Balance of Terrors
Pollution, fishing and global warming are taking their toll on coral, with reefs dying and becoming coated in slimy algae. Volcanic eruptions, on the other hand, sometimes bring blessings to the anthozoans - they keep them cool in hot times.
The turn of the year from 1997 to 1998 did not mark a good turning point for the world's corals. Around 16 percent of all reefs worldwide were severely damaged or even died at that time. The reason: An extreme El Niño event heated them up too much in many regions. Because corals feel most comfortable at water temperatures of around 26 degrees Celsius, if they exceed the 28 or even 30 degrees Celsius mark for a longer period of time, coral bleaching sets in - a process feared by marine ecologists, during which the skeletal coelenterates find their algae lodgers repel.
However, these so-called zooxanthellae provide their hosts with nutrients in exchange for phosphates and a relatively safe home. It is not clear why corals are so suddenly separated from their useful service providers: it is possible that the photosynthesis of the algae goes on strike if the temperatures are too high, or they suddenly produce toxins from which the corals have to protect themselves.
The reef builders can still feed themselves for a certain time afterwards; and when temperatures soon settle back down to normal levels, the corals absorb the algae again and the ecosystem survives. However, if it stays too warm for weeks or even months, the limited catching and filtering skills of the corals are no longer sufficient and they die. Even before death, this state of emergency is visually very clear, because the algae also give the corals their bright colors: After the banishment, a ghostly paleness remains on the reef.
Coelenteans become dangerous every three to seven years when the El Niño climate anomaly returns. Then the trade winds die down on South America's west coast, instead of cold upwelling water, a warm countercurrent from the tropics now dominates here. As a result, areas of precipitation and dryness are shifting along the equator worldwide: it rains in deserts, rainforests are burning, and many oceans are heating up because changed wind systems reduce evaporation and thus its cooling effect. El Niño means bad times for temperature-sensitive corals, and it is not surprising that several global coral bleaching events over the past 25 years have been directly linked to concurrent or recent El Niños.
The years 1982/83 and 1991 to 1994 are exceptions: Despite exceptionally strong El Niño conditions, there was no major damage to the reefs outside the Pacific - in the Caribbean, for example, where the two are otherwise almost parallel went hand in hand, as noted by marine biologists around Jennifer Gill from the University of East Anglia in Norwich. What saved the colorful structures from the otherwise apparently common heat shock?
In addition to changed ocean currents and wind systems, solar radiation is primarily responsible for the warming of the water: if it penetrates the atmosphere unhindered, it heats up the ocean; if, on the other hand, it is blocked by heavy cloud formation or a large number of aerosols, these act as a protective screen and the water remains relatively cooler.
The researchers are now investigating exactly this connection, because shortly before or during the El Niños of 1982/83 and 1991, the two volcanoes El Chichón in Mexico (1982) and Pinatubo in the Philippines (1991) erupted. They threw considerable amounts of ash and sulfur dioxide into the atmosphere, with the latter reacting with water to form droplets of sulfuric acid, which, as corresponding clouds, veiled the earth, sometimes for years. As sun blockers, they lowered regional temperatures by one to two degrees, depending on where the atmospheric currents took them.
For the corals, these moderate cooling temperatures were the deciding factors, as model calculations confirmed: El Niño periods with clean air caused almost twenty times the damage in the Caribbean compared to the two events with the volcanic eruptions. A ten percent reduction in the amount of aerosols in the air can therefore increase the extent of bleaching by a third.
The fact that the American subsea in particular benefited from the eruptions may also have something to do with the usually higher dust concentration in the atmosphere here anyway, because trade winds regularly carry particles from the Sahara and the Sahel zone. Several million tons float across the Atlantic to the west every year. Together with the volcanic ash, they can obviously bring the corals through critical warm phases - but sometimes with unpleasant long-term consequences: because deadly germs sometimes travel with the Sahara dirt. Instead of overheating, the reefs then die from introduced diseases.
