In 2001, geologist and marine scientist Nerilie Abram went out on a scientific snorkeling expedition in the eastern Indian Ocean. As part of her Ph.D. studies, she was planning to use samples drilled out of fossilized coral reefs to reconstruct the history of warm-cool cycles in the Indian Ocean over thousands of years. One stop was the Mentawai Reef just off the coast of Sumatra. But when Abram and her colleagues arrived, they made a shocking discovery: nearly 100 percent of the corals were dead!
Above: The Mentawai Islands, fringed by coral reefs, lie 160 km (100 miles) off the coast of Sumatra, on the eastern edge of the Indian Ocean. Credit: NASA Earth Observatory.
"Locals said that the coral and fish in the Mentawai reefs had all been killed when the ocean turned red in late 1997," says Abram. A red ocean likely meant a red tide. But what could have caused a red tide so destructive that it could destroy an entire, 400-kilometer long reef?
"Our research group initially started working in the Mentawai Islands because this region is vital in controlling the climate of the Indian Ocean region," explains Abram. Around the islands, a strong east wind periodically kicks up, shoving the surface waters of the ocean westward, and then cold water from deep in the ocean wells up to the surface. When the events happen, the eastern part of the Indian Ocean becomes much cooler than the western part. In late 1997, the region was at the height of a cool phase. Abram wondered if the cooling could have somehow been responsible for the destructiveness of the red tide, and she looked for evidence that it might have happened in the past.
Fossilized corals record past climates in the chemistry of their skeletons. Abram and her colleagues collected samples from the dead and fossilized corals on the Mentawai reefs dating back 7,000 years. According to their analysis, the cooling seen during the 1997 event was about 4 degrees Celsius. Cool, but not the coldest ever. About 4,400 years before present, the anomaly approached 6 degrees. None of the previous events had ever caused such massive reef death. Whatever happened in 1997 was unique.
Abram turned her investigation to the red tide. Cold water upwelling often causes "blooms" of ocean plant life, including red and green algae. In addition to producing some toxic chemicals, the algae steal oxygen from the water as they die and decompose. To study the red tide, Abram used satellite observations of chlorophyll in the ocean collected by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS).
"The chlorophyll-a concentrations detected by SeaWiFS in December 1997 show a region of elevated productivity around the Mentawai Island chain that is consistent with local observations of the red tide," says Abram.
Above: According to locals, the Mentawai reefs died in 1997 "when the sea turned red," the result of a bloom of microscopic plants including red algae. In December of that year, the Sea-viewing Wide Field-of-view Sensor recorded chlorophyll concentrations (which are proportional to the density of plant populations) that ranged from 1 milligram per cubic meter of water (green) to nearly 60 milligrams (red). In 2000, a more typical year, the chlorophyll concentration around the reefs didn't exceed 1 milligram. The amount of iron needed to support such a large red tide would have been between 10 and 50 times what local sources could have provided. Credit: NASA SeaWiFS Project.
To Abram, the massive red tide at Mentawai seemed similar to devastating red tides in the Gulf of Mexico that have been linked to Saharan dust blowing across the Atlantic Ocean. Coastal waters often have low levels of iron, an essential plant nutrient. Sudden influxes of iron-rich dust can cause algae populations to skyrocket.
If iron fertilization is what turned the Mentawai red tide so deadly, then where did it come from? Since the cooling of the ocean is driven by the winds blowing over the islands from the east, Abram looked upwind for a culprit. It didn't take long to identify one: a staggering number of fires burning in nearby Sumatra and East Taklimakan, Borneo.
In late 1997, a drought caused by El Niño was hammering Indonesia. Logging had thinned and damaged many forests, making them extremely flammable. In the midst of this exceptional drought, the people's seasonal use of fire as a way to manage their agricultural lands became disastrous. Forest fires exploded across the dry landscape and consumed millions of acres.
Above: Uncontrolled forest fires sprang up all over Indonesia in late 1997 and early 1998, especially on the islands of Sumatra and Borneo. This smoky fire occured in Borneo's East Kalimantan province. Credit: Global Fire Monitoring Center.
Using satellite observations from NASA's Total Ozone Mapping Spectrometer, Abram estimated that as much as 46 percent of the smoke from the Sumatran wildfires may have spread across the Mentawai reef area. The iron-rich ash likely 'fertilized' the algae bloom, creating a red tide that suffocated or poisoned nearly every living coral for hundreds of square kilometers.
"The combination of natural and human influences in 1997 created a situation that appears to have been unprecedented over at least the past 7,000 years," says Abram. "However, wildfires are becoming stronger and more frequent with increasing human pressures on tropical forests, and pollution and over-fishing are reducing the ability of reef ecosystems to naturally limit the extent of algae blooms. It is likely that the threat to coastal marine ecosystems from wildfires will increase in the future."
In addition to recognizing the connection between land and ocean, Abram says that we can shield reefs from such catastrophe by reducing water pollution, over fishing, and other pressures that weaken the natural defense mechanisms that reefs use to fight off algae.
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