The Heatwave of Summer 2022 in the North-Western Mediterranean Sea: Some Species Were Winners

Abstract

The warming trend of the Mediterranean Sea is a long-term process. It has resulted in a northwards and westwards range expansion and abundance increase of thermophilic species, both native and non-indigenous, and in a shrinking of the range of cold-affinity species. Marine heatwaves (MHWs) are relatively short-term extreme episodes that are responsible for spectacular mortality events in some species and have been extensively reported in the literature. In contrast, the species that benefit from MHWs (the ‘winners’) have been much less studied. A record-breaking MHW occurred in 2022 in the north-western Mediterranean Sea. We focus on three ‘winner’ species, the thermophilic green macroalgae Penicillus capitatus and Microdictyon umbilicatum and the endemic seagrass Posidonia oceanica. Penicillus capitatus, which is mainly present in the area as an inconspicuous turf of entangled filaments (espera stage), produced the erect paintbrush-like stage where sexual reproduction takes place. Microdictyon umbilicatum, usually uncommon, bloomed to the point of clogging fishing nets. Finally, a mass flowering of P. oceanica occurred in late August–September, followed the following year (April–May 2023) by the extensive production and dissemination of fruits and seeds. Both processes, the long-term warming trend and one-off heatwaves, both ‘losers’ and ‘winners’, shape the change in structure and functioning of Mediterranean ecosystems.

The Changing Biogeography of the Ligurian Sea: Seawater Warming and Further Records of Southern Species

“…Scuba divers reported on the recent occurrence of two other warm-water groupers, namely the white grouper Epinephelus aeneus (Geoffroy Saint-Hilaire 1817) [101] and the dogtooth grouper Epinephelus caninus (Valenciennes 1843) [102]. Not only fish but also warm-water algae and invertebrates, including exotic species, were seen to conform to such temporal pattern [27,31,32,36,93]. Even the only alien seagrass to have penetrated the Mediterranean Sea, Halophila stipulacea (Forsskål) (Ascherson 1867), showed a similar trend [103][104][105]: first recorded at Rhodes, SE Aegean Sea [106], it spread only throughout the eastern Mediterranean until the 1980s-1990s (first rapid warming period) to eventually enter the Tyrrhenian Sea in the early 2000s (second rapid warming period) [107]; between 2018 and 2022 (third rapid warming period), it reached NE Sardinia [108], W Corsica [109], and Cannes on the French part of the Ligurian Sea [110].…”

“…However, this astonishing biodiversity is presently threatened by climate change. Seawater warming favors the establishment of exotic species of (sub)tropical origin [21][22][23] and drives endemic species to the brink of extinction [24]; frequent marine heat waves (discrete periods of extreme local seawater warming), in particular, are causing mass mortality of native species [25][26][27]. These threats are even more evident in the Ligurian Sea [28], located at the north-western corner of the Mediterranean.…”