By dropping dead pigs into the ocean, researchers learn how long marine life can tolerate low oxygen zones in order to get their meal.
- Pig cadavers dumped on the ocean floor are helping researchers understand how animals can cope in low-oxygen zones.
- Sudden fluctuations in oxygen levels seem to be most detrimental to marine life.
- Low oxygen "dead zones" in the oceans are growing due to agricultural runoff and changes in ocean currents.
Drop a dead pig on the ocean floor and within three weeks it is picked clean to the bone.
That's an important finding for forensic researchers who need to know how fast cadavers in an ocean can disappear thanks to scavengers. The information can point to time of death. Oddly enough, watching pigs disintegrate underwater has also proven informative for marine researchers investigating low-oxygen zones in the ocean.
So-called ocean "dead zones" are cropping up in oceans all around the world as fertilizers and climate change trigger processes that rob waters of oxygen.
When forensic researchers dropped a series of dead pigs into the ocean, marine researchers piggy-backed on the study and, using underwater cameras, kept a close eye on what kinds of animals fed on the disintegrating dead pigs and how long these scavengers could tolerate low-oxygen zones.
"I think the results are really important in understanding what to expect of organisms that live on the sea floor as they encounter these dead zones," said Kim Juniper of the University of Victoria in British Columbia who also uses the underwater observatory but who was not a part of this study.
The best known dead zone lies at the mouth of the Mississippi River in the Gulf of Mexico, where fertilizer from agricultural lands upstream stimulates the growth of microorganisms that deplete the water's oxygen when they are consumed by marine bacteria after they die, suffocating other life.
Researchers are also finding that climate change is altering ocean circulation in some places, increasing the stratification of waters and leading to new dead zones in some places where new, oxygen-rich water is no longer flowing in.
"There's a lot of concern," Juniper said. "From the fisheries point of view, we catch a lot of crabs and shrimp and the coastal zone is becoming more and more important for aquaculture. These are quite vulnerable to oxygen depletion events. We need to know more: How do organisms react to them?"
The new study, presented yesterday at a meeting of ocean science researchers convened by the American Geophysical Union in Portland, Ore., begins to address this question.
Researchers have so far deposited four pigs in Saanich Inlet on Vancouver Island, a naturally occurring low-oxygen zone. Once a year fresh water comes through that replenishes the oxygen, but then it drops over time.
"The really fascinating thing is there is a wonderful meal for scavengers but the oxygen is much lower than they are supposed to be able to cope with." said Verena Tunnicliffe of the University of Victoria, who led the study. "We were very surprised to see how far they pushed their limits."
Normally crabs, shrimp and starfish hang out at shallower depths, where there are higher oxygen levels. But the presence of the dead pig drew these creatures in. Three days after the pig was placed, an octopus even arrived to take part in the feast.
"The same individuals stay there for days and days and days," Tunnicliffe said. "It's only when the oxygen gets below a level of 0.5 milliliters per liter that the crab and shrimp disappear."
The last scavengers to vacate were squat lobsters, which are actually a type of crab.
The researchers have studied four pigs so far. The first two were put down at slightly higher initial oxygen levels than the third pig. Both of the first two pigs were hit by sharks, but not dismembered. This created openings for smaller scavengers to attack. These pigs were stripped bare within three weeks.
The third pig was put down when oxygen levels were extremely low and nothing happened to it for three months.
"When the oxygen came up, the crab and shrimp went to town," Tunnicliffe said.
Once larger animals created openings in the pig, it took three to four weeks for it to be picked clean.
Pig number four, deployed just last week, was dropped in more oxygenated waters, and it was torn apart by sharks and completely disappeared.
The team has discovered that, more than the absolute oxygen level, it's the fluctuations that determine when the animals abandon the pig. "As soon as it gets unstable, the animals can't cope anymore," said Tunnicliffe.
"The worrisome thing is that these dead zones are going down below these (threshold oxygen) values and they are going so fast that the major kills are going to happen," she said.
"They're here to stay for the immediate future," Juniper said of the dead zones. "All we can hope to do is avoid having more. What we will probably end up with in these zones is reduced biodiversity."