Impacts of the first deep-sea seafloor massive sulfide mining excavation tests on benthic communities

Abstract

Japan undertook the first ever tests of deep-sea seafloor massive sulfide (SMS) excavation in 2017 in the Okinawa Trough. This study examines infauna from several nearby stations before and up to 3 yr after disturbance. Distance from excavation, current direction, seafloor topography, and modeled and observed deposition were used to classify the level of impact of each station. Metal concentrations were analyzed, as were nanofauna (2-32 µm), meiofauna (32-300 µm), and macrofauna (>300 µm). Elevated Cd, Pb, Hg, Zn, Fe, and Cu were confirmed as indicators of sedimentation from the SMS extraction. Benthic communities appeared altered by the disturbance test, with different size classes showing different levels of response and recovery. Nanofaunal and meiofaunal abundances appeared to take several weeks to show impacts from the disturbance and may have returned to pre-test levels within 1 yr, but changes to nematode community structure persisted longer. In contrast, macrofaunal abundances and diversity appeared to decrease immediately, and possibly remained depressed compared to pre-test levels at impacted sites at least 3 yr later. In addition, meiofaunal nematode:copepod ratio and macrofaunal percent composition of polychaetes, along with several nematode taxa, may serve as useful bioindicators of SMS mining. The small scale of disturbance requires caution when extrapolating to full-scale mining, but these results suggest current direction and topography greatly influence the extent of mining impacts; in addition, several metals may be useful for identifying the mining footprint. Biological results indicate that larger macroinfauna may be less resistant and resilient to mining impacts than smaller meiofauna and impacts from even small-scale mining activities may persist for at least 3 yr.