C.M.1993/C:58
SESS.V
THE CHANGES OF THE HYDROGRAPHIC CONDITIONS IN
THE BALTIC PROPER DUE TO 1993 MAJOR INFLOW TO THE
BALTIC SEA
by
Hans Dahlin"', Stig Fonselius# and Björn Sjöberg#
•
"Swedish Meteorological and Hydrological Institute, 5-60176 Norrköping
#Swedish Meteorological and Hydrological Institute, Oceanographical Laboratory,
5-40314, Göteborg
Abstract
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The changes in salinity in the Arkona basin, the Bornholm basin and
the Eastern Gotland basin caused by the 1993 major salt water inflow
to the Baltic sea were studied during several expeditions with the
R/V Argos between March and August and for shorter periods also
with current meter moorings. It was found that the deep water in the
Bornholm basin was exchanged in the beginning of March and tha t
the old Bornholm Deep Water was flowing over the Slupsk Furrow
into the large Baltic central basin. The water arrived into the Gotland
Deep at the beginning of April. The average velocity of the inflowing
water was estimated to be 9 cm/s. The changes in the Eastern Gotland
basin were studied by help of diagrams showing the variations of
salinity, oxygen/hydrogen sulphide, nitrate and phosphate in the
water body at the Gotland Deep [rom 1990 to 1993. It was conc1uded
that the inflow was not large enough to improve the oxygen
conditions in the bottom water of the Easter Gotland basin for a
longer time. The conditions in the southern Baltic proper have
improved considerably. Opportunities for new inflows are good due
to the still extremely low salinity of the bottom water. Because most
of the hydrogen sulphide has now disappeared, such inflows will
have good effects in the whole Baltic proper.
II
The salt water inflow in Ianuary 1993
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In January 1993 a major inflow of salt wJter was recorded in the
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Oresund and in the Belt sea. The temperature of the inflowing salt
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water was below 5 C in contrast to the i'nflow in 1976-77 which
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brought unusually warm water into the Gotland Deep (>7<'C). The
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size of the iriflow was estimated to be about 300 km3 (Matthäus 1993),
1
(Häkansson et al. 1993). In the Gotland D~ep the first signs of the
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inflow were observed the 1 of April. Thus it took a little more than 2
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,
months for the water to reach the Gotland 1,Deep.. If we estimate the
distance between the Darss sill and tne Gotland Deep to be
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approximately 600 km going through the deepest connections, and
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estimate the time for the salt water to flow this
, way to be 75 days (l5th
January to 1st April) we get a mean wateT velocity of 9 anis which
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corresponds to about 1/2 -1/3 of the velocity of a gravity current.
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The effects in the Arkona basin and in the Bornholm basin
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The "Argos" carried out an expedition in the southwestern Baltk sea
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the 2nd-5th of March in order to study the inflow in the Arkona
basin and the Bornholm basin (Fig. 1). Ais an, effect of the inflow new
water had penetrated into the Arkona and ,~ornholm basins (Fig. 2).
Remnants of the "old" bottom water in the Bornholm basin had been
lifted up to the sill level (around 60 m) and, was streaming in to the
large central basin through the Slupsk [urrow. The bottom salinities
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were the highest recorded since the inflow in 1976. The amount of
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"new" water in the Arkona basin was estimated to be approximately
25 km3, with an average salinity of 22 psuJ. The average salinity in
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,
,
the bottom water of the Bornholm basiri (below
65 m) was ci little
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above 18 psu and consisted partIy of "oId" water (14-15 psu) and
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"new" Arkona water (22-23 psu). This means that approximately 40I
60 km3 water had flown in to the BornhoImjbasin since the inflow in
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January. Figs 3a and 3b show the salinity and temperature variations
in the Bornholm basin in a section through the central part towards
SE. We can see ho\v water with high salinity is flowiilg over the
Slupsk sill (3a). The temperature of this water(3b) is in the eastern
part above 6-7
°c,
iildicating that this fs "old" Bornholm basin
bottom water, which is forced over the sill into the central basin. The
"new" water had a temperature below 5 oe. Tltis water is found at the
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bottom of the basin below the old water.
To be able to follow the new water filling the Bornholm Basin a
transmitting buoy with salinity and temperature sensors on 50, 60, 70,
80 and 85 meters depth was deployed in the central Bornholm Basin
(BY 5), fig. 4. However due to misjudgment of the velocity of the
entering water, which was very high compared to earlier estimates
and modelling calculations most of the bottom water was already
exchanged when the buoy was on site. Fig. 5 shows the first week of
salinity measurements, which deinonstrate the new bottom water,
the old bottom water around 70 meters, the old bottom water passing
the depth of 60 meters and the low saline surface layer above.
April 21 three current meter moorings were placed in a cross section
in the Stolpe Furrow. Each mooring had two Aanderaa RCM-7
current meters near and 5 meters above the bottom. The moorings
were recovered in the beginning of September. Registrations of
current, salinity and temperature from the period before the first
change of memory are shown in fig. 6 and 7. The figures show that
still during May high saline water, 12 - 14 psu was transported east
through Stolpe Furrow.
The effects in' the Eastern Gotland basin
Fig. 8 shows the salinity viuiations below 225 m in the Gotland Deep
in the Eastern Gotland basin from 1963 to 1993. The salt water inflows
in 1969, 1977, 1983 and 1993 can be seen. The 1983 inflow was not large
enough to renew the old bottom water with oxygen containing water.
It can also be seen that in spite of these inflows, the salinity of the
4
t
boüom water 1s decreasing.
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Fig. 9a shows the salinity variations at the Jtation BY15, the Gotland
Deep, from January 1990 to August \1993 and Hg. 9b the
oxygen/hydrogen sulphide variations during the same period. The
first signs of ihe inflow were observed duririg the cruise of "Professor
, ! '
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Albrecht Penck" from Warnemünde the ls,t of April (Penck 1993a).
The salinity at 236 m was reported to be 11.6 psu and thin layers with
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oxygen were observed in the hydrogen sulphide contairiing bottom
water. The "Argos" visited the Gotland Deep tWo weeks earlier, the 15
,
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of March and no signs of the inflow could be observed at that
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occasion. The salinity at 225 m was only 11 psu and the hydrogen
sulphide values were extremely high in the stagnant water. Five
weeks later, the 21 of April, when "Argos"I returned to the Gotland
Deep the bottom saÜnity had increased to 11'.67, but there was still no
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signs of oxygen below 100 m. depth. The reason was probably that we
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took bottle sainples and did not use a continuous oxygen recorder.
The hydrogen sulphide concentration wak very low dose to the
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bottom and a maximum was found at 200 ;n, 4.8 mlll; expressed as
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negative oxygen (Fonselius 1969). The next "Argos" expedition 'was
the 4th of June. The bottom salinity was~ li.70 and all hydrogen
sulphide
had disappeared from the water. Oxygeri was present in the
.
whole water column; In the deep water more than 1 mllI oxygen was
found from 175 m down to the bottom. BetJeen 140 m and 160 m the
oxygen values were below 1 inl/l indicating that the new water had
penetrated below the olcl water lifting itj up. The R/V Professor
Albrecht Perü:k reported still Hydrogen stilp~ide betWeen 130 and 195
m (Penck 1993b). We can also see that the s~liriity had incrE?asecl to 11
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at 175 m. Fig. 10 shows a map of the hydrogen sulphide distribution
in the Gotland basin. We can see that hydrogen stilphide is presEmt in
some parts of tne Easterri Gotland Basin a~d especially in the Färö
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Deep north of the' Gotland Deep. In July the saliriit}' in the bottom
water had decreased to 11.60 and the ox'ygen coricentration had
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decreased to values below 1 ml/i
at all depths
below 125 m. Lo\...
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concentfations of hydrogen stilphide were found at 150 m and 175 m
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(Fig. 11). The "Argos" again visited the ~otlarid Deep the 11 oE
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August. The salinity of the deep water was almost unchanged; the
oxygen values of the deep water had decreased and the hydrogen
sulphide values at 150 m and 175 m had increased. Obviously the
inflow was not large enough to improve the oxygen conditions for a
Ionger time.
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Nutrient concentrations are also good indicators of salt water inflows.
Stagnant hydrogen sulphide containing water does not contain
nitrate and has high concentrations of phosphate. Nitrate and nitrite
are in a reducing environment transformed into inorganic nitrogen
gas and ammonia. Phosphate, which in oxygen containing water is
coprecipitated together with ferri hydroxide, is in hydrogen sulphide
containing water again dissolved, when the ferri-hydroxide is
reduced to ferro ions and sulphide. When oxygen again appears in
the water we immediately find nitrate and the phosphate
concentration decreases.
Figs 12a and 12b show the nitrate and phosphate variations from
1990-1993. Since we have not seen the nutrient results of the
"Penck", these are not included in the figures. From the nitrate
diagram (Ba) we can see that nitrate appeared in the Gotland Deep
bottom water in June and that the values had increased in July and
August. In July only traces of nitrate was found in the hydrogen
sulphide containing water at 150 and 175 m. In August the nitrate
had completely disappeared at these depths. The phosphate diagram
(Bb) shows a phosphate maximum of 6 J.1M in the old water at 200 m
in April. In June and July all values in the deep water were low with
minimivalues around 200 m. In July and August a maximum could
be found around 150-175 m.
Conclusions
From these findings we may draw the conclusion that the inflow in
January was not large enough to improve the conditions in the
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bottom water of the Eastern Gotland Basin i for a longer time due to
the large amounts of hydrogen sulphide present there, which react
with the oxygen. Very soon we will again f~nd hydrogen sulphide in
this basin. Because of the small increase in salinity in the deep water
there may, however, be good opportunities for new inflows in the
near future. We have now got rid of most of the hydrogen sulphide
and therefore even smaller new inflows may have positive effects on
the oxygen conditions. The most positive e!fect of the inflow is that
the oxygen conditions in the Baltic proper south of the Eastern
Gotland basin have improved considerably: These areas are of great
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importance for the spawning of cod.
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References
Fonselius, S. 1969. Hydrography of the Ba1tic~ Oeep Basins IH. Fishery
Board of Sweden, Hydrography No. 2~.
J
Häkansson, B., B. Broman and H. Oahlin 1993. The Flow of Water
and SaH in the Sound Ouring the Baltic Major Inflow Event in
!
January 1993. leES C.M. 1993/C:57
Matthäus, W. 1993. Salzwassereinbruch in die Ostsee. Geografie
i
Aktuell. Forschungsberichte. Geografische Rundschau Jg. 45
(1993), H. 7/8.
Penck 1993a. Monitoring Cruise 02/93 20 March
to 7 April 1993 Kiel
,
Bight to northern Gotland Sea. Cruise report No. 3/93, R/V
Prof.Albrecht Penck. Baltic Sea Researc~ Institute,
Warnemünde.
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Penck 1993b. Monitoring Cruise 05/93 4 MayJo 26 May 1993 Kiel
Bight to northern Gotland Basin. Cruise Report No. 4/93. Ibid.
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0
0
0
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0
0
•••
0
0
0
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Fig. 1. Map showing the hydrographical stations visited during
the expedition 2·5 of March.
56
10-l'i PSU
15 -20
>
20
1.-----::;-:---.......l~~--.-Jt!.:::::-~----_:I=:----_J.I7~.------
Fig. 2. The distribution of salinity dose to the sea bed.
5~·
.,_ ....•..
,
Fig. 3a. A salinity secHon from the central parts of the
Bornholm basin towards SE.
•
50
J 00
•
l-4.
I naut. lIIif
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Fig 3b. The same section as in Fig. 3a showing temperature.
SEAWATCH SALTIC
ARGOS SUELLITE
TRANSlIlmR
METEOIIOlOGICAl
SENSOR
,j. ·']l ~"",,,m"
40 m
--1
20 m
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8
.t. (' t'
1
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r _.
Floating llne. &0 m
tr.wt 110.'.'
V
V
OPTlCAl
SENSOR
SUBSURFACE
BUOY
-!
CURRENT
ara'_
•
lAND STOII.\GE UNIT
WAVE HEIGHT
/SEJISOR _.' . _
IIfTER
~
rope 20 mm
~MPERATUTlEI
COIlIlUCTIVITY
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l".".,.
t·
5m
~
.c":
~
'1
'11_
1
...
•
~ ~ 1000 ~g
;,5,
PAOFtlE
SENSOR
PIlESSURE
SENSOR
+
Anchor welsht
Fig. 4. Seawatch marine environmental da ta buoy (TOBIS)
Salinity Bomholm Deep
20
i·
•
18
16
14
+---_.
I~
-SSOm
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- _..• S60m
12
10
8
6
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I/\··~·'
,
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)
o
-S70m
-:::'{I'P'
v
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0>
4
2
r-""
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-.
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1993-03-25
1993-03.26
1993-03·27
1993-03-28
1993-03-29
1993-03-30
1993-03-31
1993-04-01
00.00
00.00
00.00
00.00
00.00
00.00
00.00
00.00
Fig. 5. Salinity, Bornholm Deep
S80m
S85m
Stolpe Furrow 69m Lat: 551315
360
i
.
-
330
300
.
...
'i...
oe
l
.
l;Il
c:
240
210
180
150
.!O
120
Q
.! -•
,.J
.
...
.
1-'-- :=-.
li •
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::~
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_..:.f:
:
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_r
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,~
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:t
,
'[1
I~
-.j,.
...
270 C•=.
0
.~
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Long:I~689
:-\-, I
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:= i-
oe-
..
....
-.'
.ofF.
i:..':.
~
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:~
;
:
:
90
60
•
30
0
93-04-21 00.00
93-05-01 00.00
93-05-11 00.00
93-05-21 00.00
93-05-3100.00
93-06-1000.00
93-06-200000
Fig. 6. Current, Stolpe Furrow
Stolpe Furrow 69m Lat: 551315 Long:I~689
14 -----...,....----...,....---.-~---------------...,
12
-+-.-IHkRllI::..lI!..----'l~----_l__jil_-~
•
.....r_fl~W'\-.......- - l - - _ F = - - - -
10 -!-....;..--=---.l------l-----!------+---f----'--------!
8
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6
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4
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Tamp
-
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t
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1
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2
o
93-04-2100.00
II
93-05-01 00.00
,
93-05-1100.00
93-05-2100.00
93-05-31 00.00
Fig.7. Salinity and temperature, Stolpe Furrow
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93-06-1000.00
93-06-2000.00
Colc solini1y
e
GOTLAND DEEP
1S
I
(> 200 m)
,
-,
Ie
Ie
I-
1....
-
Ie
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I-
I-
13
Ie
Ie
.-""
~
C
I-
-a
U)
Ie
12
Ie
III-
11
Ie
I-
II-
10
1980
•
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1970
1980
1990
Year
Fig. 8. The salinity below 225 m in the Gotland Deep 1963-1993.
BY 15 Salinity PSU
250
.-t.t.!i" - - - - - ,
1
2
J
,
5
I
6
... " I
12
7
8' 9
10
11
2
J'.
5
6
7
1 991
S '9
1
1 992
1990
BY 15 OxygenlHydrogen Sulphide mlll
~I
Ie--7
I
i
I
8
,~
:c· 11
112
e
1990
1 d Deep 1990-1993.
th~'~~~:~iations
1990-1993.
Fig.9.
]'In ity /hydrogen
variations in
a.
sulp
b. The
The sa
oxygen
1
.,.1---- --....:..------------;
UIF ARGOS
93-0S.01 - 06·18
Oxygen concentraUon less than 2 mlJl
Ares wfth hydrogen sulphide
} - - - - - - 1 !!'
Fig. 10. The distribution of hydrogen sulphide in thc bollom
watcr ill the Eastern Gotland Basin in JUlle 1993.
1
-30
- -90
-90 .
Oxygen
-210
-;J/~
'-
. -210
--J
Fig. 11. The distribution of oxygen/hydrogen :;ulphide in the Baltic
Proper. Isopieth interval is 1 mI/I.
-~70
a
50
.
j"'-
U
\~
J
f\"~ /
~
SV 15 N03IJmolll
f/
j/-
/
/
./
,.. 1
.~~
-----.......-8::::
~
~\----
2
/
[\"
~,~
t4~
~
".~'tf
~10
~
e:-.,e---
~
I~~
': L .,.'.,_,"'
" I , , , ' , ' , , , , • ' , , • " ',. '" ' " I , , , , , , , , , '. ' , , • , • ''" ," ,
1990
"I ,I, I , , ,
1992
1991
i
,
Ji.'"'"
",!
1993
SV 15 P04IJmoill
o ,----...,........,..----r---,,-------II---;,-,;,---'111--,/--,1--11----:------1
.,"
P·1
,-----
50
.' ..... _ .... ,.
:
6.5
\
"
0,5
0,'...
,
----l--==::-- '---- --
_-=~
--'ODi
I ~-
'50L..~
,,!
/
I
'_:"' __ 1
'"
/
\
\
'0:1
\
1
I
/0.5
f
"
-
.'
~.5I
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I'
1 \,
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-0.1
".. . . . -'--~
\, /I~
- ~ __--------."
---------. . / --V
L
,--/
J"~J
\ \ ./<~"
5
'\"-"~ ~) L:~: ~
J"" "i" '.', '. '. '-'"',,1,''' ,'.', '.','. '. '.. ',,'U I, "" '.', '." '. '. '..'" ,,,I,,,' ,'.', '.', '. '. ',"'" '"
1990
1991
1992
Fig.12.
a. The nitrate variations in the Gotland Deep 1Q90·1993.
b. The phosphate variations 1990-1993.
1993