The Baltic Sea today

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  • This article was downloaded by: [University of Auckland Library]On: 06 December 2014, At: 04:31Publisher: RoutledgeInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

    International Journal of EnvironmentalStudiesPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/genv20

    The Baltic Sea todayBernt I. Dybern aa Institute of Marine Research , Lysekil, SwedenPublished online: 22 Dec 2010.

    To cite this article: Bernt I. Dybern (1971) The Baltic Sea today, International Journal ofEnvironmental Studies, 2:1-4, 59-69, DOI: 10.1080/00207237108709445

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  • Intern. J. Environmental Studies, 1971, Vol. 2, pp. 59-69 Gordon and Breach Science Publishers Ltd.Printed in Exeter, England

    THE BALTIC SEA TODAY

    BERNT I. DYBERN

    Institute of Marine Research, Lysekil, Sweden

    (Received October 8, 1970)

    The Baltic Sea receives many different pollutants and the quantities are continually increasing. Special topographicaland hydrographical features due to the fiord-nature of the sea, a marked halocline, especially in the Baltic proper,and temporary thermoclines, restrict the water exchange with the sea outside as well as between the upper andlower water layers. This makes the Baltic very sensitive to pollution and many coastal areas are seriously affected.Toxic substances have been shown to accumulate also in animals from the open sea and the oxygen deficiency condi-tions which occur in the deeper water layers may partly be due to the discharge of nutrients from land. Thesenutrients stimulate the productivity of the upper water layers; this is followed by an oxygen uptake from the deeperlayers when sinking organic matter undergoes degradation.

    INTRODUCTION

    The Baltic Sea is a large fiord with a very restrictedwater exchange with the sea outside it. An excessivefresh-water supply makes the whole water volumebrackish but there is, especially in the southern andcentral parts, a considerable difference in salinitybetween the surface and deep layers. A permanenthalocline is formed between these, so that the waterexchange between them is restricted. During Sum-mer a temporary thermocline may also act as abarrier against the vertical exchange in the upperwater layers.

    Due to the slightly increased salinity of thedeeper water layers during the present century theinfluence of the halocline has been strengthened.There has also been a small increase of the meantemperature during recent decades.

    The restricted vertical exchange makes the waterof the deeper parts poor in oxygen, especially inthe central Baltic. The oxygen content has decreasedsuccessively during the present century, particu-larly after World War II. This is due to the streng-thening of the halocline, but another reason maybe increased breakdown of organic matter sinkingdown from the surface layers owing to the rise intemperature. There is, however, strong evidencethat the increased quantities of organic wastesdischarged from land greatly account for the pro-nounced oxygen deficiency, and even H2S-produc-tion, which characterizes the development duringrecent years.

    The increased discharge of sewage and industrialwastes has negatively influenced many coastal

    areas, and damage to recreation areas, marine lifeand fisheries has occurred in many places.

    Toxic substances, such as DDT and PCB, havebeen shown to accumulate in the Baltic due to thesluggish movement of water out of the sea, andthe concentrations in certain fish-species and othermarine animals are much higher than in correspond-ing animals in the Skagerrak area. Mercury pollu-tion has affected some coastal areas.

    Oil pollution is now a big problem. Radioactivepollution is under control.

    The increased pollution by oxygen-demandingsubstances and, especially, by toxic compoundsand oil is alarming and will continue to increase dueto the sensitive hydrographical conditions, unlessextensive precautions are taken. To make effectivepollution abatement possible international co-operation in the form of joint research work andbilateral negotiations will have to be developed.

    TOPOGRAPHY

    The Baltic Sea covers an area of about 370,000 km2.It is thus smaller than the North Sea (about575,000 km2). The main difference between thetwo seas, however, is that the Baltic, in fact, is alarge fiord with very narrow and shallow entrancesthrough the Danish sounds and the Oresundbetween Denmark and Sweden (Figure 1). Thegreatest sill depth is only 17-18 m (above the sill ofDarss between Denmark and the German Demo-cratic Republic). This, of course, renders the waterexchange between the Baltic Sea and the North

    59

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  • 60 B. I. DYBERN

    FIGURE 1 The Baltic Sea.

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  • THE BALTIC SEA TODAY 61

    BALTIC PROPERALAND BOTHNIAN

    SEA SEA

    Land sortDeep

    400

    FIGURE 2 Longitudinal section.

    Sea rather difficult, which is of extreme importancefor the understanding of the special features of theBaltic.

    Inside the sounds there is a series of deeperbasins, as shown in Figure 2, the deepest being theLandsort Deep (459 m). Between the Aland Seaand the Bothnian Sea there is evidence of a narrowchannel about 70 m deep (not shown in Figure 2).The greatest depth of the Gulf of Finland is almostexactly 100 m.

    The coastal areas are generally relatively shallow.Especially the coasts of Sweden and Finland aresplit up into thousands of small embayments andislands. The Russian, Polish and German coastsare, on the other hand, more open. This renderswater exchange between the coasts and the open seasomewhat different in different parts of the sea.

    Figure 1 shows the main subareas and basins inthe Baltic. The names of these will be used frequent-ly below. When the terms "Baltic" and "BalticSea" are used, they mean the whole sea area betweenthe 54th and 66th parallels.

    SALINITY

    Many rivers discharge fresh water into the Balticand there is also a rather large supply in the form ofprecipitation. The fresh water mixes with the seawater to form a thick surface layer of water of lowsalinity. Part of this flows out as a surface currentthrough the entrance sounds, causing, below it, aweaker deep current of more saline water in theopposite direction. The total water balance is notknown in detail but has been roughly estimatedas follows:1

    Runoff + Precipitation -f- Inflow = Evaporation500 km3 200 km3 500 km3 200 km3

    + Outflow1000 km3

    The outflow of low salinity water is thus abouttwice as great as the inflow of more saline water.

    The inflowing water is successively mixed withthe more brackish water. Therefore, salinitydecreases inwards into the Baltic. At the same timeit also decreases from the bottom to the surface.Table I gives some mean values for differentsubareas and the Kattegat.

    TABLE I

    Mean salinity values in % for the surface and bot-tom waters in different parts of the Baltic and in the

    Kattegat

    Subarea

    Bothnian BayBothnian SeaGulf of FinlandGotland DeepBornholm DeepBight of Kiel (Belt Sea)Central Kattegat

    Surface

    1-43-43-56-87-8

    16-1721-22

    Bottom

    3-44-58-9

    11-1315-1721-2233-34

    Between the low salinity surface water and thedeeper more saline layers there is a permanenthalocline which is most marked in the southernand central basins. Figure 3 shows typical hydro-graphical conditions in the Gotland Deep duringsummer. The halocline in this area lies at about50-70 m depth. In the more shallow southern Baltic

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  • 62 B. I. DYBERN

    and the Gulf of Finland it rises to depths between15 and 30 m.

    The halocline is of great importance, since ithinders to a considerable extent the exchangebetween the upper and deeper water layers. In theBothnian Sea and the Bothnian Bay, where thesalinity differences are small, the influence isconsiderably less than in the Baltic proper.

    1 2 3 I* 5 6 7 8 9 10 11 12 13 14 15 16 17 %., C.m[/l

    100

    200-

    p^ml/l; tC

    GOTLAND DEEP

    JULY 22.1961

    ,7.. S

    FIGURE 3 Salinity, temperature and oxygen conditions inthe.Gotland Deep, July 22, 1961 (from Fonselius14).

    During recent decades there has been a certainincrease of the salinity, especially in the bottomlayers. For example, in the Gotland Deep thisincrease is about l-2%0, which is rather large whenit is borne in mind that the salinity is, on the whole,rather low. The increase has strengthened thehalocline and has certainly sharpened its effect, too.

    Salinity conditions, especially in the Baltic proper,are not quite stable. The flow of more saline wateroccurs at irregular intervals through the entrancesounds. Due to this, deep water salinity maytemporarily increase, but there is, as a rule, a moreor less rapid return to the "normal" conditionsthrough mixing of the new water with the morebrackish water.

    TEMPERATURE

    During Summer surface water layers down toabout 20 m depth warm up (cf. the temperaturecurve in Figure 3). A temporary thermocline isformed and below it the temperature is low and, onthe whole, remains rather stable for the wholeyear (about 3-6C). The thermocline disappearsduring the winter when the surface temperaturessuccessively become lower than the deep watertemperature; ice covers big areas during winter.

    The thermocline is of great importance since itprevents much of the exchange between the waterlayers at various depths. During autumn and springhomothermic conditions prevail, and the hindranceis thus only a temporary phenomenon, but mayplay an important role, e.g. in the coastal areas.

    Also, the mean temperatures of the Baltic haveincreased slightly during the present century. Theincrease is of the order 1-2C in the Baltic properand seems to be connected with a similar increasein the North Sea area.

    MAIN CURRENT SYSTEM

    The currents of the Baltic are known only imper-fectly. The general circulation is counterclock-wise,and the subareas belong partly to this system, andpartly they have local current systems.

    Since there are almost no tides in the Baltic, tidalcurrents do not play any role. Winds and differencesin the air pressure may, on the other hand, stronglyinfluence the direction and velocity of currents. Incoastal areas, especially in archipelagos, the currentsmay be very complicated.2

    OXYGEN

    Dead organic matter sinking from the layers nearthe surface, where it is produced, is successivelybroken down on the way to the bottom. This takesoxygen from the water. Even if the breakdown isslow in the cold deep water layers, these will bedepleted in their oxygen content, unless new oxygenis provided. As mentioned above, the halocline isan obstacle against water exchange. This stronglyinfluences the oxygen conditions in the deep basinsof the southern and central Baltic, the oxygencontent below the halocline being considerablylower than above it (cf. the oxygen curve in Figure3). Since the halocline is less pronounced in theBothnian Sea and the Bothnian Bay, conditions aresomewhat better there. In several coastal areasmuch of the oxygen is used up during Summerbelow the temporary thermocline, but generallythe oxygen deficit is more or less eliminated duringAutumn due to the vertical water circulation whichis then possible.

    In the open sea, below the halocline, even smallchanges in the conditions may worsen the oxygensituation. In fact, the oxygen content of the deepbasins has successively decreased during the present

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  • THE BALTIC SEA TODAY 63

    century. The magnitude of this decrease has beendemonstrated by Fonselius3 and Figure 4 shows,for example, the decline in the Landsort Deep. Itwill be seen that oxygen values have increased tozero, especially during the last 20 years. Similarconditions have been shown to exist in other deepbasins in the Baltic proper; in the Gulf of Finlandand the Bothnian Sea there has been a certainoxygen decrease in the deep waters during thesame time.

    The decline has not been quite regular. Fonseliusand Rattanasen4 have shown that since the 1950s,periods when the deeper layers of the GotlandDeep have contained at least some oxygen havealternated with others when the bottom water hasbeen entirely devoid of oxygen with an accom-panying production of H2S.

    The last oxygen-free period started about 1965,being the worst one hitherto, and all oxygen wasused up over large parts of the sea-bed not only in

    3.0

    2.0

    1.0

    0

    *

    -

    I

    \

    i

    Vi '

    1900 10

    FIGURE 4halocline inFonselius3).

    Mean values of dissolved oxygen below thethe Landsort Deep from 1900 to 1967 (from

    10

    60'

    F.R.G. \ G.D.R._T

    Riga

    U.S.S.R.

    55'

    10 20* 25'

    FIGURE 5 Bottom areas covered by H2S-containing sea water. Shaded areas show the situation in January 1969,fine-shaded areas the situation in December 1969 (from Francke and Nehring15).

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  • 64 B. I. DYBERN

    the Gotland Deep but also in other deep basinsof the Baltic proper, causing production of H2S, asshown in Figure 5. The figure also indicates, how-ever, that conditions improved during 1969. This isdue to a heavy influx of new saline and oxygen-containing water through the entrance sounds. Thenew water pushed, so to speak, the old H2S-waterin front of it, an...