SESAME : Ecosystem Pressures and Stresses

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Ecosystem Pressures and Stresses

Over the past century the population around the Black Sea regions has expanded substantially, and it is, thus, no wonder that human activities have contributed in major ways to the Black Sea?s plight. Pollution of all sorts, invading plant and animal species introduced through human activities (such as shipping) and overfishing have all taken a toll on this once-healthy sea.

The Black Sea environment is fragile and vulnerable. Even in undisturbed conditions, species diversity is three times less than that in the Mediterranean, whereas very few species attain very high abundances. Furthermore, the wide diversity of biotopes provides favourable conditions for invasion of alien species . Since the 1960s, the Black Sea ecosystem has deteriorated from a higher biodiversity ecosystem with rich biological resources to a low biodiversity ecosystem dominated by a dead-end gelatinous food chain. A number of reports focusing on the state of the environment of the area have identified these pressures as: eutrophication through agriculture, industrial activity and inputs of insufficiently treated sewage; contamination through input of harmful substances, and especially oil products; reduction of fish stocks; and introduction of alien species. Fishery overexploitation coupled with adverse environmental conditions, such as manipulation of hydrological regimes of outflowing rivers for example, effectively restructured the food web and impacted the fish stocks of the Black Sea; a well known example is the collapse of the anchovy stocks, one of the most important commercial species in the whole Black Sea as well as other valuable fish commodities. Temperature, atmospheric pressure, wind and river run-off series (1915-1993) were found to be significantly correlated with most of biological, anthropogenic and fish stock indices. Recent evidence from the late 1990s revealed recovery signal at different levels of the Black Sea ecosystem. However, the persistence of some indicators of environmental pollution calls for greater caution. An irreversible collapse of the Black Sea ecosystem may result with additional pressure.

The Mediterranean Sea is less fragile than the Black Sea (oligotrophic sea, having one of the richest flora and fauna diversity of the world ocean); nevertheless it is very sensitive to climate variability, displaying significant local features, which are linked both to short and long-term atmospheric forcing, as well as to teleconnections with global atmospheric events. A number of recent studies based on oceanographic observations have shown that the Mediterranean Sea is in a non-steady-state. A marked long-term warming trend and salinity increase in the deep water of the Western Mediterranean have been detected since 1960 and appears linked to processes of global climatic change. It is interesting that this trend of increasing temperature constitutes the first measured effect of the Green House Effect change. Most dramatically, hydrographic observations, conducted over the last decades in the eastern Mediterranean, have revealed a major transition event (EMT) consisting of a change in the eastern Mediterranean deep-water source that shifted from the southern Adriatic to the southern Aegean Sea.

Anthropogenic and climatic forcing has also impacted the biotic components of the Mediterranean ecosystem. Records of decadal changes in the ecosystem often indicate human effects as the main forcing agent, while climatic forcing, which displays oscillatory variation, is of secondary importance.

The major pressures in the Mediterranean have been identified as: pollution related to urbanisation and industrial activities (e.g. sewage and urban runoff, solid wastes, industrial effluents, eutrophication, harmful algal blooms, hydrocarbon and oil spill pollution); overexploitation of fisheries resources; and invasion of exotic species. The nutrient concentrations and ratios have changed and freshwater discharges have been altered. At the same time, the biodiversity of the area has being undergoing significant change and coastal eutrophication has reduced species richness, favouring increasing appearances of HABs . Exotic species have been introduced and established into the ecosystem mainly of subtropical-tropical origin, favoured by temperature increase (tropicalization of the Mediterranean). In addition, a northern extension of the warm-temperate species against the recession of the cold-temperate ones was observed, suggesting a meridionilization trend. All the above had an impact on the distribution of commercial fish species whereas a significant number of exotic species have become valuable fishery assets in the Levantine area. Uncontrolled fishing had a negative impact on the Mediterranean ecosystem causing overexploitation of fisheries resources and decline of the mean trophic level and structural changes in the fish populations. Many cases of biodiversity loss of non-target species and habitats caused by trawling have also been documented. These biological changes have obvious socioeconomic consequences particularly on the fishing industry and tourism.