Tropical algae are advancing through the Mediterranean at an unprecedented rate

The results of recent research demonstrate that allochthonous marine macrophytes (macroalgae and non-native marine plants) of tropical origin are spreading in the Mediterranean Sea at a much faster rate than marine macrophytes of temperate origin during recent decades. The results of the study suggest that future warming of the Mediterranean could continue to favor the expansion of these species.

The study was carried out by an international team led by the Mediterranean Institute for Advanced Studies (IMEDEA), a joint center of the University of the Balearic Islands (UIB) and the Higher Council for Scientific Research (CSIC), in Spain.

Over the last two centuries, there has been an increase in sea temperature due to global climate change. However, uncertainty has persisted in recent years about whether this increase in temperature really favors the expansion and impact of invasive species in the Mediterranean Sea.

The Mediterranean has become a critical point for the arrival of alien species, those that have been moved beyond their native range due to human activities, due to the extensive maritime routes that connect the Atlantic with the Indian and Pacific oceans. , as well as the opening of the Suez Canal.

These two factors have led to marine macrophytes, which include both macroalgae and marine phanerogams, becoming one of the most abundant taxonomic groups of non-native species in the Mediterranean Sea. Their adaptability to changing conditions and their ability to colonize new habitats have contributed significantly to their expansion in this region.

Two centuries of observations

In the study, the research team compiled observations of the presence of non-native marine macrophytes in the Mediterranean Sea over the last two centuries. They calculated their expansion rates (area invaded by each species per year) over time and the relationship between these expansion rates and the thermal conditions of the species in the distribution range.

“The results indicate that invasion rates have increased over time, and that since the 1990s those of tropical and subtropical species have especially accelerated, surpassing those of temperate and cosmopolitan macrophytes,” comments Marlene Wesselmann, IMEDEA researcher and first author of the work. “In particular, the highest expansion rates have been observed in allochthonous macrophytes that are exposed to minimum temperatures 2 to 3 degrees Celsius higher in their native range than in the Mediterranean Sea,” adds Wesselmann.

Asparagopsis taxiformis is one of the species of tropical marine macrophytes present in the Mediterranean Sea. (Photo: Carlos Alejandro Morell)

“We compared the water temperature to which these species are exposed in their native range with the temperature to which they are exposed in the Mediterranean Sea, and observed that the majority of these species experience considerably lower minimum temperatures in the Mediterranean than in its native range,” explains Núria Marbà, also a scientist at IMEDEA. “This tells us that most of these tropical and subtropical species are not limited by the colder winter temperatures of the Mediterranean Sea, probably due to the plasticity of their minimum thermal tolerance. And together with the increase in the temperature of the Mediterranean Sea during recent decades, especially in summer, the thermal conditions for its growth and expansion may have improved,” adds the researcher.

Marbà points out that “on the contrary, the settlement and expansion of macrophytes of temperate origin could be limited in summer or during heat wave events, since thermal conditions can exceed their upper thermal tolerance limits, which do not show much plasticity”.

“These results suggest that future warming will increase the thermal habitat available for non-native thermophilic species in the Mediterranean Sea and will continue to favor their expansion,” concludes Iris Hendriks, IMEDEA researcher.

The work has been carried out in collaboration with the Balearic Oceanographic Center (IEO) of the CSIC and with the University of Galway in Ireland.

The study is titled “Increasing spread rates of tropical non-native macrophytes in the Mediterranean Sea.” And it has been published in the academic journal Global Change Biology. (Source: IMEDEA / CSIC)