Baltic Sea

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Local environment

Definition and basic facts

Figure 1:Baltic Sea and its Drainage area[1]
The Baltic Sea – a shallow, epicontinental sea of volume 21,700 km3 , with a drainage area of ~1.7 million km2 - is one of the largest brackish seas in the world. Drainage runoff (660 km3 per year) exceeds ten times precipitation (if exclude evaporation) and makes up 2% of sea volume. Baltic is situated between 100 E ~54oN and ~310E to 660N (close to Arctic Circle) being under influence of mild Atlantic climate in south-west end to harsh continental – boreal climate in Bothnian Bay area. It is connected with Atlantic Ocean through system of very narrow and shallow straits (Great Baelt, Øresund and Little Baelt).
Table 1. Basic data of the Baltic Sea not included in the text (after Bonsdorff et al., 2002[2]).
Parameter Measure

415 000km2


1300 km



Maximum depth


Sill depth


Residence time of water


Stratification and circulation

Figure 2:Salinity gradient. S.Weslawski,

Restricted connection with open seas, together with high riverine inflow, results in something like backwater effect of water in the whole basin – In Gulf of Bothnia water level is 36cm higher than in North Sea, in middle part of Baltic Proper by 18 and in Kattegat by 10cm. This generate strong current in Sound system. Inversely enters weaker (roughly twofold) current bearing saline (<34PSU) waters from North Sea. Such balance of fresh vs. Saline waters – characteristic for estuaries – cause permanent strong salinity gradient. At entrance – in Kattegat – salinity of surface waters is around 20PSU but in Bornholm Basin decrease to 8PSU and even lower at North and East periphery (Fig.2). This estuarial feature is expressed also in vertical stratification of the water column and permanent halocline is present along whole Baltic Sea, it is located at 40-60m at Western end and 60-80 at Northern part. As an effect of such stratification is existence of two types of water masses – relatively light freshened surface – and heavier more salty deep waters.


Figure 3: Inhabitants of sandy coasts. S.Weslawski,

Southern part of the Baltic Sea is fringed mainly by coasts built of sedimentary material – leftovers after glacial time. Composed mainly of sand, are vulnerable to mechanical stress of wind and wave action and due to its mobility is not conducive to seaweeds and vascular plant development, also only infaunal or meibenthic animals can endure here (Fig.3). Coastline here is rather straight and exposed, only in estuaries or supports shelters for flora .

Figure 4: Inhabitants of rocky coasts. S.Weslawski,

In contrast Northern and North-East Baltic (partly Gulf of Finland, and Bothnia and Swedish coast) are mainly composed of rocky substratum, shoreline is well developed with fiords, coves and archipelagos - superb shelter for vegetation and associated fauna (Fig.4).

Primary production

Characteristic feature of primary production in Baltic Sea is its seasonality and spatial variability. Spring bloom – main pulse of new organic matter in higher latitudes starts in February March in Westernmost while in Northern part, in May. Annual primary production vary from 500 [gC*y-1 *m-2] in Danish fjords to [50 gC*y-1 *m-2] in open waters of Western part of Baltic Proper (Rydberget al. 2006 [3]) in Gulf of Gdańsk annual primary production is as high as ~200[gC*m-2y-1] (Lorentz et al. 1991[4]) and well below 100[gC*m-2y-1] in Northern part of the Sea (Kangas et al. 1993,[5]). This decreasing trend reflects both salinity and climatic gradients. Characteristic feature for Baltic is third, summer bloom of Cyanobacteria - in addition to typical for temperate seas Autumnal bloom.

Specific biodiversity issues

Figure 5: Decline of number of taxa along salinity gradient. S.Weslawski,

The marine–brackish–limnic conditions in the area established about 8000 years, and the current regime in terms of salinity and basic climatological conditions (covering six zones, from the temperate to the subarctic) is about 3000 years old (Bonsdorff 2006[2]. In compare with other similar seas , due to its brackish characteristic, Baltic Sea is inhabited by animals, protists and plants representing relatively lesser number of taxa, but sometime represented by many individuals. While most diverse marine ecosystems consists of 800 taxa of macrofauna per 10 m2 - in Baltic one can expect no more than 30 taxa. But from other side, the flora and fauna of the Baltic is unusual in that there are areas in which freshwater, brackish water and marine species co-exist. For example, the freshwater plant - common reed (Phragmites australis) and the marine wrack (Fucus vesiculosus) can be found side by side or fresh water species of phytoplankton exists beside “fully marine” ones (~20 marine, 20 fresh and 30 brackish water). Biodiversity follows differences of environmental factors (see above) in different part of the Baltic Sea. In Skagerrak, where a water salinity is as high as 30 PSU, some 1500 marine animal species and 154 seaweeds live. But in so far proximity - in the southern part - where salinity is much lower (10 PSU) only ~ 150 animal and less than 50 benthic alga species can be found (Kautsky et al., 1990.[6])



From drainage area populated by over 80 millions people[7] Baltic Sea receive load of phosphorus as high as 32,9*103[t*y-1] and 661,8*103[t*y-1] of nitrogen (means for decade 1994-2004) [8].
Figure 6: Oxygen depleted and anoxic zones at the bottom. S.Weslawski,
These loads shows generally decreasing trends but in Baltic Proper and Gulf of Bothnia were still increasing while in other parts decreased. High waterborne nutrient load (4kg phosphorus and 83kg nitrogen per capita) results in high winter concentrations in water column - >15μg*dm-3 inorganic phosphorus and >60μg*dm-3 inorganic nitrogen in prevailing area of the sea only in Gulf of Finland and some coastal zones these values are higher [9]. Excess of nutrients cause, beside spring and autumnal primary producers blooms (mainly diatoms), summer dominated by cyanobacteria and dinoflagellates blooms(causing sometime “red tides”). High and rapid biomass production results in high flux not consumed organic carbon fall passing by lid of halocline and in decomposition processes cause oxygen depleted “death zones” - currently covering about 25% of total area of Baltic Sea bottom[10] (Fig. 6).

More visible – for sea shore visitors – is eutrophication effect in the coastal zone. Here apart cyanobacterial and red tides massive development of filamentous algae – mainly Ectocarpaceae – forms awfully looking algal mats, in late phase of development floating and finally ending at the shore or at the bottom covering all living organisms competing for oxygen during decomposition.

Climate Change

One of the expression of the Climate change is temperature increase, both over land and in the sea. The warming trend for the entire globe was about 0.05oC/decade from 1861–2000, while the trend for the Baltic Sea basin has been somewhat larger, 0.08oC/decade. [11] Increasing temperature over drainage area would cause mean annual river flow from the northernmost catchments - in the southernmost ones possibly decrease. Seasonally, summer river flows would tend to decrease, while winter flows would tend to increase, by as much as 50%. Mean annual sea surface temperatures could increase by about 2oC to 4oC by the end of the 21st century. Ice extent in the sea would then decrease by about 50% to 80%. The average salinity of the Baltic Sea is projected to decrease by between 7% and 47%. [12]

Alien species

Figure 7: Origin of the alien species. After Leppäkoski & Olenin 2000[13] )
Alien, invasive or non-native species – are ascribed to species colonizing Baltic Sea. Up to date, between 98 (Leppäkoski & Olenin. 2000 [14] ) to 120[15] non-native species are noted here. “Invaders” comes from freshwater compartments as well as from marine ones from Europe and all over world (Fig. 7). Some introduced intentionally, some "fulfilling" natural expansion (amplified by Climate change or eutrophication) or unintentionally transported by ships – attached to hulls (ie. North American - Balanus improvisus) or with ballast waters (ie, New Zealander - mud snail Potamopyrgus antipodarium) (Leppäkoski & Olenin. 2000 [14] ). Once invasive species reach Baltic Sea it can spread horizontally with speed 40 to 480km*year-1 depending on species (see Leppäkoski & Olenin 2000).


  1. balans
  2. 2.0 2.1 Bonsdorff, E., C. Ronnberg & K. Arnio, 2002, Some ecological properties in relation to eutrophication in the Baltic Sea. Hydrobiologia 475/476, pp 371-377 Cite error: Invalid <ref> tag; name "Bonsdorff" defined multiple times with different content
  3. Rydberg. L, G. Ærtebjerg , L. Edler .2006. Fifty years of primary production measurements in the Baltic entrance region, trends and variability in relation to land-based input of nutrients. Journal of Sea Research 56, pp.1–16
  4. Lorenz Z, J. Nakonieczny, S. Ochocki, H.Renk. 1991. Primary Production and Chlorophyll in the Gulf of Gdansk in 1987-1988. Acta Ichtyologica Et Piscatoria Vol. XXI Supplement pp. 117-124
  5. Kangas, P., Alasaarela, E., Lax, H.-G., Jokela, S., C., Storgård-Envall. 1993. Seasonal variation of primary production and nutrient concentrations in the coastal waters of the Bothnian Bay and the Quark. Aqua Fennica, Vol. ( 2) . pp. 165-176
  6. Kautsky H., Eddy van der Maarel. 1990. Multivariate approaches to the variation in phytobenthic communities and environmental vectors in the Baltic Sea. Mar. Ecol. Prog. Ser. 60: pp.169-184
  8. HELCOM Indicator Fact Sheets 2005. Online.
  9. The Baltic Marine Environment 1999-2002. 2003. Baltic Sea Environment Proceedings No. 87, pp. 1-47 ( _l_id=79889&folderId=377779&name=DLFE-36818.pdf
  10. Bonsdorff, E., C. Ronnberg & K. Arnio, 2002, Some ecological properties in relation to eutrophication in the Baltic Sea. Hydrobiologia 475/476, pp 371-377
  11. Climate Change in the Baltic Sea area Draft HELCOM Thematic Assessment in 2006. HELSINKI COMMISSION Baltic Marine Environment Protection Commission
  12. Climate Change in the Baltic Sea area Draft HELCOM Thematic Assessment in 2006. HELSINKI COMMISSION Baltic Marine Environment Protection Commission
  13. Leppäkoski E., S. Olenin. 2000. Non-native species and rates of spread: lessons from the brackish Baltic Sea. Biological Invasions 2, pp 152-163
  14. 14.0 14.1 Leppäkoski E., S. Olenin. 2000. Non-native species and rates of spread: lessons from the brackish Baltic Sea. Biological Invasions 2, pp 152-163

See also

  • Baltic Marine Environment Protection Commission (Helsinki Convention)
  • An exercise in comparing the pelagic and benthic macrofauna species diversity in Arctic, Antarctic and Baltic sites using the taxonomic distinctiveness index [1]
  • Values of, and threats to, marine and coastal habitats in the southern Baltic [2]
  • Shallow sandy sublittoral: the ecological treasure of the southern Baltic Sea [3]
  • Daily sea surface temperatures rom the late 1800s to the early 2000s implications for biodiversity in the Baltic Sea [4]
  • Application of benthic indices to assess biodiversity in the southern Baltic Sea [5]
  • Alien Species Directory [6]

[[Category:Coastal and marine habitats]

The main author of this article is Wiktor, Jozef
Please note that others may also have edited the contents of this article.

Citation: Wiktor, Jozef (2020): Baltic Sea. Available from [accessed on 18-07-2024]