Tropical Algae Spreading Unprecedentedly in the Mediterranean

• According to a study led by the Mediterranean Institute for Advanced Studies (IMEDEA-CSIC-UI), tropical marine macrophytes have spread at a faster rate than temperate ones over the past two centuries.

• Scientists attribute this expansion to the adaptability of these species to changing conditions and their ability to colonize new habitats, highlighting their potential impact on the region.

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 demonstrated that allochthonous marine macrophytes (macroalgae and non-native seagrasses) of tropical origin are spreading in the Mediterranean Sea at a much faster rate than temperate-origin marine macrophytes over the past few decades. The findings of the study, published in the journal Global Change Biology, suggest that future warming of the Mediterranean could further favor the expansion of these species.

Over the last two centuries, there has been an increase in sea temperature due to climate change. However, uncertainty remains regarding whether this temperature increase truly promotes the expansion and impact of invasive species in the Mediterranean Sea.

The Mediterranean has become a critical hotspot for the arrival of allochthonous species, those transported beyond their native distribution range due to human activities, facilitated by 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 led marine macrophytes, including both macroalgae and seagrasses, to become 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 significantly contributed to their expansion in this region.

Photo: Halimeda incrassata. Author: Carlos Alejandro Morell

Two Centuries of Observations

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 within their distribution range.

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

"We compared the water temperature at which these species are exposed in their native range with the temperature they experience in the Mediterranean Sea, and we observed that most of these tropical and subtropical species experience considerably lower minimum temperatures in the Mediterranean than in their native range," explains Núria Marbà, also a scientist at IMEDEA. "This indicates that most of these species are not limited by the colder winter temperatures of the Mediterranean Sea, probably due to the plasticity of their minimum thermal tolerance. Together with the increase in Mediterranean Sea temperature over the last decades, especially in summer, this may have improved the thermal conditions for their growth and expansion," adds the researcher.

Marbà points out that "in contrast, the settlement and expansion of temperate-origin macrophytes may be limited in summer or during heatwave 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 for allochthonous thermophilic species in the Mediterranean Sea and will continue to favor their expansion," concludes Iris Hendriks, researcher at IMEDEA.

The work was conducted in collaboration with the Balearic Oceanographic Center (IEO-CSIC) and the University of Galway (Ireland).

Reference

Increasing spread rates of tropical non-native macrophytes in the Mediterranean Sea

Marlene Wesselmann, Iris E. Hendriks, Mark Johnson, Gabriel Jordà, Frederic Mineur, Núria Marbà