Ecosystems of the Baltic Sea Since the Last Glaciation
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Climate Science. Please check back later for the full article.
Holocene aquatic ecosystems such as the Baltic Sea evolved in northern freshwater, brackish and marine environments fringing the former glaciated areas in Eurasia and North America. Their key ecological environmental factors such as temperature, salinity, and nutrient levels are disturbance driven; ultimately regulated by climatic conditions, primarily regulated by temperature and runoff (which affects both the salinity and nutrient concentrations). First studies concentrated on relict species, that is, species that arrived (from both freshwater and marine ecosystems) in the early Baltic Sea, presumably not being common there during the first freshwater phase, the Baltic Ice Lake. The subsequent aquatic environments: Yoldia Sea, Ancylus Lake, and the Litorina Sea are all named after a species of mollusks, characteristic of geological sediment strata of the event in question and indicating physical and chemical characteristics of their living environment.
Indicated large-scale ecosystem developments are from initial glacial turbidity to clearer water with increasing primary production (also enhanced by increasing temperatures), however, eventually leading to self-shading and other consequences of eutrophication today. This could also be characterized as evolution from oligotrophy to eutrophy. From a food chain point of view there has been a shift from the grazing chain to the microbial loop. Thus large top predators (pelagic fish, mammals, and birds) at the end of the chain have lost their previous predominance while filtering top predators in the microbial loop (the jellyfish) have benefited. Another large scale change has been from low (freshwater) biodiversity to increased (marine) biodiversity.
The present-day Baltic Sea ecosystem is a direct descendant of the Litorina Sea. That stage also started the change from primeval to a man-regulated ecosystem characterized by high concentrations of pollutants and nutrients with a change from perennial to annual macrophytes (and associated nutrient budgets), and increased speed of non-native species arriving. Thus, an increasing pace of man-made ecological change could also be named as one large trend in recent the Baltic Sea ecosystem.
The single most important ecosystem driver in Holocene brackish water environments is the runoff, which regulates, for example, the salinity, and consequently the distribution of both freshwater and marine plants and animals. The future of these ecosystems globally depends on the proceeding of the climate change (natural and anthropogenic, of which the latter may exceed in speed all the previous changes). In the Baltic Sea this is likely to depend on regional factors, such as (1) the salinity, which is regulated on, one hand, by the eustatic sea level rise, and on the other hand by isostatic land level changes; and (2) the runoff that controls both the salinity and leaching of nutrients to the sea. However, there are numerous complications, for example, projected increasing windiness may lead to increasing mixing and environmental conditions favoring diatoms instead of Cyanobacteria and the changes due to industrial fishing and land-based pollution and nutrient loading may appear surprisingly fast. Thus, without more sophisticated and extensive cross-disciplinary oceanographic modeling, it would be simplistic to try and see whether the Baltic Sea will develop toward a freshwater or marine direction in coming decades.