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 communi ties on large spatial and temporal scales. As a first approach to inve stigating the effects of large-scale sediment disturbance from deep-se a mining, the environmental risk assessment experiment DISCOL (DISturb ance and ReCOLonization) was started in 1989 at 4150 m depth in the tr opical southeastern Pacific Ocean. A specially constructed disturber w as towed 78 times through a 10.8 km(2) experimental field. The reestab lishment of the impacted macrofaunal assemblages in the disturber trac ks, with particular emphasis on the Polychaeta, was monitored three ti mes over 3 yr. Mean macrofaunal densities (> 500 mu m, 0-5 cm sediment depth range) in undisturbed sediments ranged from 77 to 122.8 inds./0 .25 m(2) at the three sampling times. After the impact, the animal abu ndances in the plow tracks were reduced to 39% of undisturbed densitie s. 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 we re comparable to those of undisturbed sediments. Dominance shifts in p olychaete 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 w ithin the Polychaeta did not allow clear interpretation of long-term d isturbance effects, but Hurlbert rarefaction, used in a nonstandard wa y, demonstrated significantly reduced ''diversity'' after 3 yr. The re establishment of a semi-liquid surface sediment layer is proposed as a potentially controlling factor for the reestablishment of the macrofa unal community after physical disturbance. Although the DISCOL experim ent did not fully simulate full-scale industrial impacts, it has provi ded insights into macrofaunal recolonization following large-scale dis turbance of deep-sea environments. (C) 1998 Elsevier Science Ltd. All rights reserved.