Monitoring of a sediment plume produced by a deep-sea mining test in shallow water, Málaga Bight, Alboran Sea (southwestern Mediterranean Sea)

In this study different experimental designs for monitoring of sediment plumes produced by deep-sea mining are presented. Plumes of sediment stirred up from the seabed by mining machines are considered to represent a major environmental pressure which may extend far beyond the actual mining area. Two industry field tests with the scaled mining vehicle Apollo II of Royal IHC conducted in a relatively shallow setting offshore southern Spain provided valuable insights for anticipated monitoring of nodule mining activities in the deep Pacific. Although the tests were performed in only 300 m water depth, much less than the depth where future deep-sea mining will take place, the weakly stratified bottom water, tide-dominated near-bed currents with mean magnitude of around 5–10 cm s⁻¹, and gently sloping seabed covered with fine muddy sediment provide a good analogue to operational conditions in the deep sea. The plume of suspended sediment mobilised by the mining vehicle was monitored with turbidity sensors deployed on a ship-operated CTD system and on a static array of moored sensors and monitored visually using a ship-operated ROV. It was found that the generated sediment plume extended no >2 m above the seabed close to the source (<100 m) but increased in height at greater distance. Furthermore, turbidity values decreased rapidly with increasing distance to the source. Even though plume monitoring suffered interference from bottom trawling activities in neighbouring areas, a distinct turbidity signal generated by the mining equipment could still be distinguished above background turbidity at 350 m away from the source. From the experience gained in shallow water, recommendations are made on how a combination of sensors operated from moving and moored platforms may be a suitable and successful strategy for monitoring man-made sediment plumes in the deep sea.