Warm, acidic, oxygen-free water: life in the ocean is having a hard time

He. She dead trio They are called, the triple killer. Acidification, warming and low oxygen level. There are three trends in the world’s oceans, which have dire consequences for marine life. The World Meteorological Organization reports that acidification and warming will reach new records in 2021 Last week in the report The state of the global climate 2021† Surprisingly enough, the report didn’t say anything about the low oxygen level either.

Together, this deadly trio causes so many changes in the oceans that not all plants and animals can adapt to them. “The major climate change has started in the seas,” says marine biology professor Hans-Otto Portner of the Alfred Wegener Institute in Bremerhaven. He cites corals in warm water as the clearest example. They are already suffering greatly from rising temperatures and the increasing frequency of sea heat waves. A large part of the species is expected to become extinct this century due to increased warming. “While we know coral reefs are home to a quarter of marine biodiversity,” Pörtner says. In addition, they protect the coasts from wave action.

Read about ocean heat waves: And the sea is getting hotter and hotter

Overfishing

On top of existing pressures on marine life: overfishing, pollution, coastal development, and nutrient run-off from agriculture and cities.

But how has global warming changed the oceans so far? And what are the consequences?

The IPCC has reported that the oceans have risen an average of 0.88 degrees Celsius at the surface since the early 20th century. His last case report from last August. Of those 0.88 degrees, the bulk – 0.6 degrees Celsius – has occurred since 1980. The oceans are warming because they are absorbing nearly all of the excess heat (more than 95 percent) trapped under a dense “blanket” of greenhouse gases. Piles above the ground. Otherwise, it would have been uninhabitable long ago. Water is a good insulator of heat.

As on land, the number of heat waves at sea is increasing. Its frequency nearly doubled in the twentieth century. It also became more intense and lasted longer.

Warming is also causing sea level rise. When water heats up, it expands. This so-called thermal expansion represents 38 percent of the total sea level rise of 20 cm (1901-2018).

Acidification has to do with the massive amounts of extra carbon dioxide₂ Released by humans into the atmosphere through the combustion of fossil fuels. The oceans absorb a quarter of it. This affects the chemistry in seawater. bicarbonate concentration (HCO₃^†) and hydrogen ions (H .).^†) Increases. The latter causes an increase in acidity. In other words, the pH drops. But the carbonate concentration (CO₃^2-) in the ocean water drops. Whereas all kinds of organisms need it to build a calcium skeleton.

The Intergovernmental Panel on Climate Change has dealt with acidification in The report was published last February† Since the pre-industrial era, the average pH of the upper ocean waters (up to 200 m depth) has decreased from 8.2 to 8.1. This sounds like a bit, but it’s a logarithmic scale. For marine life it means a big change. Scientists are seeing acidification at ever greater depths.

Stronger contrast

The decrease in the oxygen level is partly due to the increased stratification of the water. The upper layer rich in oxygen becomes sweeter due to the increased flow of melt water from the ground, and due to increased precipitation. The contrast with deeper salt water that lacks oxygen becomes stronger. “The mixing of these layers decreases, so that oxygen does not easily penetrate into the deeper waters,” says Caroline Slomp, professor of marine biochemistry at Utrecht. In coastal areas, it is important that nutrient discharges to the sea, from agriculture and urban areas, stimulate algal growth at the surface of the water. When the algae die and drown, they are broken down by bacteria, a process that consumes oxygen. Since there are more algae, the degradation and oxygen consumption increases. As a result, the areas devoid of oxygen – dead zones – Grow in size. “Many organisms, such as fish, swim around,” says Stomp.

According to the Intergovernmental Panel on Climate Change (IPCC), the concentration of oxygen in the upper thousand meters of the oceans decreased by 0.5 to 3.3 percent between 1970 and 2010. The size of areas where the concentration of oxygen is negligible is expanding in many places.

What are the consequences?

For example, in response to rising sea water temperatures, Nathaniel Bendoff, professor of oceanography at the University of Tasmania, notes that many organisms migrate. “They are heading towards the poles,” he says via the video link. Species often live within a certain temperature range, which can vary from one species to another. As the water temperature rises, their ideal habitat changes. This so-called climatic velocity, which is the rate at which spatial gradients of temperature change, now averages 21.7 kilometers per decade in the upper 200 meters of the oceans, according to the Intergovernmental Panel on Climate Change. “The species you can try to move with,” Bendoff says. But this is difficult for many restricted organisms, such as corals. Pörtner adds that species around the equator are moving away, but something isn’t taking place everywhere, precisely because it’s getting too warm. “We are seeing a decline in biodiversity.”

smallest living creatures

Bindoff points to another trend: living things are getting smaller because the water’s temperature rises. “We see it in fish, for example.” He explains this because at higher temperatures metabolism increases. It requires more energy for maintenance operations, which reduces growth opportunities.

This decline will continue with further warming. This is why it is expected that ocean biomass will also decline – and with it fish catches (apart from the effects of overfishing). The extent of this reduction depends on how much humans continue to emit greenhouse gases. If humans can limit warming to 1.5 to 2°C, the reduction is expected to be 5 percent by the end of the century. But if the current trend of global warming continues, the biomass decline could reach 15 to 20 percent.

Their house is made of the mineral aragonite. And aragonite dissolves sooner in acidification

Geert Jean Richart Concerned about snail wing

One animal group that has already been clearly affected by acidification is the wing snail, says Gert Jan Reichart, head of ocean research at the Netherlands Institute for Sea Research. “Their house is made of the mineral aragonite.” To construct an external skeleton, organisms use either aragonite or calcite. “And aragonite dissolves faster in acidification.” But there are also organisms that actually begin to produce more lime in an acidic environment. “It’s not a black and white photo,” Richart says.

This is the hard part. The ocean is a vast area, with so many different combinations of temperature, nutrients, salinity, acidity, oxygen levels, and so many different species with varying sensitivities and adaptability, that it is almost impossible to make general statements. The Intergovernmental Panel on Climate Change also writes about compounding impacts. For example, warming and lowering oxygen levels are mutually reinforcing, because warming increases metabolism, which requires more oxygen, while being less available in areas where the oxygen level is lower. But the IPCC also notes that warming could mitigate the negative effects of acidification. She cites an example, a snapper that occurs in Australia and Asia, which benefits from a combination of acidification and warming. But the Intergovernmental Panel on Climate Change is writing about a combination of acidification, warming, and lowering oxygen levels, which will limit the habitats of many fish and invertebrates. This combination plays, for example, in five partially closed seas: the Persian Gulf, the Red Sea, the Black Sea, the Baltic Sea and the Mediterranean. There, a significant death of marine life was observed. But there are also other human influences at play here – tourism, poaching and pollution.

little good

In any case, the four scientists interviewed for this piece do not expect much of the changes that began in the oceans. Coral is already suffering from rising temperatures and sea heat waves. “The acidification weakens their structure,” says Bendoff.

IPCC was established in 2019, in oceans special report, graph showing the risks to marine ecosystems – Bendoff was one of the lead authors of that report. Warm water coral reefs are most at risk, followed by kelp forests and seagrass beds, two ecosystems that also have a rich biodiversity.

Updated version of the chart noon last september in The nature of climate change, with Pörtner as one of the authors. It also contains animal populations that are already at high risk with a temperature increase of 1.5°C. These are winged snails at high latitudes, bivalves (such as mussels and oysters) at temperate latitudes, and krill at high latitudes. Krill is a shrimp-like species that lives in the polar regions and is an important food source for whales, penguins and all types of fish.