Polymetallic nodules, as the name suggests, are globular structures that contain a variety of metals. These nodules are found on the seabed and are usually rich in valuable metals such as cobalt, manganese, nickel, and copper. Terrestrial mining has severe impacts on land ecosystems, and some think deep-sea mining might be a lucrative alternative. However, mining for manganese nodules involves massive disturbance to the uppermost surface sediments. Despite this, several nations around the globe are actively scanning the seafloor to look for the mining sites. But are we ready to mine these habitats without ascertaining impacts on the organisms living on and around these nodules? How long will it take for the seafloor to recover from the impacts of mining? A team of scientists led by T.R Vonnahame is working to answer some of these questions.
DISCOL is a unique DISturbance and ReloCOLization experiment that is being carried out at the Peru Basin (4150 m deep, roughly the distance from California to Florida) in the South Pacific. In 1989 the site was first ploughed in an area of 11 km2 to replicate the effects of deep-sea mining on the seabed. Ploughing mimics, the mining activity whereby heavy gears are pulled along the seafloor, non-selectively. Scientists have gone back to the site in 1992, 1996, and, most recently, in 2015 to study the impacts of ploughing by comparing it to three undisturbed sites nearby. In the current study, the scientists have particularly focused on the microbial community. They focused on microbes to ascertain if the number of microbes and their activities are sensitive to seafloor disturbance and could further be utilized as indicators of disturbance.
On returning to the DISCOL site, the scientist could easily identify the original plough track even after 26 years. As surprising as this may seem, this is common in disturbed deep sea beds because it takes really long for the sediment to sink to such depths. DISCOL was divided into four zones of disturbance: i) minor disturbance outside the tracks; ii) furrows (depression) created by ploughing; iii) ridges from ploughing; and iv) areas of deep ploughing, these were the areas where surface sediments that usually contain a lot of tiny organisms were lost entirely. Many things organisms need to survive, like food (carbon) and oxygen, were quite different at the four locations. The first significant takeaway of the experiment is that disturbances impact food and oxygen for these microbes, and conditions can take a really long time to return to normal.
To compare the long term and the short-term impacts of mining, the scientists ploughed the experiment site five weeks prior to the sampling. Food particles from the water column settle on the surface sediments, and this results in an increase in the number of microbes in the surface sediments, but mining activity disturbs the surface sediments. This study found the fresh (5 weeks), and the 26-year-old disturbance showed a decline in the number of microbes. The reduction in bacterial numbers at the fresh disturbance was 50%, and even after 26 years, the number of bacteria was lower than undisturbed sediment by 30%. This is due to the slow buildup, from 20 to 30 thousand years, of food particles at the seafloor. Mining activity removes not only the bacteria but also their food, preventing new communities from taking over.
Interestingly, the scientists noted that, despite the reduction in the numbers of microbes, no new microbes took over at both the disturbance sites, i.e., the species of the microbes remained the same. The decline of the microbial community is also particularly alarming because microbes form the base of the food chain on the seafloor. Smaller organisms depend on microbes to break down the bigger food particles and make them easier to eat. Because microbes break down larger particles, usually with the help of oxygen, scientists also measured oxygen uptake at the disturbed sites and found a decline compared to undisturbed sites. This tells us that the microbes are not breaking down food particles effectively at the disturbed sites. Therefore, a decline in bacterial population and their activity could mean the scarcity of food for other organisms higher in the food chain.
What does this mean for deep-sea mining?
Current technologies disturb the surface layer over hundreds and thousands of square kilometers per year. This would mean loss of ecological functions at a large scale and would take several decades to recover from. The authors suggest that the disturbance mainly depends on the technology being employed to mine the seafloor, and therefore future ecologically sustainable technologies should avoid removal of the bio-active surface layer, which harbors and supports a variety of organisms.
Hi, I am a Ph.D. student at the National Institute of Oceanography, India. I am currently studying the particulate and dissolved organic matter dynamics in the central and eastern Arabian Sea. I am also interested in the effects of climate change on marine systems, as well as outreach and science communication. My interest in science communication stems from the lack of effective scientific outreach in my country and I wish to contribute to improving that. In my spare time, I like to read non-fiction and learn about things I didn’t know existed.