Microbes disturbed by a seafloor experiment 26 years ago still haven’t recovered
Deep-sea mining may damage underwater ecosystems for decades

Microbe communities living in the seafloor off Peru haven’t bounced back from a deep-sea mining experiment 26 years ago. The populations are still reduced by 30 percent in this part of the South Pacific Ocean, researchers report April 29 in Science Advances.

From 1989 to 1996, the DISturbance and reCOLonization, or DISCOL, experiment plowed grooves into the seafloor to mimic deep-sea mining for valuable metal-bearing rocks. The lumps of rock, known as polymetallic or manganese nodules, contain economically important metals such as copper, nickel and cobalt.

To recover the nodules, miners dredge the seafloor, scraping off much of the top layer of sediment along with the rocks. Researchers have long expressed concern about how this might affect deep-sea ecosystems. But there is little data about the effects of deep-sea mining on the ocean environment — and particularly on the microbes at the base of the food web, which cycle the nutrient nitrogen between seafloor and bottom waters.

Scientists last assessed DISCOL’s effects in 1996. So in 2015, microbial ecologist Tobias Vonnahme, now of The Arctic University of Norway in Tromsø, and colleagues devised a new test, comparing the 26-year-old plow tracks with five-week-old tracks they dug into the seafloor.

Cell counts of microbes in the younger tracks were reduced by about 50 percent compared with undisturbed areas; in older tracks, cell numbers were reduced by about 30 percent. Due to slow accumulation of sediment in the deep sea, regions disturbed by mining could take more than 50 years to fully recover, the team says.

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