The ocean sustains all life on our planet. It provides food to eat and oxygen to breathe while playing a key role in moderating our climate. But marine life is increasingly threatened by climate change. The ocean is warming dramatically, affecting its ability to sustain life.
The scorching temperatures seen around the Mediterranean this year are indicative of rising global temperatures. This is expected to continue into the next century, depending on how much CO₂ we continue to emit. The International Energy Agency reported that global energy-related CO₂ emissions increased by 6% in 2021 to reach their highest level on record.
What is a sea heat wave?
The Mediterranean has been subjected to intense thermal conditions in recent years. This has hit another milestone again this year, with sea temperatures hitting a record 30.7C off Corsica.
A marine heat wave is a prolonged period of abnormally high sea temperatures compared to the seasonal average. Their frequency has doubled since the 1980s.
Due to the delay between the completion and publication of ecological work, the most comprehensive study we have on Mediterranean marine heat waves covers the period 2015-2019.
The study found sea temperatures recorded in the Mediterranean during the period were the highest since recording began in 1982. Across nearly a thousand field surveys conducted, researchers found that 58% of them contained evidence of widespread mortality of marine life, closely linked to periods of high heat.
The research provides insight into the future ecological impacts of marine heatwaves elsewhere. This is important because substantial temperature increases are predicted for tropical and polar regions.
While the ocean acts as a large carbon sink, we still face increases in sea surface temperature of 1-3°C before the end of the century. Linked to this global warming, marine heat waves of increasing frequency and intensity.
Much of the research on marine heat waves reveals that they particularly affect certain habitats, including coral reefs, seagrass beds and algae. Marine heat waves have been shown to be responsible for the loss of up to 80% of the population of certain Mediterranean species between 2015 and 2019.
A mass mortality event is a single, catastrophic incident that quickly wipes out a large number of species. About 88% of these events in the Mediterranean were associated with hard seabed inhabitants, such as corals. However, seagrass beds and the more diverse soft seafloor community were also severely impacted, accounting for 10% and 2% of these events, respectively.
Death in shallow water
More than two-thirds of the deaths of marine organisms occurring on the hard seabed occurred in the shallowest waters. Marine environments between 0 and 25 meters deep are subject to particularly intense warming and are home to some of the most biodiverse ecosystems in the Mediterranean, made up of coral-like organisms. Other research estimates that marine heat waves are responsible for the loss of 80-90% of Mediterranean coral density since 2003.
Founder species tend to be habitat-forming organisms and are therefore essential in structuring an ecosystem. They serve as a nursery, provide protection from predators and serve as a food source. Founder species are essential for maintaining biodiversity and their loss will affect other species. As founding species, the loss of corals, seagrass and algae is of particular concern.
It is not just intense heat stress that causes mortality episodes. High water temperatures are associated with the proliferation of pathogenic organisms, such as bacteria, fungi and viruses. This can further reduce the ecosystem’s ability to adapt to extreme heat, contributing to further ecological damage.
Migration of marine life
In addition to promoting the widespread death of marine life, marine heat waves often trigger migrations. Warm-water invasive species will move to warmer areas, replacing species that escape rising temperatures. Anecdotal evidence suggests that the exceptional temperatures seen across the Mediterranean this summer could lead to significant mass migration.
In Greece, scientists have observed an increased abundance of invasive species from warmer waters. This includes lionfish and silver-cheeked toad, both of which are poisonous and can cause considerable ecological damage.
Some research even suggests that invasive species from the eastern Mediterranean, where native populations have collapsed, will soon become the only ones able to sustain ecosystems.
There have also been sightings of non-native barracuda off the southern coast of France. The invasion of predatory species, which find new prey while facing fewer predators, could significantly alter the functioning of Mediterranean ecosystems, most likely towards a less rich form with less species diversity.
However, while anecdotal evidence is plentiful, research into the ecological effects of marine heat waves is still in its infancy. Further sound scientific studies are needed to develop modeling of realistic future scenarios.
In some branches of the scientific community, the recent intensity and frequency of marine heat waves suggests that we have reached a “climate endgame”. This means preparing for all the consequences of widespread mortality of marine species if emissions are not reduced. This year’s likely devastating Mediterranean marine heat wave will only fuel these discussions.
This article was originally published on The conversation by John Spicer at the University of Plymouth. Read the original article here.