Dusty and disordered
So deep down and far away from the strong currents on the sea surface, it is not so surprising that even small-scale traces of the DISCOL experiment were still visible. “However, also the biogeochemical conditions had undergone lasting changes,” stresses Antje Boetius. According to the researchers, this is mainly due to the fact that the plough destroys the upper, active sediment layer. It is ploughed under or stirred up and carried away by the currents. In these disturbed areas, the microbial inhabitants can only make limited use of the organic material that sinks to the seafloor from upper water layers. As a result, they lose one of their key functions for the ecosystem. Microbial communities and their functions could thus be suitable as early indicators of damage to deep-sea ecosystems caused by nodule mining – and of the extent of their potential recovery.
Disturbance in another dimension
All mining technologies for manganese nodules currently being developed will lead to a massive disturbance of the seabed down to a depth of at least ten centimetres. This is comparable to the disturbance simulated here, but in completely different dimensions. Commercial deep-sea mining would affect hundreds to thousands of square kilometres of seabed per year, while all plough tracks in the DISCOL combined only covered a few square kilometres. The damage to be expected is correspondingly greater, and it would be correspondingly more difficult for the ecosystem to recover, the researchers stress.
“So far, only few studies have dealt with the disturbance of the biogeochemical function of deep-sea floors caused by mining,” explains Boetius. “With the present study, we are contributing to the development of environmental standards for deep-sea mining and pointing out the limits of seabed recovery. Ecologically sustainable technologies should definitely avoid removing the densely populated and bioactive surface layer of the seabed”.