Effects Of Deepsea Mining For Manganese Nodules On The Abyssal Megabenthic Community
Authors: Bluhm Hartmut 
Published: October 2008

The nearly total removal of manganese nodule During commercial deep-sea mining will destroy the hard-bottom habitat of the bottom fauna and result in the formation of a pure soft-bottom community of lower diversity. In 1989 the first large-scale and long-term experiment in the abyssal tropical southeastern Pacific Ocean (DISCOL) was initiated. A 10.8 km2 area was disturbed using a specially developed plough- harrow device, to study the reaction of the deep sea fauna to disturbance similar to deep-sea mining. The recolonization of the disturbed area and its megabenthic community was monitored during three cruises of the German research vessel SONNE by an Ocean Floor Observation System with TV and â??photo-on-command' capabilities The results of the image-analysis at four stages of the DISCOL disturbance/recolonization study demonstrate not only a direct impact within the disturber tracks, but also an apparent impact on areas which were not primarily disturbed Commercial mining will create a sediment plume which will be drifted away by near bottom currents. The reduction of this effect is a task for the development of future nodule collector vehicles Pre-pilot mining tests and pilot mining operations should be accompanied by environmental studies the method developed for monitoring environmental effects on megabenthic communities provides good results

INTRODUCTION

Commercial deep-sea mining for manganese nodules will affect the deep-sea benthos in a harmful and long-term way1,2,3. The removal of manganese nodules will not only destroy the hard-bottom fauna; but also the sediment cloud, created during nodule sampling by the collector vehicle, will impact the surrounding areas.

In 1989 a long-term, large-scale, disturbance and recolonization experiment was started in the tropical southeastern Pacific Ocean to mimic the impact of commercial mining and to achieve a better understanding of the rate, sequence, and direction of benthic community re-establishment after severe anthropogenic disturbance. The DISCOL (Disturbance and Recolonization Experiment in a manganese nodule area of the Deep South Pacific Ocean) experiment is funded by the FRG Bundes- minister fÜ Forschung und Technologies

Close to a German manganese nodule claim, a circular area of 10.8 km2 was selected as the QISCOL experimental Area (DEA). Its central position is 07Â°04.4' S and 88Â°27.6' W (Figure1). The average water depth is between 4140 and 4160m. The mean manganese nodule coverage in this area is about 5-10 kg/m2. The nodule size ranges ges between 8 and 15 cm in diameter. The nodules are quite different from those found in the Clarion-Clipperton Fracture Zone of the North Pacific. The â??cauliflower' type nodules of the DEA have an ellipsoidal to spherical, and in some cases, discoidal shape with a botryoidal to knobby surface. The nodule surface is rough, porous and cavernous. The pores and fissures of the nodules are inhabited by a diverse infauna4.

The DEA was subdivided into eight pie-shaped sectors, each consisting of a central and peripheral region separated by a 350 m wide buffer zone (Figure 1)0In these regions an intensive sampling program was established to obtain information on bacterial, meiofaunal, macrofaunal, and megafaunal communities during the experiment. Baseline samples were taken to evaluate the natural undisturbed standing stock of the benthos5.