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Abyssal epibenthic megafauna of the Clarion-Clipperton area (NE Pacific): Changes in time and space versus anthropogenic environmental disturbance

Radziejewska T., Stoyanova S.

Oceanological Studies

2000

Vol. 29, No. 2

83-101

Abyssal epibenthic megafauna was used as one of biological indicators of the effects of anthropo-genic sediment disturbance in the Benthic Impact Experiment (BIE) carried out within 1994-1997 by the Interoceanmetal Joint Organization (IOM) in the Clarion-Clipperton Fracture Zone (NE Pacific). Analysis of bottom photographs collected from 14 phototransects measuring about 2-10 km in length, made during three cruises, yielded data on the composition and abundance of the epi-benthic megafauna in nodule-bearing (N) and nodule-free (NF) areas of the bottom. The latter included a site at which experimental sediment disturbance, mimicking that produced by nodule mining, was induced. Megafauna was studied before (October 1994, July 1995) and immediately after the disturbance (July 1995), a follow-up survey being performed in April 1997, i.e., 22 months after the original disturbance. Characteristic differences in megafaunal composition and abundance between the two habitat types (N vs. NF) were recorded, the N bottom megafauna being consistently more abundant and dominated by sessile invertebrates (mainly sponges), while the NF megafauna showed a preponderance of mobile deposit feeders (holothurians). The megafaunal abundance in both habitat types showed differing patterns of temporal changes: while the abundance in the NF areas declined sharply immediately after the disturbance and markedly increased 22 months later, the N megafauna abundance was observed to increase from one survey to the next. Due to the fact that the sediment sampled in April 1997 showed evidence of a substantial phytodetritus input, the increase in the megafaunal abundance, recorded in both habitat types at that time, is assumed to reflect mainly the increase in the benthic food supply, superimposed in the NF transect sections on natural processes of recolonisation, proceeding in the disturbed areas of the bottom.