The Effect of Climate Change on the Mediterranean Sea

The Mediterranean Sea is a sea with specific identifiable water masses in each sub-basin and at different depths that have been well recorded. The Mediterranean sea was chosen as the final analysis of ecosystems in this paper because of its wide biodiversity and scientific opinion of being a “miniature ocean” by physical oceanographers (Myers et al. 2000). The circulation of water through the Mediterranean is determined by incoming water from the Gibraltar straights (Atlantic water) and the sinking of waters caused by influx from the three coldest areas of the sea; the northern Adriatic, the Gulf of Lions, and the North Aegean sea. A study performed by Lejeusne et al. (2009) examined the past thirty years and discovered that the traditional flow of water in the Mediterranean Sea has been disrupted due to climate change. Deep-water temperatures from the northwestern Mediterranean have been experiencing a general warming of 0.0048 °C per year since the 1980s. The Eastern Mediterranean transient has encountered temperature and salinity increase as well as increased stratification in the water column. Further research has shown that the Eastern Mediterranean Transient has begun to affect the water circulation and species ecosystems there as well. At shallow depths two types of climate-driven effects have been observed: a warming trend and an increase in the frequency of anomalous events like storms and anomalous annual temperature spikes. Rosemary Kulp
Myers, P.G., Mittermeier, R., Mittermeier, C., Fonseca, G., Kent, J., 2000. Biodiversity hotspots for conservation priorities. Nature 403, 853–858
Lejeusne, C., Chevaldonne, P., Pergent-Martini, C., Boudouresque, C., Pe´rez, T., 2009. Climate change effects on a miniature ocean: the highly diverse, highly impacted Mediterranean Sea. doi:10.1016/j.tree.2009.10.009.

The organisms of the Mediterranean Sea are divided into three biogeographical provinces, the western basin, the eastern basin, and the Adriatic Sea, with each province subdivided along latitudinal patterns (east-west, north-south). Subtropical species are more abundant in the southern parts of these provinces while more temperate species dominate the northern parts. Increasing sea surface temperature and higher frequency of storms has lead directly to an increasing abundance of thermotolerant species and the disappearance of stenothermal species acclimated to colder temperatures. Not surprisingly the major shifts in stenothermal populations have occurred in the colder climate of the Northwestern Mediterranean, the coldest area of the Mediterranean Sea. An example of the changing biodiversity occurring in the Northwestern Mediterranean sea involves the Hemimysis speluncola, a cave-dwelling species which collapsed and re-distributed Northward under the temperature anomalies of the 1990’s. Subsequent studies confirmed a higher thermotolerance for the newly dominant H. margalefi compared to the cold stenothermal H. speluncola, unable to move north and now restricted to the cold regions of the Gulf of Lions and the northern Adriatic. An important note to consider is that current geographical context of the Mediterranean Sea makes it impossible for shallow-water temperate species already trapped in the northernmost, coldest parts of the different basins to migrate or disperse northwards to mitigate temperature changes which will lead to large scale extinction of these trapped species as the waters continue to warm. Significant range expansion is also occurring with two-thirds of the mobile species recorded, as warm loving species, like the ornate wrasse Thalassoma pavo andorange coral Astroides calycularis, found usually along the eastern and southern shores of the Mediterranean Sea, have been recently recorded distributed more north and westward over the decades (Lejeusne et al. 2009).
 The expansion of thermophiliac species from the eastern Mediterranean into the northwestern Mediterranean is important because of the additional organisms introduced through the Suez Canal from the Red Sea. Most of the species introduced into the Mediterranean are of tropical origin, and have been traditionally confined to the easternmost Levantine shores, but the warming of the Mediterranean allows them to progress westwards and northwards, through the whole eastern basin, with some now reaching the Adriatic and the western basin (Galil and Zenetos 2002). It is expected that the southern parts if the Mediterranean Sea will become more colonized by sub-tropical species from the Southern Atlantic and Red Sea, while the Northern parts of the Mediterranean will be populated by current southern and eastern native species of the Mediterranean (Lejeusne et al. 2009).
Climate change and subsequent increases in the severity and frequencies of storms and anomalous temperature spikes can exceed organisms range of thermal tolerance, which leads to diseases and mass mortality. Mediterranean temperate sessile invertebrates such as sponges and corals experienced unprecedented disease outbreaks and mass mortalities within the past decade. After being initially affected, bare skeletons become exposed and are then colonized by microorganisms and eventually macroscopic organisms. Colonized skeletons can remain attached for years, unless detached by storms, and the slow regeneration rate of corals and sponges cannot counterbalance immediate and delayed effects of disease outbreaks, dramatically altering benthic and coastal seascapes within the Mediterranean Sea (Lejeusne et al. 2009).
Since the Mediterranean operates under low levels of nutrients, the benthic ecosystems are under strong nutritional forcing with the summer season traditionally carrying the lowest levels of nutrients. With increased temperatures, the combination of thermal stress and food shortage results in mass mortality events will likely disrupt summer season benthic–pelagic coupling. Cascading effects will likely contain planktonic communities, especially copepod assemblages, which have a strong influence on pelagic ecosystem fluxes due to their role of a biological carbon pump to deeper waters. Plankton populations impact fish recruitment in Mediterranean marine ecosystems and will certainly be influenced by the arrival of new species in these ecosystems, possibly disrupting or completely altering current ecosystem functioning (Lejeusne et al. 2009).
It is important to note that marine invertebrates and fish are not the only species concerned by such expansions. The last decade has seen increasing reports of toxic dinobionts in coastal areas, with possible consequences on human pathology. These dinoflagellants are thermophiliac, exotic species like Gambierdiscus toxicus, and the main causative agent of ciguatera poisoning[1], normally has a tropical or subtropical distribution but have been recently reported from Crete (Lejeusne et al. 2009). Range expansion of the palytoxin-producing dinobiont Ostreopsis ovata in the Mediterranean have also been documented, with definite consequences on human health documented (Kermarec et al. 2008) For both cases increases in water temperature are believed to be the direct result of these species proliferation in the Mediterranean.

In all four ecosystems climate change has resulted in species endangerment with the understanding that barring genetic adaptation the biofood web range-extension northward of current mobile species will occur as stenothermal species that cannot migrate into colder water will die out and ecosystems are reconstructed with invasive themroresiliant species. Increasing sea levels are expected to invade on current real estate properties as primary plant producers and nursery grounds for many commercially valued fish and shrimp flood with sea-water. Further research is required to adequately prove these theories, but based on these analyses a clear north-south gradients is revealed with the impact of invasive species being a key factor to future ecosystem predictions. Another clear issue to consider is the implications of warmer water on the proliferation of potentially harmful bacteria pathogens and microbial ecosystems in relation to human health and commercial consumption of tropical and subtropical fish.

[1] Ciguaterra poisoning is a nonbacterial food poisoning that results from eating fish contaminated with the ciguatoxin, produced by dinoflagellates such as Gambierdiscus toxicus which live in tropical and sub-tropical waters. The toxin is believed to block acetylcholinesterase activity and characteristics of poisoning include vomiting, diarrhea, tingling or numbness of extremities and the skin around the mouth, itching, muscle weakness, pain, and respiratory paralysis. No specific treatment has been developed.

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