Context:
- Frequent heat waves cause huge damage to underwater ecosystems and researchers expect more of them in the future.
- Extreme heat has plagued the Mediterranean for weeks. Wildfires raged across at least nine countries in the region from Algeria to Greece. But the soaring temperatures are not only a danger for people and ecosystems on land; they’re also harming marine life.
Why are high sea temperatures a problem?
- In a warming world, marine creatures are in danger of suffocating. Gases like oxygen and carbon dioxide dissolve better at colder temperatures, so that means the warmer the water; the less oxygen is available to breathe.
- Conversely, higher temperatures also cause an increase in metabolism, which in turn means animals have to breathe even more than usual.
- The rise in temperature accelerates metabolism, and the organisms need more food to maintain this metabolic rate.
- Algal blooms are more common in hotter waters too. Such blooms can further deplete oxygen levels and produce toxins harmful for fish, marine mammals and birds, for instance.
What species and ecosystems are worst hit by marine heat waves?
- High water temperatures are most harmful for animals living at the bottom of oceans, lakes or rivers. These benthic species include corals, mussels, sponges, starfish and plants like sea grasses, and are often attached to rock or solid ground. They can’t migrate when it gets too hot.
- Scientists observed mass deaths of benthic species along thousands of kilometers of Mediterranean coastline between 2015 and 2019.
- Many benthic species are crucial to the marine ecosystem. They filter the water and keep seas, rivers and lakes clean by eating dead organisms. Some species are an important food source for other creatures or are harvested by humans. Benthics like soft corals, seaweed and seagrasses provide some of the main ocean habitats.
Is heat good for any animals in the Mediterranean?
- Jellyfish, on the other hand, are thriving because of higher temperatures, as well as nutrient run-off from farms and sewage. Overfishing and loss of fish habitat mean the jellyfish have few or no predators. When currents push the animals together, the Mediterranean turns into a crowded jellyfish hotspot.
- The sea also hosts around 1,000 invasive species — the highest number in the world. While this is not directly linked to climate change or rising temperatures “such conditions clearly favor species introduced from warmer seas”. Alien species can have a major impact on ecosystems.
What does extreme heat in the Mediterranean mean for people?
- Fishermen are catching fewer familiar species and instead are finding more invasive fish which they have difficulty selling.
- Habitat loss could also lead to an overall decline in fish populations, while disappearing seagrass means coasts will be more exposed to future storms. This could also have a knock-on effect for tourism because divers will be less likely to visit an impoverished underwater landscape.
What can be done?
- Humans must stop emitting greenhouse gases.
- Combatting the growth of algal blooms which are worsened by run-off from agriculture, wastewater and industry.
- UN goal of protecting 30% of the world’s oceans by 2030 will directly benefit the Mediterranean. So far, just 8% of the sea is protected.
- Need to increase the number of strictly protected areas where fishing, diving and boating are not allowed. Even if marine protected areas won’t make the water cooler, well protected areas are recovering faster and better from human disturbance.
- Some invasive species like Halophila stipulacea Ascherson, originally native to the Red Sea could potentially help seagrass beds survive in a smaller part of the Mediterranean and continue to provide some of their essential ecosystem services.
- Hope for the native Neptune grass. Studies show that the plants can deal with rising Mediterranean temperatures if they are intentionally exposed to heat as young seedlings.