LOS ANGELES — For several years now, one question has held the key to understanding just how much we should worry about the hundreds of tons of DDT that had been dumped off the coast of Los Angeles:
How, exactly, has this decades-old pesticide — a toxic chemical spread across the seafloor 3,000 feet underwater — continued to reenter the food web?
Now, in a highly anticipated study, researchers have identified tiny zooplankton and mid-to-deep-water fish as potential links between the contaminated sediment and the greater ecosystem.
For the first time, chemical analyses confirmed that these deep-sea organisms are contaminated by numerous DDT-related compounds that match similar chemical patterns found on the seafloor and animals higher up on the food chain.
“This DDT pollution happened several decades ago, there’s no new source, it’s been banned … but this old source is still polluting the deep-ocean biota, which is really alarming,” said Eunha Hoh, whose lab at San Diego State’s School of Public Health led the study’s chemical analysis. “We’re not talking about zooplankton collected in 1960 — we’re talking about zooplankton collected now, in the deep ocean, that is still polluted with DDT.”
Hoh’s team had already found significant amounts of DDT-related chemicals in present-day dolphins and coastal-feeding condors. But even though DDT has clearly been accumulating at the top of the food chain, how the DDT reached these animals has been somewhat of a mystery. Key questions remain on whether it has been coming from more shallow sources (such as the Palos Verdes Shelf Superfund site, where DDT had been discharged for years via the sewer system), or from the deep-sea sediment itself.
“It really (hits home) this concept that nothing is untouched,” said Lihini Aluwihare, a chemical oceanographer whose lab at the University of California, San Diego’s Scripps Institution of Oceanography helped piece together the many multi-disciplinary aspects to the study. “Establishing the current distribution of DDT contamination in deep-sea food webs lays the groundwork for thinking about whether those contaminants are also moving up through deep-ocean food webs into species that might be consumed by people.”
The study, published Monday in Environmental Science & Technology Letters, is one of many research efforts sparked by a 2020 Los Angeles Times report that detailed the little-known history of ocean dumping off the Southern California coast — and how the nation’s largest manufacturer of DDT had for years disposed of its waste at sea.
One team of scientists, in an attempt to map and scan the seafloor for DDT-related waste, discovered instead a multitude of discarded military explosives from the World War II era. Another team unearthed records showing that barrels of radioactive waste had also been dumped at sea.
And during an urgent investigation into old and forgotten records, the U.S. Environmental Protection Agency discovered that from the 1930s to the early 1970s, 13 other areas off the Southern California coast had also been approved for all manner of dumping — including the disposal of various refinery byproducts and 3 million metric tons of petroleum waste.
As for the DDT, which is short for dichlorodiphenyltrichloroethane, scientists have so far confirmed that much of what’s still sitting on the seafloor remains in its most potent form and is buried barely 6 centimeters deep — raising concerns about just how easily it could remobilize and spread by reentering the food web.
In a world dominated with concerns over microplastics and “forever chemicals,” DDT persists as an unresolved problem — long after the pesticide was banned in 1972 following Rachel Carson’s book “Silent Spring.”
With this latest study, researchers sought to demonstrate how the chemical is still likely making its way up from the deep seafloor by coming into contact with zooplankton, which get eaten by deep-sea fish, which then get eaten by midwater fish and marine mammals higher and higher up the food chain.
Hoh joined forces with Aluwihare’s lab at Scripps, where a microbiology team also provided sediment analysis and a deep-sea biologist helped determine which organisms to sample — and where across Southern California to collect them.
In addition to zooplankton, which are a window into the base of the food chain, one type of fish, myctophids, proved to be key.
Also known as lanternfish, myctophids are tiny, unassuming fish that travel remarkable distances from the deep ocean all the way to the surface. (One of the most abundant and widespread fish in the world, myctophids make up roughly 65 percent of all deep-sea biomass.) The researchers methodically ground up each fish sample, extracted the lipid, and assessed the contamination with an unprecedented level of scrutiny.
The findings have been sobering: Wherever they looked, they found DDT.
