Deep-sea macrofaunal impacts of a large-scale physical disturbance experiment in the Southeast Pacific

Future human use of the deep sea potentially threatens benthic communities on large spatial and temporal scales. As a first approach to investigating the effects of large-scale sediment disturbance from deep-sea mining, the environmental risk assessment experiment DISCOL (DISturbance and ReCOLonization) was started in 1989 at 4150 m depth in the tropical southeastern Pacific Ocean. A specially constructed disturber was towed 78 times through a 10.8 km2 experimental field. The reestablishment of the impacted macrofaunal assemblages in the disturber tracks, with particular emphasis on the Polychaeta, was monitored three times over 3 yr. Mean macrofaunal densities (>500 μm, 0–5 cm sediment depth range) in undisturbed sediments ranged from 77 to 122.8 inds./0.25 m2 at the three sampling times. After the impact, the animal abundances in the plow tracks were reduced to 39% of undisturbed densities. Polychaeta (48.6% of undisturbed densities) were less impacted than Tanaidacea (28.0%), Isopoda (18.5%), and Bivalvia (9.3%). Abundances of most higher taxa increased rapidly in the tracks, and after 3 yr were comparable to those of undisturbed sediments. Dominance shifts in polychaete families level were observed in the early post-impact phases. Significant displacement of macrofaunal depth distributions after 3 yr indicated sustained disturbance effects. Single species reactions within the Polychaeta did not allow clear interpretation of long-term disturbance effects, but Hurlbert rarefaction, used in a nonstandard way, demonstrated significantly reduced “diversity” after 3 yr. The reestablishment of a semi-liquid surface sediment layer is proposed as a potentially controlling factor for the reestablishment of the macrofaunal community after physical disturbance. Although the DISCOL experiment did not fully simulate full-scale industrial impacts, it has provided insights into macrofaunal recolonization following large-scale disturbance of deep-sea environments.