4. Meiobenthos as a Component of Anthropogenic Disturbance Assessment in the Abyssal Pacific Environment

Effects of disturbance in the marine environment are assessed, in situ or in the laboratory, based on various indicators, including measures of change in benthic community attributes (abundance, biomass, composition, diversity). The benthic organisms used usually represent the macrobenthos, but the meiobenthos is increasing frequently recommended for such assessments. In anticipation of the polymetallic nodule extraction from the abyssal Pacific areas, a number of field experiments were conducted in which seafloor alteration resembling that accompanying nodule mining, or effects similar to those expected from mining activities, was simulated using various devices: a test miner (the 1975–1980, with a 2006 follow-up, DOMES experiment in CCFZ), a plough-harrow (the 1989–1996 DISCOL experiment in the Peru Basin, S Pacific), and a Benthic Disturber (Benthic Impact Experiments or BIEs: the 1991–1993 US-Russian Joint BIE, the 1994–1997 JET, and the 1995–2000 IOM BIE, all in CCFZ). The severity of impact was assessed by analysing, more or less comprehensively, changes in meiobenthic community-related variables which included qualitative (taxonomic composition, with a finer resolution in nematodes and harpacticoid copepods) and quantitative (abundances of total meiobenthos and of the key taxa, relative abundances) characteristics. Attempts were also made to assess the degree of recovery from the disturbance by re-sampling the disturbed areas at various time intervals post-disturbance. Meiobenthic communities were observed to be affected by the disturbance, reduced abundances immediately post-disturbance being the major community-level manifestation of impact. Effects observed during follow-up studies differed considerably ; although, in most cases, the overall community recovery was recorded, sometimes as early as several months after the disturbance, the composition of both nematode and harpacticoid taxocoenes was altered. The causes underlying the alteration are difficult to be unequivocally explained. The patch mosaic effects which could have been at play could have been accompanied by effects of some natural phenomena such as episodes of phytodetritus sedimentation known to affect deep-sea meiobenthic communities.