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.
The Baltic Sea catchment area extends from the upper course of the Elbe in the Czech Republic to northernmost Lapland where the Tornionjoki river (Sw. Torneälven = Lake Torne) marks the border between Finland and Sweden today. This article concentrates on the coastal regions of the sea and discusses a mutual dialogue between climate and man. Oxygen isotope 18O and hydrogen isotope 2H in the layers of polar ice sheets indicate climate change in the time span of thousands, even tens of thousands, of years. In northern areas, much climatologic information is based on polar ice drilling data from Greenland. The influence of climate changes on human subsistence is clearly visible in pollen data from the numerous ponds and swamps in the Baltic Sea coastal zone. Accelerator mass spectrometry (AMS) dating of carbon remains in archaeological materials (such as the crusts of ceramic pieces) are used to build detailed chronological sequences. Human adaption to conditions dictated by nature is usually interpreted as innovation and progress in prehistory. But numerous raw materials, once used, cannot be replaced, while land exploitation is often followed by side effects such as erosion and eutrophication. For example, the Neolithic “revolution”—the beginning of crop cultivation and large-scale cattle breeding—is an example of such changes in southern Europe from ca. 6,000 bce. Between ca. 200 bce and 100 ce, during the Early Iron Age, the climate was relatively warm here. Local iron production expanded in the Baltic Sea region and allowed effective slash-and-burn crop cultivation for the first time in prehistory. Since then, human activity has caused damage to forests all around the Baltic Sea. A colder phase followed in 100–600 ce. Even today slight changes in annual temperature have great impact on subsistence in areas with harsh climate conditions, such as close to the Polar Circle. An abrupt and radical fall of temperature surely caused severe difficulties. Hunter-gatherers had to find secondary food resources while societies which were strongly dependent on one single base for economy, like agriculture, had even greater difficulties. In the southern part of the Baltic Sea sphere, considerable areas of land were under cultivation at that time. Harvest failures led to famines. A climate catastrophe, probably caused by volcanic eruption, adversely impacted urban, peasant, nomadic, and hunting populations all over the northern hemisphere in 535–536 ce. Recent archaeological studies and AMS samples have proven there was a demographic crisis in the northern part of the Baltic Sea. Soon after 600 ce, the climate became milder again, and the following centuries were warmer than almost any period during Holocene: the warm phase from 800 to ca.1050 ce perfectly matches a historical and archaeological era: the Viking Period. The Middle Ages and early post-medieval period were relatively mild and human-friendly times. But this was followed by the so-called Little Ice Age, dated approximately to 1275–1870. With the beginning of industrialization in mid-19th century, human impact on climate became obvious all over the globe, and the Baltic Sea region is no exception.