lCES
C.M. 1986/C:7
Hydrography·Commlttee/5ess. Q
ON LONG-TERM VARIATIlONS OF SOHF. HYDROGRAPHICAL PARAMETERS IN
THE DEEP BASINS OF THE BALTIC PROPER
BY
ST1G H. FONSELIU5
Swedlsh Meteorologieal and Hydrologieal Institute
Oeeanographieal laboratory, Göteborg
P.O. Rox 2212
5-403 14 Göteborg, 5weden
ABSTRACT
In the Baltie Sea long-term series of salinity, temperature and
dissolved oxygen' observations from the maln stations exist. The series
begin at ·the end of the last eentury. Nutrients eoncentrations have
been measured at these stations ~fter W.W.II. Phosphate measurements
began in the 19503 and nitrogen compounds and silicate have been
measured sinee the end of the 1960s. By help of these saries it has
been possible to study long-term trends in the surface water and to
follow the large variations in the deep water, caused by inflows of
Kattegat water, with high density along the bottom of the Baltlc
proper.
Unfortunately no similar long-term serles of biologieal parameters
exist. Therefore it 15 diffieult to draw reliable conelusions
regarding biologie al trends and variations and to couple biologieal
results to the hydrographieal changes.
"
•
In the present pap~r series of density and oxygen measurements from
the surfaee to the bottom at the station BY 15, the Gotland Deep are
shown. Inflows of water with high density below the haloeline are
shown to cause stagnation and hydrogen sulphide formation in the bottom water of the deep basins. The stagnation periods are broken by new
inflows, improving agaln thc oxygen conditions. DIagrams for phosphate
variationsfrom 1967 and nitrate variations from 1968 to present time
in the Gotland Deep are also shown. The aeeumulation of phosphate and
dlsappearancc of nitrate in thc stagnant water are demonstrated.
Trends of some blo1ogical parameters are briefly discussed. The
observed eutrophieat ion of the Baltic Sea surface water and posslble
effects on reproduction of f1sh and also thc diffieultics encountcred
in trylng to conneet fisheries statlsties to hydrography are finally
diseussed.
ON LONG-TERM VARIATIIONS OF SOME HYDROGRAPHICAL PARAMETERS IN
THE DEEP BASINS OF THE BALTIC PROPER
BY
STIG H. FONSELIUS
Swedlsh Meteorological and Hydrologieal Institute
Oeeanographicnl laboratory, Göteborg
P.O. Box 2212
S-~03 14 G~teborg, Sweden
In the Ra1tie Sea we find long-term Baries of measurements of sa1inity, temperature and dlssolved oxygen beginning at the end of the
last eentury. These series have been taken at the main deep stations
rather regularly. The work has bcen carried out by marine laboratories
from different countries Burrounding tho Baltic Sea. When the lCES was
founded In 1902 thc work was by agreement divided between the Ealtic
Sea countries in order to try to cover the main decp stations at least
once every yea~. In the beginning the work could not be carried out
very regularly. For some years data are completely miBsing. Often only
one expedition was carried out annually. Unfortunately the werk hadto
be Interrupted
durlng the two World Wars (W.W.), due to military
operations and lack of research ships. Therefore we find gaps of
several years in thc ser1es du ring the war periods •.
•
After W.W. 11 the amount of observations has increased enormously and
all the main stations are now covered at least four times annually.
Afte~ the
war, observations of nutrient concentrations have been
introduced in all Baltic Sea countries. The analytical methods have
been worked out. Intercalibrations have been carrled out In the frame
of ICES und we now have quite rel1ablc ser1es of. phosphate measurements slnec 1954. In the middle of the 19605 nitrate and nitrite
analyses were introdueed. Also silicate and ammonia have becn measured
on a routine basis since the beginning of the 1970s (Fonselius 1986).
Fig. 1 shows a map of the Baltic.Sea with the stations discussed In
the paper marked. Fig~ 2 shows thc variations of density in the deep.
water of the Gotland Deep (BY 15) from 1883 to 1985. The 'density 1s
rna1nly dependent of the salinity in the deep water, where the temperntura variations are low and a d1agram of thc salinity would be very
similar to .the density diagram. In thc figura we can see that an
inflow of Kattegat water with high density occurred durlng W.W.I and
that thc denslty then decreased. The high peak in 1928 1s caused by a
sIngle set of measurements and 1s obvlously wrong. The measurements
were carried out by the "Skagerak" and the next day thc same station
was occupied by the ,.Aranda" , which got quite normal results. The
"Skagerak" values have to be reduced with 0.5 salinity units from thc
surfaee to thc bottom. My raason for this correctlon i8, that the oxygen values of the decp water were extrernely low. An inflow of high
saline water would have contalned oxygen. Fig. 3 shows the oxygen
variations in thc Gotland Deep in thc same manner. We can see periods
wlth hydr~gen sulphide formation (the black arens 1n the fig.). These
3
are always corresponding to periods with decreastng density. Tho oxygen peaks correspond to inflows of water with high density. If we
return to fig. 1 we can see that thera was another large water inflow
In 1952. Thls Inflow was the largest ever obscrvad in the Baltic Sen.
It has been descrlbed in detail by Wyrtki (1954). Both the above monttoned inflows caused hydrogen sulphide formation in the bottom water.
After the 1952 inflow, increasing periods with stagnation and hydrogen
sulphide formation have been obscrved (fig. 3). The inflow obviously
was a catastrophe for the Baltic Sea. It has often been clairned that
eutrophicatlon 1s responsible for the hydrogen sulphide formation in
the Baltic Sea (Larsson et al. 1985). The stagnation pcriodn, howcver,
are not caused by eutrophicat1on and the increasing occasions with
salt water inflows followed by stagnation, is the prirnary renson for
the bad oxygen
conditions
and
hydrogen
sulphide
fo~ation.
Eutrophication r.~y have increased the oxygen utilization, eausing more
hydrogen sulphlde formation.
. ,
•
During stagnant, conditlons phosphate is accumulated in the stagnant
deep water and when hydrogen sulphide 1s formed, phosphate is released
from the sediments (Fonsellus 1969). Nitrate is also accumulated, as
long as oxygen is pro3ent in the' water. ~oen the dissolved oxygen
reach values below 0.5 mlll nitrate begins to be reduced to nitrogen
gas and disappears completely from the stagnant ~~ter during reducing
conditions, when hydrogen sulphide 1s ro~ed. Fig. 4 shows tho'phosphate variations in the Gotland Deep in the whole water muss from 1967
to 1985. We can see in the figure the accumu1ation of phosphate and
the seasonal variations in the surface layer. Fig. 5 shows in the same
manner the nitrate variations from 1968. We can see that the nitrate
disappears in the deep water cornpletely or almost cornpletely. This 1s
due to the analytieal technique, which 13 not very aecurate.Often tho
zero results are reported as <0.1 and sometimes valucs are given with
two decimals even below 0.10 unol/l, whlch i~ meanlngless. In the
surfa'ce layer the nitrate disappears eompletely during the sumrr:er
period, indicating that nitrate 1s one of the production limlting
factors.
•
From these figurcs 1t is difficult to see if the nutrlcnt levels in
the surface water have incrcased since the 19605. Nehring et al (t985)
has shown that,the nutrient concentrations in the whole surface layer
of the Baltie'Sea has increased during that period and that the salinity of the ~~ter also has increased. Figs. 6, 7 and 8 show examples
of this increase In,the Gotland Deep, as annual mean values at 40 'm
depth for salinity, phosphate and nitrate during the period 1960-1985.
Unfortunately we do not have corresponding long-term observations of
biologlcal parameters. We do not know if the primary production of the
water has inereased, we also do not know if the blomass has increased.
There are some signs of an increase of the zooplankton biomass, but
also contradictory rcsults have been 'obtained (Wulff et al 1986).
Schulz (1985) has shown that the chlorophyll a has increased in the
southern Baltie Sea from 1975 to 1983. He eoüld not find satistieally
significant Increases for pri~ary production and zooplankton biomass
during the same period.
One or the difrtculties in blo1og1cal long-term investigatlons is,
that gear and methods have been changed several times and that the
results therefore are not comparable.
4
For fisherics long serles of eateh statlsties cxist, but the 'results
are hardly useful for conelusions rega:rding ehanges in thc flah stock.
If we look at thc eonditions in thc Paltie Sea, eod was not caught in
larger arnounts before W.W.II. It was not considered eommercially
interesting. During W.W.II thc German trawling fleet was due to the
war eonditions exeluded fro~ its normal fishing grounds In the North
Sen. Therefore it moved into the Ra.ltie Sea. This gives a high response in eateh statistles, but of course the result depends on new
fishing methods, trawllng was not usual in the Baltie Sea at that
time, and of course ~n inercased effort (Mayer and Kalle 1950),
(Otterlind 1986). Otterlind also points out other faetors, which heve
inereased thc eatehes after W.W. 11 as inereased fishing efforts mainly by Poland and USSR uslng new large trawllng fleets, transfer of
fishing efforts from the North Sea to the Baltie, ehanged market
eonditions ete. (fig. 9).
•
•
It fs not easy to conncet variations in flah enteh to hydrographie
eonditions. We have to remember that there are saveral links in the
food ehain between thc phytoplankton, whieh first 1s influeneed by
ehanges in nutricnt eonditions i~ the surfaee water and eommercial
fish, whlch 13 ineluded in flsherles statistics. Hydrographie ehnnges
do not only have etfcets on the nutrient eondltlons. stagnation of the
deep water may lead co anoxia and .hydrogen sulphide formation in the
Baltie baslns. Thls may influenee the cod reproduet10n 1n thc Baltie
Sea. Important spawning areaa are the Bornholn Basin and thc Gdansk
Rasin, where anoxia ~ay develop. Svansson (1985) has tr1ed to eonneet
good year elasses of eod with thc hydrographie conditions in the
Bornholm Basin (fig. 10). From the flg. we ean see that very good
elasses developed in 196 1., 1972, 1976' and possibly 1977. Good elasses
dcvcloped 1n 1963, 1966, 1979 and 1980. Otterlind (1986) additional1y
points out 1977 and possibly 1981 as years for good elasses. Very bad
was the elass of 1968. The very ~cod elasses eoincide with inflows cf
new ~~ter, whieh improved the oxygen oondltlons in the whole southern
Baltle Sea, thus Improvlng the spawnin;g rcsult. One rnay or course
point out that eod eaught in an area does not neeessarily origlnate
from that area. Cod 1s a mlgratory fish, but the Inflow of new water
tnflucnees the whole Baltle proper and thercfore it should not matter
if the cod originates from thc areas ~~st or east of Bornholm.
But oceasional improvements of thc oxygen eonditions should not eause
the observed largc inereasing of the eod stock. That should probably
be eonneeted to the inereasing cutrophication of thc surfaee water,
regardlcss If this i5 eaused by natural or anthropogenie proccsses.
5
Referenees:
Fonselius, S. 1969: Hydrography of the Balt1e Deep Basins III. Flshery
Board of Sweden, Series Hydrography, No 23.
Fonsellus, S. 1986: On long-term variations of dissolved oxygen in the
deep water of the Baltte Sea. Raltie Sea Monitoring Symposium, Tallinn, USSR. 10-15 March 1Q86.
P~oceedlngs. (in print). Manuscript.
Larsson, U., R. Elmgren and F. Wulff. 1q85: Eutrophication and the
Baltie Sea: Causes and Consequenees, Ambio, Vol.
111:1.
•
Meyer, P.F. and K. Kalle. 1980: Die biologische Umsti~~ung der Ostsee
in den letzten Jahrzehnten - eine Folge hydrographischer Wasserumschichtungen. Archiv fur Fisehereiwissenschaften. 2:1-9.
Nehring, 0., G. Aertebjerg, A. Trozosinska, S. Fonselius, P. Tulkki,
P. Alenlus, V. Tervo, M. Bannus, V. Astok and A.. K.
Yurkovaskis • 1q86: First Periodic Assessment of
the State of the Baltte Sea, Chapt. 2. Nutrients.
Balt1e Marine Environment Proceedings. (in print)~
t1anuscript.
.
Otterlind, G. 1986: On Fluctuatlons of the Baltte cod stock. J. Cons.
Int. Explor. mer. (in print). Manuscript.
Sehluz, S. 1985:
"
Ergebnisse ökologischer Untersuchungen 1m pelagischen Ökosystem der Ostsee, Akad. der Wissenseh,
der DDR Inst. fur Meeresk. Rostoek-Warnemunde.
Printed manuseript. 185 pp.
Svansson, A.. 1985: Fiskerihydrografl. Medd. Havsfiskelab. Lysekil,
Nr 307. Inst. Hydrogr. Res. Götehorg Series, No
31, 121 pp.
Wulff, F., G. Aertebjerg, G. Nicolaus and A. Niemi. 1986: First perlodle Assessment of the State of the Baltlc Sea.
Chapt. 4. Pelagic Biology. Baltle Marine Environment Proceedings. (in prlnt). Manuserlpt.
Wyrtkl, K. 1954: Der gros se Salzeinbruch in die Ostsee 1m November und
Dezember 1951. Kieler Meeresforschungen, 10:1.
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