The upper integration level was
therefore chosen 5–10 m below the sill level. The volume
mean oxygen consumption rate, CONS, is then:
CONS=DEPL+DIFF
of the upper (lower) boundary of the basin water. The
second term on the left side of Equation (1) is the
vertical diffusive oxygen flux into the basin water, where
As is the horizontal area, Ks is the eddy diffusivity, and
(dO2/dz)s is the vertical gradient of dissolved oxygen at
the upper integration level. The oxygen consumption
rate is equal to the sum of the two terms on the left side
of Equation (1). On the right side of the equation,
CONS is the volume mean oxygen consumption rate
and Hb is the mean depth defined by Vb=A(s) Hb,
where Vb is the volume of the basin water.
(ml l "1 month "1)
(2)
DIFF is the contribution from the diffusive flux of
oxygen into the basin water (where DIFF can be written
DIFF=Ks (dO2/dz)s/Hb) and can be estimated from our
data provided the vertical diffusivity (Ks) and the vertical oxygen gradient (dO2/dt)s close to sill level are
known. Ks was determined using the density decrease
during stagnation periods, applying an equation for
density analogous to Equation (1). The density decrease
during stagnation periods is assumed to be due solely to
vertical diffusion. DIFF is estimated to be lesser than
10% of CONS.
592
J. Aure et al.
Qf
Coast
Fjord
Brackish water
Fc (fjord)
Coastal
water
Intermediate water
Fc (coast)
Fc (coast)
Sill depth
(Ht)
Stagnant basin water
Oxygen consumption
Figure 2. Schematic diagram of circulation and fluxes of organic matter in a fjord with extensive water exchange above the
sill depth.
Table 2. The observed volume mean oxygen consumption (CONSobs) and calculated by Equation (3) (CONSEq3). Ratio between
CONSobs and CONSEq3 (Cobs/CEq3).
Sill
basin
CONSobs
(ml l "1 month "1)
CONSEq3
(ml l "1 month "1)
Cobs/CEq3
R2-3
R-5
R4
S1-2
L1
A1
T4
T2
T3
0.21
1.15
0.63
0.78
1.0
1.35
1.28
1.0
0.94
0.15
0.81
0.4
0.53
0.56
0.93
0.85
0.66
0.58
1.4
1.42
1.57
1.47
1.78
1.45
1.51
1.51
1.62
In Table 2 we have listed the observed volume mean
oxygen consumption rates during summer and autumn
1990, after correction by DIFF. There are several
possible error sources influencing our determination of
the mean oxygen consumption. The hypsographic function may have some error. Water samples may be taken
from slightly erroneous depths due to ship drift, and
internal waves may distort the property fields at the
point of sampling. The analyses of water samples have
errors and small, undetected advective exchanges of
basin water which violate our requirement of stagnation
may have occurred. A rough estimate of the magnitude
of error in CONS indicates that it may be of the order
of 15%.
An empirical model developed by Aure and
Stigebrandt (1989), based on observations from 30 different sill basins of fjords located at the Norwegian west
coast (Møre and Romsdal), gives the relationship
between the volume mean oxygen consumption rate
(CONS) in sill basins, the supply of organic matter
minus the buried and exported fractions (Fc), and the
mean basin depth (Hb).
Fc decreases with increasing sill depth (Ht) and the
relationship was shown to be approximated by:
Fc =(a"b)Ht
(g C m "2 month "1)
(4)
a=5.4 (g C m "2 month "1) and b=0.07 (g C m "2
month "1 m "1)
(Ht<50 m)
ì=2.43 [ml O2 g "1 C] is the conversion factor, ì, for
complete oxidation of organic matter composed according to the Redfield ratios. In low oxygen water, denitrification may reduce the effective value of ì. All
calculations of oxygen consumption is based on oxygen
concentrations above 2 ml l "1 to avoid effects of
denitrification.
Figure 3a shows a high linear correlation between the
observed versus the calculated mean oxygen consumption rate given by Equation (3), but the mean oxygen
consumptions in the sill basins along the Skagerrak
coast appeared to be about 1.5 higher (S.D.=0.11) than
at the Norwegian west coast (Table 2). The observed Fc
(a)
1.4
1.2
1.0
0.8
R = 0.98
a = 0.05
b = 1.43
0.6
0.4
0.2
0.0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
CONS (Eq 3) (ml l–1 month–1)
593
0.22
0.20
Stn R2
100 m
0.18
0.16
0.14
0.12
0.10
0.08
1930 1940 1950 1960 1970 1980 1990
Year
Figure 4. Oxygen consumption rate (dO2/dt) at 100 m depth in
the sill basin R2 (Fig. 1) from 1930–1994.
8
(b)
Fc (g C m–2 month–1)
Oxygen consumption (ml–1 month–1)
CONS observed (ml l–1 month–1)
Eutrophication assessment of coastal waters
R = 0.88
a = 8.47
b = 0.11
7
C m "2 yr "1 at the Norwegian west coast and PT =187 g
C m "2 yr "1 at the Skagerrak coast. These calculations
indicate that the observed difference in PE could be
explained by PT which is some 40% higher at the
Skagerrak coast.
6
5
Long term variations
4
3
0
5
10
15
20
25
Ht (m)
30
35
40
Figure 3. (a) Observed volume mean oxygen consumption rate
(dO2/dt) in the fjordic sill basins at the Norwegian Skagerrak
coast plotted against volume mean oxygen consumption rate
computed from Equation (3); (b) Fc in the fjordic sill basins at
the Norwegian Skagerrak coast plotted against sill depth (Ht).
values, as at the Norwegian west coast (Equation 4),
decrease linearly with increasing sill depth (Fig. 3b). The
empirical constants a and b in Equation (4) for the
Skagerrak coast in 1990 were, respectively, 8.5 (g C m "2
month "1) and 0.1 (g C m "2 month "1 m "1).
The new or export production (PE) is often defined as
the weight per unit surface area of particulate organic
matter sinking out of the photic zone during the production season. Wassman (1990) showed that the flux of
carbon into the aphotic zone in the boreal coastal zone
increases with increasing total primary production (PT)
by the power of 1.41:
PE =0.049 PT1.41
PT =8.49 PE0.71
(g C m "2 yr "1), or
(g C m "2 yr "1)
(5a)
(5b)
By using Equation (4) and if, for instance, the depth of
the photic zone (H=Zp) is 20 m, PE in the observed
fjordic basins at the Norwegian west coast and
Skagerrak coast is calculated to be, respectively, 48 and
78 g C m "2 yr "1. Equation (5b) gives PT =132 g
In Østerfjord (Stn R2–3) oxygen, temperature, and salinity have been observed every autumn since the late
1920s. The basin water at Stn R2–3 normally has a
stagnation period of several years (Aure and Danielssen,
1993) and yearly observations of oxygen are often
sufficient to compute the oxygen consumption rate in the
basin water. The sill of this basin is situated close to the
coast (Fig. 1). The oxygen consumption at 100 m depth
in the period from 1930 to about 1980, 70 m below sill
depth, had a mean value of 0.13 ml l "1 month "1 and a
standard deviation of 0.02 (Fig. 4). After 1980, an
abrupt 50–60% increase in the oxygen consumption rate
to about 0.21 ml l "1 month "1 was observed. A similar
50% increase in the oxygen consumption rates seems to
have occurred after 1979 in the coastal basin Ærøydypet
(Stn Æ1) (Fig. 1). The mean oxygen consumption rate at
100 m depth, about 50 m below sill depth, increased
from 0.31 ml l "1 month "1 between 1975 and 1979 to
about 0.46 ml l "1 month "1 after 1990 (no observations
were available in the 1980s). The calculated new production of 78 g C m "2 yr "1 in 1990 was also about 50%
higher than the average new production of 50 g C m "2
yr "1 in the inner Skagerrak for the period 1957–1982, as
presented by Stigebrandt (1991).
In German Bight waters the nitrate concentrations
increased by about 100% during the 1980s compared to
the long term average of the decade before. Phosphate
concentrations increased in the sixties and seventies,
plateauing to about twice their former level, and then,
contrary to the nitrate pattern, decreased after about
1982 (Hickel et al., 1993). Due to the general cyclonic
J. Aure et al.
nature of water circulation in the North Sea, much of
the anthropogenic nutrients in the German Bight are
transported into the Skagerrak and Kattegat by the
Jutland coastal current.
Estimates of the annual transport of total anthropogenic nitrogen into Skagerrak from the southern North
Sea, Baltic Sea, Kattegat, and local sources are of the
order of 600–700 000 tonnes (Anon., 1993). In relation
to the estimated natural nitrogen transport of about
1.7 million tonnes yr "1 in the upper 50 m along the
Skagerrak coast, anthropogenic input has increased
annual nitrogen transport by about 40%.
According to Andersson and Rydberg (1993), an
increase in oxygen consumption greater than 100% has
occurred in the Kattegat deep water since 1971 and
has reduced its mean oxygen concentration during late
summer from approximately 4.5 ml l "1 in 1971 to about
3 ml l "1 in the period 1985–1990. Reduced annual
minimum oxygen concentrations and increased oxygen
consumptions have also been observed in the basin
water of some Swedish fjords in Skagerrak (Rosenberg,
1990). Increased large scale eutrophication was
suggested as the reason for these changes.
The significant influence of mean basin depths and sill
depths on the volume mean rate of oxygen consumption
(and Fc), and the general 50% increase in oxygen consumption in the observed fjordic sill basins after 1980,
indicate that the volume mean rate of oxygen consumption in the basins was dominated by the trophic state of
the Skagerrak coastal water. There may be several
reasons for the relatively small influence of local factors
on oxygen consumption in the basin waters. The main
reasons are the short distance from the coast to the sills
(Fig. 1), the relatively small surface areas and volumes
above sill level (Table 1), and an intensive water
exchange between the fjords and the coastal area,
enhanced by intermediate water exchange mainly driven
by large fluctuations in the density field along the
Skagerrak coast (Stigebrandt, 1990). These factors result
in a short residence-time of water above sill levels which
tend to make conditions above the sill levels similar to
those outside the coastal water. The influence of anthropogenic nutrients in the observed fjords is too small to
cause any significant local effects (Baalsrud et al., 1991).
The increase in eutrophication in the Kattegat and in the
inner Skagerrak, which can be related to an increase in
anthropogenic nutrient input (Anon., 1993), seems to
have also increased oxygen consumption and transport
of organic matter into the fjordic sill basins along the
Norwegian Skagerrak coast. Calculations indicate that
this increase could be explained by a 40% increase in
total primary production in the Skagerrak coastal zone.
The calculated 40% increase in primary production is in
accordance with the estimated increase of nutrient transport along the Skagerrak coast due to anthropogenic
input.
Reduction in oxygen minimum (ml l–1)
594
0.0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Oxygen minimum (ml l–1)
Figure 5. Reduction in the oxygen minimum concentration
(DO2min) as a function of the original oxygen minimum concentration, with 50% increased oxygen consumption and
O2i =7 ml/l.
Effect of increased oxygen consumption on
minimum oxygen concentration
The minimum oxygen concentration attained in a sill
basin, O2min, depends on the oxygen concentration, O2i,
of the renewed basin water, the time scale for basin
water exchange, Te, and the time required to reduce the
oxygen concentration to zero in the basin water, To.
According to Aure and Stigebrandt (1990) O2min is given
by:
Where dO2/dt is the mean oxygen consumption rate
(Equation 3).
From Equation (6a) it follows that when To >Te,
O2min will be greater than zero and when To <Te the
oxygen content in the basin water will at times be
exhausted and hydrogen sulphide (H2S) will appear.
Using the observed 50% increase in mean oxygen
consumption, the reduction in minimum oxygen
consumption (DO2min) is expressed as:
DO2min = "0.5 (O2i "O2min)
"1
(ml l "1)
(7)
where O2i (~7 ml l ) is the oxygen concentration of
the renewed basin water and O2min is the originally
minimum oxygen concentration. Equation (7) and
Figure 5 show that the decrease in the minimum oxygen
concentration is greatest in sill basins with naturally low
minimum oxygen concentrations. Such natural low
levels in many of the sill basins along the Skagerrak
coast make them very sensitive to increased
oxygen consumption. After a 50% increase in oxygen
Eutrophication assessment of coastal waters
consumption, sill basins with minimum oxygen concentrations originally between 3.6 and 2.0 ml l "1 will reach
minimum oxygen concentrations below about 2.0 ml
l "1, which is thought to be the lower tolerance limit for
fish and benthic infaunal species.
Conclusions
The volume mean rate of oxygen consumption of
nine coastal close fjordic sill basins at the Norwegian
Skagerrak coast in 1990 was about 50% higher than
in similar basins along the Norwegian west coast. This
indicates approximately a 40% higher level of total
primary production at the Skagerrak coast.
The observations are in accordance with an empirical
model developed for the Norwegian west coast, where
the volume mean oxygen consumption rate in sill basins
is derived as a function of sill depth and mean basin
depth.
The dependence on topographical factors and the
general 50% increase in the level of oxygen consumption
indicate that the rate of oxygen consumption in the
observed sill basins is determined essentially by the
trophic state of the Skagerrak coastal water. The short
residence time of water above sill level in the observed
fjords, due to intensive water exchange between the
fjords and the Skagerrak coast, and small influence of
local anthropogenic nutrients, support this assumption.
Long-term observations of oxygen in sill basins
bordering the Skagerrak coastal water suggest that
increased oxygen consumption occurred during the first
part of the 1980s and seems to have been related to
increased large scale eutrophication in the Kattegat and
the inner Skagerrak during this period.
Reduction in minimum oxygen concentrations due to
increased oxygen consumption is shown to be greatest in
sill basins with naturally low minimum oxygen concentrations. The naturally low concentrations in many of
the sill basins along the Skagerrak coast make them
particularly sensitive to the observed increase in oxygen
consumption.
595
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