sea ​​heat waves in Mediterranean Sea lead to mass mortality of marine species. This alarming finding is the first time after five years of study by 30 research groups from 11 Mediterranean countries. Researchers also Greek scientistswhich record the first catastrophic consequences of rising sea temperatures in the southern Aegean.

A study published a few days ago in scientific journal Global Change Biology, shows that in the period 2015-2019. successive sea heat events have caused the mass death of at least 50 marine species up to 45 meters below the surface. Heatwaves have affected 90% of the Mediterranean, from the Aboran Sea to the coast of the Aegean and Levantine Seas.

From the Greek side, from the Department of Oceanography and Marine Life Sciences of the University of the Aegean, Professor Stelios Katsanevakis and PhD Research Fellow Maria Sini and Associate Professor of the Department of Environment at Ionian University Vassilis Gerovasilou. The multinational research team was coordinated by researcher Joaquim Garrabu of the Spanish Institute of Marine Sciences.

“As it turned out as a result of research, depending on the intensity and duration of thermal phenomena in the Mediterranean, the frequency and intensity of mortality of marine organisms increased.” says Mr. Katsanevakis to K.. “The ‘stable’ organisms, that is, those that do not move to avoid high temperatures, such as Posidonia grasslands and soft corals known as gorgonians, suffer the most. Benthic organisms are also affected, such as gastropods such as sea snails. Fish that can move tend to go deeper these days.”

The impact of rising temperatures on marine organisms varies. “It is important to understand how different species respond to different levels of heat exposure. However, this is particularly challenging due to the fact that marine species respond differently to exposure to extreme conditions at different spatial and temporal scales,” says Mr Gerovasiliou. The high frequency of sea heat waves does not allow nature to recover. “In recent years, these incidents have been continuous and often long in duration and intensity, covering much of the Mediterranean. As a result, populations of various organisms do not have time to recover, which leads to local extinctions. For example, for several years the edible sea urchins, which we also know in Greece, disappeared from the shores of Israel, which did not seem to withstand high temperatures. On the contrary, an invasive species of sea urchin with larger spines, which is inedible, is spreading in our country.”

quick change

As a result, the changes taking place in the Mediterranean during this period are very fast. “The mass die-off in the Mediterranean is a phenomenon similar to coral bleaching, which has been repeatedly recorded and destroyed large areas of the Great Barrier Reef in Australia. Therefore, these phenomena, unfortunately, do not concern one-time exceptions, but are already part of the new reality in the context of climate change,” emphasizes Ms. Sinis.

The increase in the temperature of the Mediterranean Sea could not but affect the Greek seas. “The usual maximum summer temperature in Kastelorizo ​​is 29.3 degrees Celsius, in Crete 27 degrees, and in the islands of the northeast Aegean it does not exceed 25 degrees. In case of sea heat, these prices are exceeded,” says Mr. Katsanevakis. “Rising sea temperatures, combined with overfishing and the spread of invasive species, are causing seafloor desertification in many places, especially at shallow depths. To explain the role of the invaders, two alien fish species, the German and wild salps, are herbivores: they deprive the reefs that are the core of marine biodiversity, supporting many species of flora and fauna. Thus, the reefs gradually turn into bare rocks, devoid of life. In contrast to the South Aegean, the situation in the North Aegean is currently better. “The seas are colder because of the latitude, and also because of the Black Sea, which is colder than the Mediterranean. In addition, there are fewer alien species. That is why the seas there today are the richest in biodiversity in our country.”