The Baltic sea in geological terms is a young (10,000 years ago) but is the largest brackish water ecosystem in the world, with its 415.000 km2 and an average depth of 52m (range between 25 and 459m). it is formed by different seven sub-basins in terms of surface, volume, depth and salinity. It is relatively closed sea, open to the North Sea and Atlantic Sea through two straits near Denmark (Kattegat and Skagerrak). In this zone, the salinity is higher than the rest of the sea, more than 30 practical salinity units. In the east coast is about 5-7,5 psu and in the north, in front of Finland coast is less than 5 psu. The average salinity is 7 psu, really low compared with the average salinity of world oceans that is 35 psu. There is a positive water balance, because freshwater enters the Baltic Sea from more than 200 rivers, land runoff and precipitation. Due to the fact that is a non-tidal sea, the stratification of the waters and its low renewal (25-40 years), favour pollution problems. As winters are cold with long periods of ice cover, the physical, chemical and biological decomposition of hazardous substances are slow, favouring their concentration and bioaccumulation.
The coastal zone comprises three types of habitats. Near the offshore, the hard bottom communities composed by rocky substratum with a high biodiversity. Located in the North-Eastern, are formed by macroalgae as a keystone specie (its huge abundance play a critical role in maintaining the organization and diversity of the community) and bladder wrack (Fucus vesiculosus). Among fauna species dwell another key specie (providing the filtration of the waters) the blue mussel (Mytilus edulis), herring (Clupea harangus), sprat (Sprattus sprattus) and the perch (Perca fluvialities). The second type is the most predominant, the soft bottoms (mud and sand). They are vulnerable to the mechanical stress of wind and waves action. The common species are the Baltic clam (Macoma balthica), the reeds (Phragmites australis) and sea grass beds (Zostera marina) wich offers stabilization to several species as cod (Gadus morhua). At last, the pelagic community is the habitat with a few species. Phytoplankton as a primary producers, zooplankton as copedods (Acartia sp or Pseudocalanus sp).
The sensitivity of the Baltic sea also resides in its relatively low species diversity referring to few ecological interactions in the food web being more vulnerable to environmental changes. Five is the regular number of levels where the energy flows in Baltic sea food chains. The higher level predators are fishes, seabirds and shorebirds, and marine mammals. Besides the typical grazing food chain, also exist an important microbial food chain that is usually longer but less efficient. To address these topics is difficult due to its multi-species interactions, etc. In general, Baltic Sea biodiversity have been argued to follow the salinity gradient, as it is the main environmental factor defining structural and functional characteristics of aquatic biota, thus biodiversity increase towards south (with a 20-40 times higher biomass of both fauna and flora).
Ecosystem dynamics are altered through a variety of interacting, mutually reinforcing mechanistic pathways, including species’ resource acquisition traits; population densities, etc. It is pivotal to know what are the mainly sources of pollutants in the Baltic Sea area.
Among the biological stressors, are the invasive species (also named as alien, non-indigenous, exotic or invasive). According to Working Group on Introductions and Transfers of Marine Organisms (ICES-WGITMO) reports, there are 121 alien species registered, which 33 of them has been find out that had ecological impacts in the Baltic Sea. Shipping is the most important vector transporting aquatic organism trough the discharge of ballast water o by fouling on hulls. Alien species is a biogeographical category. Alien species are those that have moved beyond their natural geographic range of habitat, of any kind of phyla, from microorganisms to plants and animals. In other terms, they are a subset of established non-native species which have the potential to spread elsewhere, causing significant harm to biological diversity, ecosystem functioning, socio-economic values and/or human health in invaded regions. For example, according to the study of EC 2008 IAS Impact Assessment, alien species causes to fisheries and aquaculture installations, over 162 million euros per year in damage costs. Invasive species often increase pool sizes, for example biomass, and promote accelerated flux rates.
Among the most harmful invaders are a dinoflagellate (Pfiesteria piscicida), an American comb jelly (Mnemiopsis leidyi) and an Asian clam (Corbicula fluminea). But also there are invasive engineering species as the Bay barnacle (Balanus improvises) adn the Conrad’s false mussel (Mytilopsis leucophaeata). Among the abiotic stressors, there are contaminants with different origin and nature but they all remain specially in the sea floor but, also can enter into the food chain, remaining stored in the fatty tissue of animals. It can be summarized as: radionuclides (13Cs and 90Sr. Related with nuclear power plants); heavy metals (steel, titanium, Cd, Fe, Al, Zn, Cu. Related with minery, and metallurgia industries); oil and petrolatum derived hydrocarbons (PAHs, polycyclic aromatic hidrocarbons. Related with refineries and boat spills); chlorinated chemicals (bleached kraft pulp. Related with pulp and paper mills); POP’s (Persistent organic pollutants as halogenated hydrocarbons such PCBs, DDT or dioxins) and nitrogen and phosphorus (causing eutrophication. Related with farming, agriculture and atmospheric deposition (NOx). It promotes phytoplankton and algae blooms, as the summer blooms of cyanobacteria recorded since 1885. Decomposition of these organic phytoplankton matter consumes oxygen, causing hypoxia or even anoxia).
According to the Intergovernmental Panel on Climate Change (IPCC) the global mean surface temperature of 2,4-6,4ºC by 2100. Otherwise the salinity are diminishing. These abiotic parameters structuring the species composition of food webs and biodiversity in the Baltic.