Nitrogen-rich runoff from sewers and farms is fueling a rise in invasive seaweed that, when consumed by the reptiles, may trigger an otherwise dormant herpes virus. This virus in turn causes the often fatal growths.
The cauliflower-like tumors—which can sprout on a turtle's eyes, mouth, joints, and internal organs—have contributed to declines in the 4-foot-long (1.2-meter-long) turtles. Listed as endangered by the International Union for Conservation of Nature, green sea turtles are found in the world's tropical and subtropical waters. (See sea turtle pictures.)
In some parts of Hawaii, where green sea turtle strandings occur regularly, as many as 90 percent of stranded dead or dying turtles discovered have been afflicted with the disease, according to study leader Kyle Van Houtan, an ecologist with the National Oceanic and Atmospheric Administration's Pacific Islands Fisheries Science Center in Honolulu.
Wherever the turtle strandings occur, there is often evidence of sewage and invasive algae, Van Houtan said.
Until now, however, hard evidence that pollution factors into the turtles' disease has remained elusive, he noted.
Polluted Areas Are Turtle-Tumor Hot Spots
Van Houtan and colleagues analyzed human activities on land to calculate a "nitrogen footprint" for each watershed—an area that drains into a water body—on the Hawaiian islands of Oahu, the Big Island, and Maui (see map). The team also studied 28 years' worth of data on green sea turtle strandings on the islands.
Comparing the two data sets, the scientists found that diseased turtles strand more in areas with high nitrogen runoff from agriculture, sewers, and cities.
These turtle-tumor hot spots are "the places that I wouldn't necessarily want to go surfing after a rain … because of the nasty stuff that would show up" in the ocean, Van Houtan added.
Nitrogen Awakens Herpes Viruses
The scientists don't think the nitrogen-loaded runoff causes tumors directly. Instead, there may be a chain of interactions that starts with a nitrogen-fed boom in nonnative seaweed, at least in Hawaii.
Nitrogen acts as a fertilizer in oceans, and is a main cause of oxygen-sucking algae blooms called dead zones.
Around the Hawaiian islands, several seaweed species were either accidentally introduced or deliberately harvested for food crops and later "escaped" cultivation and spread into the wild.
For instance, hookweed—native to the Mediterranean, the Caribbean, the Indian Ocean, and the Philippines—was introduced to Hawaii in 1974. Gorillo ogo and spiny seaweed came from the Pacific and Indian Oceans and Guam, respectively.
The seaweed absorbs the extra nitrogen and converts it into an amino acid—the building block of protein—called arginine. When turtles eat the seaweed, arginine awakens dormant herpes viruses in the turtles' bodies that generate the tumors.
It's possible that nonnative seaweed in Hawaii is better than native seaweed species at converting nitrogen to arginine, Van Houtan noted.
And because Hawaiian green sea turtles now rely on the invasive seaweed for food, Van Houtan suspects the animals get dosed with tumor-triggering amino acids at every meal.
Turtle Tumors Have Other Causes?
Alonso Aguirre, a wildlife epidemiologist with the environmental nonprofit EcoHealth Alliance in New York, praised the study for confirming the link between pollution and sea turtle tumors.
But he also said that the tumors' cause may be more complicated.
The pollution-virus link is "a very simplistic pathway to explain the whole disease," Aguirre said, adding he suspects other factors, such as water temperature, and possibly additional viruses, are at play.
Regardless of exactly how the tumors form, Aguirre said the study shows that "the turtles … are telling us that something is happening to the oceans in a way that now, we have to pay attention."
Turtle-tumor research published September 29 in the journal PLoS ONE.