Warming of the sea favors the appearance of tropical algae in the Mediterranean Sea

An international study led by the Mediterranean Institute for Advanced Studies (IMEDEA-CSIC-UIB), a joint center of the Spanish National Research Council (CSIC) and the University of the Balearic Islands (UIB), has shown 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 over the last few decades.

The results of the paper, published in the journal Global Change Biology, suggest that future warming of the Mediterranean may continue to favor the expansion of these species.

Over the past two centuries, sea temperatures have increased due to climate change. However, uncertainty remains as to whether this temperature increase actually favors the spread and impact of invasive species in the Mediterranean Sea.

The Mediterranean has become a hotspot for the arrival of allochthonous species, those that have been moved beyond their native range due to human activities, because of the extensive maritime routes connecting the Atlantic with the Indian and Pacific Oceans, as well as the opening of the Suez Canal.

These two factors have made marine macrophytes, which include both macroalgae and marine phanerogams, one of the most abundant taxonomic groups of allochthonous 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.

In the study, the research team compiled observations of the presence of allochthonous 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' range.

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

"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 we observed that most of these species experience considerably lower minimum temperatures in the Mediterranean than in their native range," explains Núria Marbà, also an IMEDEA scientist. "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 the last decades, especially in summer, it may have improved the thermal conditions for their growth and expansion," adds the researcher.

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

"These results suggest that future warming will increase the thermal habitat available to allochthonous thermophilic species in the Mediterranean Sea and will continue to favor their expansion," concludes Iris Hendriks, researcher at IMEDEA.

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