Mass Mortality Events — ScienceDaily

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A new study carried out by teams from the Faculty of Biology, the Biodiversity Research Institute (IRBio) of the University of Barcelona and the Institute of Marine Sciences (ICM-CSIC) of Barcelona has revealed that marine heat waves associated with the climate crisis coral populations in the Mediterranean, whose biomass has sometimes been reduced by 80 to 90%.

According to the study published in the journal Proceedings of the Royal Society B, coral populations in the Mediterranean – essential to the functioning of coral reefs, one of the most emblematic habitats of this sea – may not recover from the recurring impact of these extreme episodes, with water temperatures reaching high degrees for days and even weeks.

This is the first study to assess the long-term recovery capacity of the populations of two emblematic species of Mediterranean coral: the red gorgonian (Paramuricea clavata) and red coral (Corallium red), which provide complex habitats essential to a great diversity of associated fauna. It is therefore essential to understand their resilience in the face of more frequent and intense heat waves.

Mass mortality events

The climate crisis is severely affecting marine ecosystems around the world, and the Mediterranean is no exception. In particular, the marine heat waves associated with the climate crisis cause mass mortality events in all the coastal ecosystems of this basin, and among the most affected species are the Mediterranean corals.

Knowledge about the long-term resilience of coral is still scarce, despite studies that have analyzed the immediate impact of sea heat waves on these organisms. They are long-lived species (more than a hundred years in some cases) and slow population dynamics (i.e. organisms with low rates of growth and recruitment), and researchers therefore need long temporary series (decades) to assess their recovery capacity.

As part of the study, the team analyzed the results obtained during a long-term monitoring of different populations of corals affected by a large mass mortality caused by a heat wave in 2003 in the marine protected area of Scandola (Corsega, France). In particular, they analyzed data on the state of these populations (density, size structure and biomass) collected over the fifteen years following the heat wave, by researchers from the MedRecover research group, made up of experts from the UB and the ICM-CSIC, among other centers.

Far from a recovery, the results show that all the populations analyzed have tended to collapse since they were hit by the 2003 heat wave. Fifteen years after this event, these populations are practically considered extinct by a functional point of view.

“We observed an average biomass loss concerning the initial biomass of 80% in the populations of red gorgonians, and up to 93% concerning the studied population of red coral”, notes Daniel Gómez, researcher at the ICM-CSIC.

“These data are worrying for the conservation of these iconic species, and they indicate that the effects of the climate crisis are accelerating with obvious consequences for underwater landscapes, where the loss of coral equals the loss of trees in forests”, notes Joaquim. Garrabou, also a member of the ICM-CSIC.

Recurrent exposure to heat waves

Cristina Linares, professor in the Department of Evolutionary Biology, Ecology and Environmental Sciences of the Faculty of Biology and member of IRBio, says that “we believe that one of the main reasons why we have observed these trajectories of collapse is the potential recurrent exposure to heat waves, incompatible with the slow population dynamics of these species.” In particular, during the study period (2003-2018), they recorded significant heat waves in at least four years: 2009, 2016, 2017, 2018.

“During these heat waves,” continues Linares, “temperature conditions in the study area reached extreme levels incompatible with the life of these corals, which probably caused new mortality events to decimated populations and made recovery impossible.” .

As the number and intensity of marine heat waves are expected to increase over the coming decades due to the climate crisis, the viability of many coral populations could be seriously threatened.

“However, there will probably be areas in the Mediterranean where, due to several factors, the recurrence of such climatic impacts could be lower. This makes it particularly relevant to conserve – in view of other potential impacts – these climatic refuges where the trajectories of coral populations may be more positive than observed in this study,” the research team notes.

“Nevertheless, there is an urgent need to put in place stronger measures against the climate crisis before the loss of biodiversity becomes irreplaceable”, conclude the experts.

This study was carried out thanks to the support of the Regional Natural Park of Corsica, the Plan National HEATMED project (RTI2018-095346-B-485 I00), the European projects H2020, MERCES, H2020 Futurmares, Interreg MED MPA-Engage and Interreg MED AMP -Adapt.

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