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Post-Glacial Baltic Sea Ecosystems

Summary and Keywords

Post-glacial aquatic ecosystems in Eurasia and North America, such as the Baltic Sea, evolved in the freshwater, brackish, and marine environments that fringed the melting glaciers. Warming of the climate initiated sea level and land rise and subsequent changes in aquatic ecosystems. Seminal ideas on ancient developing ecosystems were based on findings in Swedish large lakes of species that had arrived there from adjacent glacial freshwater or marine environments and established populations which have survived up to the present day. An ecosystem of the first freshwater stage, the Baltic Ice Lake initially consisted of ice-associated biota. Subsequent aquatic environments, the Yoldia Sea, the Ancylus Lake, the Litorina Sea, and the Mya Sea, are all named after mollusc trace fossils. These often convey information on the geologic period in question and indicate some physical and chemical characteristics of their environment. The ecosystems of various Baltic Sea stages are regulated primarily by temperature and freshwater runoff (which affects directly and indirectly both salinity and nutrient concentrations). Key ecological environmental factors, such as temperature, salinity, and nutrient levels, not only change seasonally but are also subject to long-term changes (due to astronomical factors) and shorter disturbances, for example, a warm period that essentially formed the Yoldia Sea, and more recently the “Little Ice Age” (which terminated the Viking settlement in Iceland).

There is no direct way to study the post-Holocene Baltic Sea stages, but findings in geological samples of ecological keystone species (which may form a physical environment for other species to dwell in and/or largely determine the function of an ecosystem) can indicate ancient large-scale ecosystem features and changes. Such changes have included, for example, development of an initially turbid glacial meltwater to clearer water with increasing primary production (enhanced also by warmer temperatures), eventually leading to self-shading and other consequences of anthropogenic eutrophication (nutrient-rich conditions). Furthermore, the development in the last century from oligotrophic (nutrient-poor) to eutrophic conditions also included shifts between the grazing chain (which include large predators, e.g., piscivorous fish, mammals, and birds at the top of the food chain) and the microbial loop (filtering top predators such as jellyfish). Another large-scale change has been a succession from low (freshwater glacier lake) biodiversity to increased (brackish and marine) biodiversity. The present-day Baltic Sea ecosystem is a direct descendant of the more marine Litorina Sea, which marks the beginning of the transition from a primeval ecosystem to one regulated by humans. The recent Baltic Sea is characterized by high concentrations of pollutants and nutrients, a shift from perennial to annual macrophytes (and more rapid nutrient cycling), and an increasing rate of invasion by non-native species. Thus, an increasing pace of anthropogenic ecological change has been a prominent trend in the Baltic Sea ecosystem since the Ancylus Lake.

Future development is in the first place dependent on regional factors, such as salinity, which is regulated by sea and land level changes and the climate, and runoff, which controls both salinity and the leaching of nutrients to the sea. However, uncertainties abound, for example the future development of the Gulf Stream and its associated westerly winds, which support the sub-boreal ecosystems, both terrestrial and aquatic, in the Baltic Sea area. Thus, extensive sophisticated, cross-disciplinary modeling is needed to foresee whether the Baltic Sea will develop toward a freshwater or marine ecosystem, set in a sub-boreal, boreal, or arctic climate.

Keywords: brackish water, ecosystem, evolution, Baltic Sea, climate change, Holocene

Introduction to Post-Glacial Baltic Sea Ecosystems

The Baltic Sea is a huge transition area between freshwater and marine conditions. Plants and animals are a mix of marine and freshwater species, with some brackish-water species. Early Baltic Sea research was largely geological, and the geological stages established were mostly (except for the first, the Baltic Ice Lake, BIL) named after biological indicator species: the Yoldia Sea (YS), the Ancylus Lake (AL), the Litorina Sea (LS), and the Mya Sea (MS). All these stages, and even previous Pleistocene glaciations (circa 2.6 mi years ago, when diversity of circumboreal taxa was formed, cf. Snoeijs-Leijonmalm, Schubert, & Radziejewska, 2017, p. 239) and Eemian interglaciation (130,000–115,000 BP), before the Holocene Baltic Sea, have contributed to the present species composition. Since the time of the BIL, the species composition has been a mix of ice ecosystem species and immigrants from adjacent freshwater (arctic, boreal, sub-boreal, and even southern) and marine ecosystems (temperate, boreal, and arctic).

Around ten thousand years ago, the warming climate had created meltwater lakes on the fringes of the glaciers. Their aquatic ecosystems were recruited from several sources. Temporary connections to the North Sea led to the development of large coastal brackish-water ecosystems. Simultaneously, freshwater biota was recruited from adjacent freshwater bodies, which had a long history of connections over the north and east of Europe to the Ponto-Caspian region (the area including Caspian and Black Sea basins), and further east to Siberia. The first developing ecosystem was in the Baltic Ice Lake. Present understanding of the post-glacial ecosystems of the Baltic Sea is partly based on observations in glacial lakes. The maximum glacier extent of the last ice age was smaller than during some previous Pleistocene glaciations. The Pleistocene refugia in lakes confronting the glaciers may have provided plant and animal species of distant origin to the developing Baltic Sea ecosystems (Segerstråle, 1976). This may explain the presence of some Ponto-Caspian, arctic marine, and even Siberian species among the immigrants of the early Holocene. There are not many species endemic to the Baltic Sea ecosystems, since at first there was only ice, and subsequently very few species have evolved there, due to the extreme environmental changes and limited fertility. However, new species have been constantly arriving, and new scientific results with modern molecular biology techniques give evidence of surprisingly rapid evolution in some cases.

Initially, biological studies of the post-glacial Baltic Sea concentrated on relict species (“glacial relict” is a general name for species, either of freshwater or marine origin, that settled the Baltic Sea in the early Holocene and have survived to the present day; cf. Snoeijs-Leijonmalm, Schubert, & Radziejewska, 2017). They colonized the developing Baltic Sea, and some are still found in many northern lakes in Europe and North America, in several cases as keystone species in their ecosystems. A parallel development seems to have taken place in all glaciated areas of Europe, Asia, and North America.

Humans arrived early in the development of the Baltic Sea area and have caused many changes in the marine environment. It may be discussed whether the most urgent anthropogenic effect so far is climate change, which affects the distribution of Baltic Sea species through osmoregulation stress for both marine and freshwater species. However, it is hard to pinpoint a single most important environmental concern, since most, if not all, ecological factors are interrelated (e.g., climate change contributes to eutrophication via increased runoff) and closely intertwine with global international political questions such as overfishing and non-indigenous species distribution due to increased commerce and traffic. The crucial point is to realize that there are two simultaneous and quite similar climatic changes going on which both may lead to increasing warming. Firstly, climate development in the Baltic Sea area has produced winter warming for the last 500 years or so (BACC, 2008). If this continues and corroborates (secondly) anthropogenic climate change, which is expected to be especially intense toward the northern part of the globe, the Baltic Sea regional outcome may be more intense and quick than projected by global climatic models.

Prehistoric Ecosystems

Initial Warming: Baltic Ice Lake—Re-Establishment of Freshwater Ecosystems

Deglaciation in the northern globe from 20,000 to 10,000 BP was due to changes in astronomical factors, most importantly earth’s orbital movement. Toward the end of the glaciation, meltwater accumulated at the edge of the glacier that covered Scandinavia and the British Isles and extended eastward over Franz Josef Land and Novaya Zemlya and even northwards to Spitzbergen. In Scandinavia, meltwater formed the BIL (which lasted c. 4400 years). The Scandinavian glacier and the BIL subsequently expanded or retreated between ca. 16,000–11,600 BP. Little life presumably reached the BIL from the glaciers (ice-associated bacteria, viruses, and some algae, e.g., diatoms). The BIL was also fed by runoff from adjacent tundra water bodies. A crucial factor in the development of the BIL ecosystem was that many early glacial periods had been even more extensive than the last. This had created conditions allowing migration of several aquatic species back and forth over vast areas that were alternatively covered or uncovered by the glaciers that supplied water to the meltwater bodies. The consensus is that the ancient glaciers thus transported, together with their meltwater lakes, aquatic ecosystems, with generations of all their organisms, back and forth from Scandinavia over the watershed border to the Ponto-Caspian area, with similar transfers with Siberia as the Siberian Ice Lake expanded and retreated (Segerstråle, 1962, 1976). Thus the BIL ecosystem acquired its combination of organisms through transport along the glacier borders, from distant freshwater and marine sources.

The turning point between the end of the glacial period and the Holocene in the Baltic Sea area was the abrupt flood discharges of the BIL to the North Sea. The last flood took place shortly after 11,600 BP, in SW direction across present-day south-central Sweden. It was followed by the Preboreal climate warming, with increasing solar radiation in northern latitudes in autumn, winter, and spring, as evidenced by pollen and diatoms in lake sediments. Arthropod microfossils have added details on Holocene climate change locally and regionally. Air temperatures south of the glaciated areas warmed to close to present values, reaching 13°C to 15°C in summer. The meltwater from the glacier was milky in color due to high content of mineral particles, limiting primary production in the BIL receiving it. BIL sediments have low carbon content and contain few paleolimnological trace fossils, except for some diatom microfossils, which indicate low temperatures (Winterhalter, 1992). Observations in comparable environments that still exist in current periglacial lakes can help us to understand the BIL ecosystem (e.g., Birks et al., 2000). While turbidity (caused by the high content of mineral particles in the meltwater) will result in limited primary production near the glacier, there may have been relatively high bacterial production in the less turbid conditions further from the meltwater inflow. Algal production, however, was probably still limited by nutrient availability (cf. Snoeijs-Leijonmalm, Schubert, & Radziejewska, 2017, p. 64, the case of the heterotrophic Bothnian Bay). Consequently, the euphotic layer must have been thin and the energy basis for developing a food chain limited. However, conditions may have changed quickly as the glacier retreated. The BIL also received water, nutrients, and biota from the rivers and runoff from unglaciated land areas to the south. Thus, we may assume (following Birks et al., 2000, p. 92) that development of littoral (the area of bottom vegetation), pelagic (open water), and benthic (deep bottom) ecosystems started in the south of the BIL (see Figure 1). Succession was primarily regulated by temperature, through its effect on the conditions in the watershed area, mainly the growing season and snow cover that affect vegetation (both terrestrial and aquatic) and soil processes (such as nutrient leaching).

Post-Glacial Baltic Sea EcosystemsClick to view larger

Figure 1. Hypothetical BIL watershed including possible location for freshwater lakes and river systems that were the origin of incoming freshwater immigrant species and the basis for the ecosystems formation in the BIL. Map by Terri Vuorinen, modified following Björck in BACC (2008).

Early biological studies of the BIL biota concentrated on glacial relicts. Large lakes such as Lake Onega are known to have supplied relict species, but minor lakes and waterways probably also contributed. Some ecological keystone species originated from North Siberian freshwater ecosystems, and some arrivals evidently had a marine origin, indicating a connection to the Arctic Ocean through the White Sea area during some previous, Pleistocene, glaciation. Sven Lovén (1860) first identified glacial relict species in the Swedish lake area. In Finland, Sven Segerstråle (1956) made the first biological studies of early post-glacial relict species and reviewed studies from the adjacent Baltic Sea, northern Europe, and the British Isles. These seminal works led to numerous studies of relict species’ distribution in Russia, Sweden, and Finland in the second half of the 19th century. Segerstråle’s (1962) review is somewhat outdated, due to the weak geological background knowledge available to him. However, he detailed possible immigration routes of a number of aquatic invertebrates such as the opossum shrimp, Mysis relicta; the isopod, Mesidotea entomon; the amphipodsGammarus spp, Pallasea quadrispinosa, Monoporeia affinis, and Gammaracanthus lacustris; and vertebrates, such as Myoxocephalus quadricornis, the four-horn sculpin. Both Lovén (1860) and Segerstråle (1962) considered immigration to the Baltic Sea from the Arctic Sea through the White Sea area likely. The current consensus is that this route did not exist after the Eemian interglacial (ca. 130,000–115,000 years ago), which would have been the latest opportunity for immigration by that route. Further glacial relict species have been identified by later studies, for example the circumpolar copepod Limnocalanus macrurus, the ringed seal Pusa hispida, and most recently the marine relict comb jelly Mertensia ovum (Lehtiniemi et al., 2013). Thus, present benthic and pelagic food chains in the Baltic Sea have origins in an obscure past, well before the latest glaciation, and were rapidly reestablished by species from glacial refugia once the deglaciation started. The immigration of species continued throughout subsequent stages of the Baltic Sea; for example, the following stage, Yoldia Sea, brought in many marine immigrants. New information on the biogeographic history is accumulating at an increasing pace, especially through genetic studies. Good examples are reports (Väinölä & Johannessen, 2017; Väinölä & Nikula, 2017) on the migration of the blue mussel Mytilus and the Baltic tellin Macoma balthica (now Limecola balthica) from the Pacific through the Arctic Ocean and White Sea into the Baltic, and on finding that the opossum shrimp, Mysis relicta, is four cryptic, rather than one circumpolar, species.

Temperature Oscillation: The Yoldia Sea—First Baltic Seawater Inflows

Initially, the developing Baltic Sea drained to the North Sea through present-day Denmark, but seawater never entered the Baltic Sea through this narrow drainage in the South (Heinsalu & Veski, 2007). As the glacier withdrew, a new drainage of the BIL was created about 11,600 BP at the level of the big lakes in Sweden (and lasted for about 600 years) (see Figure 2). Initially, it was a huge waterfall (or possibly first several smaller ones, under the ice), and once the level of the BIL reached that of the adjacent sea, seawater could enter, creating the brackish Yoldia Sea (YS, lasted for approx. 900 years, ca. 11,600–10,700 BP). This coincided with a period of climate warming, the Preboreal oscillation, when summer temperatures in northwest Russia rose to 10°C–12°C. Pollen from the Baltic countries gives evidence of broad-leaved trees, such as the lime tree Tilia, elm Ulmus, hazel Corylus, and ash Fraxinus. The Yoldia Sea was named after the Arctic bivalve Yoldia arctica (now Portlandia arctica). This species is no longer found in the Baltic Sea, but its fossils are characteristic of YS sediments influenced by the inflowing seawater. In geology, the name Yoldia is mainly used to refer to a period or a shore level (Mörner, 1995) and does not imply that Yoldia was an ecological keystone species. This probably is true for other name-giving species for other post-glacial Baltic Sea stages as well.

Post-Glacial Baltic Sea EcosystemsClick to view larger

Figure 2. A hypothesized scheme of the Baltic Sea ecological conditions at the time of the YS. The Swedish straits of YS were occasionally importing seawater, which then created a vertical stratification (with a deepwater salinity around 10 PSU, and possibly anoxia), and a horizontal biologically critical salinity zone of 5 to 7 PSU together with an counterclockwise surface circulation. Map by Terri Vuorinen, based on Heinsalu and Veski (2007) and Björk in BACC (2008).

The YS period shared some oceanographic circumstances with the present Baltic Sea and its ecosystems, such as a close cause–effect relationship between low runoff (during occasional coolings) and Major Baltic Inflows (MBIs), resulting stratification, and hypoxia (Sohlenius et al., 2001). The immigration of species continued, now with a marine origin, as demonstrated by, for example, foraminiferans and diatoms. Due to continuing land rise, the sea connection gradually became narrower and shallower, and the seawater inflows eventually ceased.

Seawater inflows started during a relatively short (200 to 250 years) cooling period, some 250 years after the drainage of the BIL (cf. Heinsalu & Veski, 2007). During this period, glacier runoff decreased, causing sinking velocities and lower water level in the outflow channels, allowing seawater inflows during episodes of higher sealevel on the marine side than in the Baltic Sea. Mörner (1995) mentions the possibility that seismic events may also have contributed to the start of the inflows, while Heinsalu and Veski (2007) stress: “A diminished meltwater discharge from the ice sheet and correspondingly decreased freshwater transport from the Baltic Basin.” The mechanism by which reduced runoff allows greater seawater inflows was independently identified for present-day Major Baltic Inflows (MBIs) by Hänninen et al. (2000) and Zorita and Laine (2000) and discussed for the YS already by Björck et al. (1996).

Three temporally and spatially separated aquatic ecosystems have been identified during the YS. The first was a freshwater littoral ecosystem that was evidently a direct descendant of the BIL, after its drainage. Several common littoral mollusk genera have been reported from this period: Ancylus, Limnaea, Pisidium, Bithynia, Physa, and Radix (Königsson & Possnert, 1988). Segerstråle (1956) notes that salinity must have been low also toward the end of the YS, when lacustrine whitefish and roach reached W. Norway. South of the glacier-fed meltwater, the inflowing marine water caused clay particles to coagulate (Heinsalu & Veski, 2007), clearing the water for pelagic primary production. The resulting marine pelagic ecosystem included some new biological keystone species. The diatom Thalassiosira baltica bloomed and created wharves in the sediment that indicate the occurrence of a vernal diatom bloom. As the bloom settled to the bottom, it sustained a benthic deep-water ecosystem (cf. Heinsalu & Veski, 2007).

After seawater flowed into the YS, it presumably moved in a counterclockwise manner, first southwards in in the Baltic proper, and then east and northwards to the Gulf of Finland. Concerning salinity, some preliminary hypotheses can be presented, following Heinsalu and Veski (2007). Seawater entered into the present northern Baltic proper, and a critical surface water salinity level of 5–7 PSU was found in the Gulf of Finland, the Bothnian Bay, and the Baltic proper. This is approximately the salinity in which the minimum (number) of species is found in the Baltic Sea (Remane & Schlieper, 1971). Bottom water salinity reached 10 PSU in the northwestern Baltic proper, and there are signs that a salinity stratification developed, a condition conducive to the development of anoxia (Sohlenius et al., 2001). With freshwater input from the glacier in the north, the YS ecosystem combined a northern glacial fjord-type system with seawater connection, and a freshwater-dominated southern part with food chains originating from the BIL and probably with continued species immigration. The picture is complicated by temporal changes in the ecosystems (cf. Heinsalu & Veski, 2007). The diatom records indicate freshwater conditions from around 11,000 to 10,700 BP. The diatoms in YS fall in three indicator groups: brackish-water, a large lake, and a lagoonal/small lake (Heinsalu & Veski, 2007), indicating that the initial freshwater glacial conditions were replaced by more brackish conditions for a relatively short period (60 to 200 years). Finally, however, land rise blocked the marine connection and the freshwater ecosystem was re-established.

Water Rising and Warming: The Ancylus Lake and the Initial Litorina Sea

Climate change toward milder, boreal conditions continued during the Ancylus Lake (AL, which lasted for around 2,200 years, after ca. 10,700–ca. 8500 BP). This period started with increased river runoff (e.g., Witek & Jankowska, 2014) and nutrient leakage, as well as diminishing winter ice which lasted for about 1,000 years, after which a transition started which is called the Initial Litorina Sea, and that lasted until 8500 BP. The glacial boundary retreated from the area of the modern boreal zone in Finland, and temperatures at latitude 70° N were close to modern (annual mean −3°C to 0°C, Heikkilä & Seppä, 2003). In central Sweden, pollen analysis indicates summer temperatures of 10.0°C–12.0°C between 10,700 and 9000 BP. During this period, pine forests were spreading in the Baltic Basin, as were spruce, Picea abies; birch, Betula spp.; poplar, Populus tremula; and gray alder, Alnus incana. The level of the Ancylus Lake continued to rise until the creation of a new drainage channel in the southern Baltic caused an abrupt water level drop. Over time, the southern part of the Baltic Sea catchment gradually flooded, while its northern part continued to rise, with new archipelagos born in present lake areas in Sweden and Finland (cf. Witek & Jankowska, 2014). This transformation of the AL is clearly seen in the diatom record.

The freshwater gastropod Ancylus fluviatilis, after which the AL is named, is not an ecological keystone species but was found in littoral freshwater ecosystems in the eastern and northern AL (cf. Witek & Jankowska, 2014). It is typical of running fresh water with high oxygen concentrations. Lists of freshwater species from sediment sampling document the presence of littoral freshwater ecosystems quite similar to those currently found around the Baltic Sea. The first evidence (Königsson & Possnert, 1988) of the presence of these littoral freshwater species in the Baltic Sea date to the previous stage, the marine YS (cf. Mörner, 1995).

The early AL provides first evidence of a complete pelagic food chain with top predators (seals and humans at the top, as the Mesolithic Sindi-Lodja archaeological site in Estonia revealed the presence of ringed seal in the diet of its human population). This site was located on a riverbank away from the seashore, but the inclusion of ringed seal (a relict species) in the diet indicates hunting on the sea ice. The marine food chain most likely included not only phytoplankton primary producers but also herbivorous and carnivorous zooplankton and pelagic fish as steps below the seals and hunters (Kriiska & Lõugas, 2009). Evidence from littoral Mesolithic human populations in south Sweden (Hansson et al., 2016) also indicates widespread use of freshwater ecosystems, but no strong connection to open sea food chains.

Sediments of the AL are homogeneous clays and indicate an oligotrophic environment with low carbon accumulation and a diatom flora typical of large clear-water lakes. There is no evidence of salinity stratification. A water level rise by around 10 m during the AL flooded southern pine forests and increased organic carbon in the sediment. The continued water level rise created the big lake areas of Sweden and Finland, which in the early stages appeared as a huge lake archipelago (see Figure 3), probably enhancing the importance of the littoral ecosystem, as typically indicated by biological analyses of the AL. While there is much evidence of the presence of a well-developed littoral ecosystem in the AL, it seems that its open waters were still typical of a large oligotrophic clear-water lake.

Post-Glacial Baltic Sea EcosystemsClick to view larger

Figure 3. Extensive archipelagos of the northern AL characterized the aquatic ecosystems. Map by Terri Vuorinen, based on Tikkanen and Oksanen (2002) and Björck in BACC (2008).

Paleolimnological diatom evidence from the Baltic Sea is extensive, as the first studies were done already in the 1920s. Witek and Jankowska (2014) describe how Southern Baltic diatom studies show shifts between species that indicate environments of different salinities (as well as planktonic vs. benthic, pH, and saprobic conditions). The AL environment was regulated by fluctuations in water level and runoff that caused alternating transgressions and regressions, for example in the Saaremaa Islands (Saarse et al., 2009).

Toward the end of the AL, climate cooling again decreased runoff from the Baltic, allowing North Sea water to penetrate the Danish Straits. Hyvärinen (1984) describes the start of the transition to the Litorina Sea on the basis of brackish-water diatoms appearing in Ancylus strata. They are distinguished by the presence of a sparse, weakly brackish component (species such as Rhoicosphenia curvata, Nitzschia tryblionella, Campylodiscus echeneis, and C. calypeus, and the characteristic genus Mastogloia), all taxa common in the littoral of the subsequent Litorina Sea and the present Baltic Sea.

This has been taken to reflect a gradual penetration of the Baltic Sea basin by seawater after the opening of the Danish Straits as a result of the rise in the level of ocean. Toward the end of the AL and the Initial Litorina Sea, there is a cooling of climate, simultaneous with flooding of the American ice lake, Lake Agassiz, into the Atlantic, with an effect on the Gulf Stream (Palter, 2015). Climate cooling again decreased the runoff from the Baltic, making way for North Sea waters to penetrate the Danish Straits. An increase in marine diatoms around or shortly after 7500 BP marks the beginning of the Litorina Sea proper.

Sea Level Rise Versus Climate Warming and Increased Runoff: The Litorina Sea—Developing and Future Ecosystems

The high salinity period of the Litorina Sea (LS) lasted around 4,500 years (ca. 7500–3000 BP), during which the Danish Straits were considerably broader and deeper than at present, as the sea level was relatively higher, facilitating seawater inflows. A salinity stratification developed at about the same depth as in the Baltic Sea. The surface salinity of the open Baltic Sea proper was between 10 and 12 PSU. The name-giving mollusk, the common periwinkle, Littorina littorea, was found above 10 PSU (its distribution changing over time). The distributions of keystone species, such as the wracks, blue mussels, eelgrass, and other components of the littoral ecosystem, resembled those still present in the Danish Straits area. At the time of the LS, the Baltic basin ecosystem definitely had a more marine character than today. However, remote parts such as in the Gulf of Bothnia developed more freshwater character. As land rise continued, the sills to the Gulf of Bothnia and Riga became shallower, gradually excluding Baltic Sea bottom water from entering these gulfs. Unless sea level rise increases, land rise will eventually cut off the Bothnian Bay and create another big freshwater lake, similar to Lakes Ladoga and Onega.

The initial phase of the LS, global sea level rise, caused increasing nutrient-rich seawater inflows from the south, while central parts were still heavily influenced by fresh water. The seawater inflows favored the development of a halocline and stagnation, and deepwater oxygen deficiency followed. This released phosphate from bottom sediments, which together with the nutrient increase due to the inflows triggered the first cyanobacteria blooms. Following the global warming climate trend, climate eventually became somewhat warmer than in modern history, as evidenced by pollen records of pine in Finland and of spruce and deciduous trees (Ulmus, Tilia, Quercus) in Russia and Sweden.

As the entrance of seawater and the salinity values of incoming water were similar to the present-day Baltic Sea, salinity isolines (imaginary horizontal lines of same salinity in the surface) were probably also similarly oriented, except that the area of critical salinity, between 5 and 7 PSU, must have been located further north and northeast during the salinity maximum. This would have given marine species a broader distribution area, and the ecosystem as a whole a more marine character, than in later years. The (regional) land rise in the north, however, continued, and eventually the sill between the northern Baltic proper and the Åland Sea became shallow enough to restrict bottom-water inflows to the Gulf of Bothnia, this reduced deep-water salinity there, allowing species of less marine character to extend their distribution.

The physical circumstances—water volume, drainage area, entrance, and the predominating westerly winds, including direction and activity of cyclone storm tracks associated with the North Atlantic Oscillation (NAO, index of the air pressure gradient between Azores and Iceland) and the Gulf Stream—have persisted since the end of the LS and have formed characteristic conditions for the developing Baltic Sea.

The presence of top predators, seals, birds, and humans, in the LS indicate that an efficient pelagic food chain had developed. The effect of humans on the faunal composition is starting to be seen, possibly as early as Viking times. The sand gaper clam, Mya arenaria, was evidently introduced to Europe already at the time of the first Viking settlements in Newfoundland. It is worth noting that the present-day Baltic Sea is sometimes referred to as the Mya Sea. In modern times, the rate of introduction of non-indigenous species by humans (sometimes called “biological contamination”) has increased greatly and has been seen as a threat to the present Baltic Sea ecosystem. The geologically young age of the Baltic Sea could as well explain that for natural reasons many species that could thrive in the Baltic Sea have not yet been established there. Thus immigration of new species is just a succession of a developing ecosystem (further discussion below).

The Present Baltic Sea, the Mya Sea—Discussions of the Structure and Function of the Marine Ecosystem

Around 3000 BP, the Baltic became significantly more stagnant and the development toward the present stage started, with a continuously sinking relative sea level.

A Unique Species Composition—Pike and Jellyfish Swimming Together Among the Bladder Wrack—But Naturally Low Species Diversity and Therefore Highly Vulnerable to Environmental Change

Despite a variety of origins (molecular studies show that some Baltic species are of Pacific origin, for example Mytilus and Macoma (now Limecola) balthica (Väinölä & Nikula in Snoeijs-Leijonmalm, Schubert, & Radziejewska, 2017, p. 240)) and that the speed of evolution can be higher than previously realized (e.g., Väinölä & Nikula in Snoeijs-Leijonmalm, Schubert, & Radziejewska, 2017; Momigliano et al., 2017).) Still, the number of species found in the Baltic Sea is low. Freshwater species have reached the Baltic via rivers and continue to do so. Many of them are confined to littoral and shallow water, such as the perch (Perca fluviatilis L.), roach (Rutilus rutilus), and pike (Esox lucius L.), which thrive among the reed beds and near freshwater sources, but can also be very numerous among truly marine flora, for example in the bladder wrack (Fucus vesiculosus L.) zone of rocky coasts in Sweden and Finland. The river-mouth ecosystems in the Baltic Sea are especially vulnerable to land-based eutrophication; there are hardly any pristine river-mouth ecosystems left in the Baltic proper due to intensive agriculture and other human activities.

In addition to impoverished faunas of arctic and North Sea origin, relicts (Segerstråle, 1966) from previous glaciations are a special group of animals in the Baltic Sea. Examples are found also in North European great lakes. They are found in relatively cool, deep water, or in the benthos, which makes them particularly vulnerable to deepwater anoxia. Radiation, light, and temperature vary seasonally in the Baltic and influence the composition of Baltic Sea fauna. The regular ice cover excludes harbor porpoises (the only whale common in the Baltic Sea) from the Gulf of Bothnia but is a prerequisite for reproduction of ringed seal, Pusa hispida L.

It is often said that its geological youth explains the low biodiversity of the Baltic Sea. The ideas that few species have had time to evolve in the Baltic Sea, and that many species that could have a niche in the Baltic have not yet arrived here, are subject of discussion. For example, studies of opossum shrimp and flounder have demonstrated several cryptic species (species that show little or no morphological interspecific differentiation but have lived long enough in reproductive isolation to evolve genetic differences) previously unknown to the Baltic Sea. Furthermore, new species arrive regularly in the Baltic Sea, almost one new species per year since the 1990s. They come mainly with increasing traffic on traditional waterways; the river and channel systems in the east and the sea traffic in the west. The naturally low biodiversity of the Baltic Sea is therefore increasing rapidly. A striking example was the arrival of the American comb jelly, Mnemiopsis leidyi, in media named the “killer jelly” due to its devastating effect on the pelagic ecosystem after its earlier introduction to the Black Sea. It has, however, stopped short of the northern Baltic proper, for unknown reasons. The discussion, however, focused attention on another relict species, the arctic comb jelly Mertensia ovum, which appears to have been long present in the Baltic Sea, but was earlier erroneously identified as the Atlantic species Pleurobrachia pileus.

Biodiversity in the Baltic Sea can be considered particularly vulnerable, since many of the species that make it up live at the boundary of their distribution, and in some cases just a few species represent a whole phylum. Examples could be priapulid worms and the comb jellies. It is implicitly assumed that with a low biodiversity comes low resilience, since after a hypothetic catastrophe, there are few species with which to start the recovery of the ecosystem. If the Baltic Sea lost two species of priapulids, a whole phylum would be lost, and the new, recovered ecosystem would be less diverse.

Salinity Sets the Limits For Fauna and Flora Distribution, Thus Regulating Biodiversity

Maintaining the osmotic pressure inside its cells in brackish water is stressing for freshwater and marine biota and often limits their distribution. Baltic Sea salinity increases horizontally, from almost fresh water in the river mouths to about 25–30 PSU in the bottom water of the Danish Straits, and the number of marine species increases in parallel. Vertically, marine species are more common at depth, where the salinity is higher. In the Baltic Sea the area with lowest number of species is between 5 and 7 PSU, which in the Baltic Sea is found north of Gotland and may be called the critical salinity range. Present understanding is that the low number of species between 5 and 7 PSU does not hold for all taxa, since some predominantly microscopic groups show high species numbers in this range. Therefore, the mechanism preventing species from adapting to the salinity of the Baltic Sea is not likely the lack of osmotic regulation capacity as such, but the relatively long generation time. Biota with short generation times (relative to environmental change) are more likely to adapt successfully and contribute to higher biodiversity. In the Baltic Sea, this has been claimed for smaller (micro-)plankton (Telesh et al., 2013, 2015). Evidently, much Baltic Sea biodiversity is still hidden in small taxa with generation times that are short in relation to environmental variability.

Not only the distribution range but also the growth of many marine species, even keystone species, is negatively influenced by low salinity. Examples of reduction in size in the Baltic Sea include the mussel, Mytilus edulis L., and the Baltic herring, Clupea harengus membras L. It is noticeable that reduction in size due to low salinity is gradual, extending over hundreds of kilometers. Such size diminution is not, however, a universal phenomenon; it seems absent, for example, in microfauna, and macroalgae such as the bladder wrack decrease in size only near the boundary of their distribution (Remane & Schlieper, 1971). Salinity changes can cause wide fluctuations of abundance; for example, the Baltic Sea cod stock has been reduced since the 1980s due to a drop in salinity combined with overfishing, and consequently sprat, its main prey, has flourished (Rudstam et al., 1994). Since the 1980s, the Baltic herring has also shown likely effects of decreasing salinity, for example decreasing weight-at-age and stocks (the latter again aggravated by overfishing, as for cod), leaving sprat as the dominating pelagic fish species in the Baltic Sea in the early 2000s. As the salinity is regulated by climatic factors and the balance between runoff and inflows largely determines the future salinity, we may deduce that salinity control and the future salinity of the Baltic Sea will remain a crucial factor determining the composition and function of the biota.

The Trophic Status of the BS is Naturally Low, Thus Eutrophication Will First Enhance Production, But Eventually Lead to Functional Changes and Loss of Productivity

Runoff, like salinity, is a key to understanding eutrophication of the Baltic. It has been thoroughly demonstrated in the reports of the Baltic Marine Environment Protection Commission (the Helsinki Commission, or HELCOM) that loading of nutrients follows closely the total runoff from rivers. The total runoff has also been shown to regulate the nutrient concentrations in the surface layer (and down to the average depth) of the Baltic Sea (Voss et al., 2011; Hänninen & Vuorinen, 2015). A common view is that the increased rainfall due to anthropogenic climate change will most likely aggravate eutrophication in the coastal areas. In a larger scale this can be discussed, since runoff in the north may increase the transport of relatively nutrient-poor water from north to south, while southern areas are expected to have less rainfall in summer times, which would reduce the loading.

As the Baltic Sea has a naturally low nutritional status, eutrophication will generally first lead to increased production. Due to the stratification of the Baltic proper, eutrophication will stimulate bottom-water anoxia, leading to a vicious circle (nutrient release from anoxic sediments will increase algal blooms, and sedimenting dead algae will in turn increase anoxia) and consequent blooms of cyanobacteria in the open Baltic Sea. Thus, eutrophication has been considered the most severe environmental threat to the Baltic Sea ecosystem. Nutrient inputs to the pelagic ecosystem feed the eutrophication itself, aggravating plankton blooms, turbidity of water, and self-shading (which reduces phytoplankton production). Finally, this leads to simplification of the food web and functional losses. There is also concern that jellyfish production may increase at the expense of copepods, pelagic fish, and top predators. Furthermore, in the benthic vegetation annual filamentous algae tend to be favored at the expense of perennial plants. This will shift the nutrient budgets from an oligotrophic, clear water environment, where the nutrients are stored in large macrophytes and circulated slowly, to eutrophic conditions, where nutrients are mainly found in rapidly reproducing and recycling planktonic algae, for example cyanobacteria. In the littoral zone, eutrophication favors small epiphytes over macrophytes and mollusks over crustaceans. The increased turbidity leads to self-shading of phytoplankton and decreasing benthic primary production, thus affecting both littoral benthic (Tolvanen et al., 2013) and pelagic ecosystems. Due to persistent stratification of the water, large volumes of bottom water become anoxic, loaded with hydrogen sulfide and devoid of macroscopic life (the central deeps of the Baltic Sea have been called “the largest desert in Europe”). The deepwater ecosystem of the Baltic Sea, which in the 1920s supported a commercial fishery, has been lost (Elmgren, 1989) for the time being.

The Baltic Sea ecosystem has been characterized as “poor in species but rich in individuals.” This is reflected in the fish catch, which makes up almost 1% of the world’s total fish catch. Fisheries have been central in the human use of Baltic Sea resources. Historically, the rise of the Hanseatic League in northern Europe was largely based on the herring fishery. Globally, almost all major fish stocks are currently overfished, and there is cause for concern also in the Baltic Sea. Still, herring in the Gulf of Bothnia is considered to be within safe biological limits, due to a relatively small fishery that respects the fishing quota and an abundant alternative source of food in the glacial relict copepod, Limnocalanus macrurus, which is not as abundant in the Baltic proper. HELCOM claims that overfishing is less severe in the Baltic Sea than in other European waters, but protective measures are still needed to ensure a sustainable use of the stocks (http://www.helcom.fi/action-areas/fisheries/management/baltfish/).

Strong Human Pressures Combined With Physical and Geological Characteristics Make the Baltic Sea Ecosystem Vulnerable

Its small water volume, restricted water exchange, stratification, and practical lack of tides, combined with a relatively large human population and intensive agriculture, make the Baltic Sea ecosystems especially vulnerable to environmental change.

Anthropogenic influence is strong in the Baltic Sea, as clearly seen not only in fisheries but also in contaminant concentrations. The concentrations of DDT, DDE, and PCBs in biota have declined steeply since their use was banned, but levels of these and many other contaminants are still several-fold higher than in the North Sea (BACC, 2008). The toxic pollution was at its most severe in the 1970s, when it hampered the reproduction of top predators, such as seals and white-tailed eagle, the populations of which had already been reduced by hunting. They have since recovered due to reduced contaminant levels and bans on hunting. The human population in the Baltic catchment is ca. 85 million, almost 20% of the population of the European Union. This, combined with the shallowness of the Baltic Sea basins (the average depth of the Baltic Sea is only some 55 meters) and the lack of tides, makes the ecosystem especially vulnerable. The preliminary 2017 HELCOM Environmental assessment states: “Nutrient inputs from land have decreased clearly, but effects are not yet reflected in the status of all sub-basins. The contamination status is elevated in the entire BS, but some improving trends are seen. Biodiversity status is inadequate for most assessed species, and continued efforts to support biodiversity are of key importance” (http://stateofthebalticsea.helcom.fi/).

It is urgent to understand the dual nature of environmental change in the Baltic Sea. On one hand, its ecosystems are still recovering from the last glaciation; on the other, there is rapid anthropogenic change and, unfortunately, these changes push the sea in the same direction: eutrophication. This process has been going on for a long time, but it seems impossible to say for how long, since modelers project changes in the future rather than in the past.

During all the stages described here, the environmental factors for the aquatic ecosystems (such as the temperature, salinity, and nutrient levels) ultimately reflect climatic conditions at global (spatial and temporal) scale of advancing or retreating glaciers. Proximately (regionally), they are regulated by temperature (depth of the thermocline, i.e., lower limit of the surface layer where the sun’s radiation is illuminating and warming the water, and where primary production is possible) and runoff (affects the halocline, or salinity stratification of the water; the redoxcline, where oxidation state changes most rapidly; and the nutricline, where nutrient concentrations change most rapidly), both of which may temporarily deviate from global trends.

Starting roughly from the 1970s, all big environmental changes (decreasing salinity, changes in the nutrient budgets and the pelagic food chain structure and function) indicate a recent system change of the Baltic Sea ecosystem. Furthermore, several prominent changes have coincided with increased runoff. They may be directly or indirectly connected as follows: increasing rainfall and runoff decreased the salinity, which happened roughly during and after the 1970s (Hänninen et al., 2000); eutrophication appeared to increase with increasing runoff as surface water total P-concentrations increased semi-simultaneously; and both the runoff and nutrients remained at an elevated level afterward (Voss et al., 2011; Hänninen & Vuorinen, 2015). The collapse of cod stocks and substantial increase of sprat also followed the increased runoff and decreasing salinity (BACC, 2008), as did the decrease in herring growth and change in zooplankton composition (Vuorinen et al., 1998; Flinkman et al., 1998; Rajasilta et al., 2006). A comprehensive modeling has apparently not been done so far, but it may be further derived that in the future northern Baltic Sea the land rise will continue, as will the land sinking in the southern Baltic Sea. Both can be overshadowed by an increasing sea level rise in the southern Baltic Sea due to climate warming, which may facilitate MBIs.

The specific ecological sensitivity of the Baltic Sea ecosystem can thus be described in terms of a small volume of water, restricted water exchange, a relatively large runoff area, and a close dependence of the variation in the quality and amount of incoming water. Practically all of the incoming water to the Baltic Sea area is derived from the Atlantic, and both the MBIs and the fresh water are coming from a westerly direction. Thus the climatic characters of the geographic area that supplies the water are crucial in regulating the aquatic ecosystem of the Baltic Sea. In addition to the above-mentioned regional environmental changes, and perhaps more importantly, the future of the Gulf Stream and the NAO in a changing climate will dictate the characteristics of future Baltic Sea ecosystems. Even if milder winters with increasing rainfall can lead to freshening of the water, and the global sea level rise can have an effect on salinity, stratification, and so on, the crucial factor will still be the air temperature and pressure over the Atlantic and Barents Seas, that is, the fate of the Gulf Stream and the NAO. The Baltic Sea is situated at the latitude of the Arctic Ocean, with the cities of Helsinki, St. Petersburg, and Stockholm all at about the latitude of the southern tip of Greenland. If the prevailing southwesterly air and seawater currents weaken, temperatures in the Baltic Sea area will again fall closer to arctic ones.

Further Reading and Acknowledgments

This presentation follows three recent textbooks on Baltic Sea science: BACC (2008), BACC 2 (2015), and Snoeijs-Leijonmalm, Schubert, and Radziejewska (2017). Several persons have contributed to early versions of this text by commenting or with publications; special thanks to Profs. Veli-Pekka Salonen, Irina Telesh, and Marek Zajączkowski. The anonymous reviewers suggested valuable edits and some literature for further reading. The book The Baltic Sea Basin edited by J. Harf et al. (Springer Verlag, 2011) should be mentioned, especially chapter 4 by T. Andrén et al. Furthermore, Sven Ekman’s Zoogeography of the Sea (Sidgwick & Jackson, 1953) is a basic textbook of relevance to the topic.

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