ABSTRACT:

Watermass properties were obtained in the Indian Deep Sea Experiment (INDEX) area during pre- (27 May -: 8 July 1997) and post- (22 July - 5 September, 1997) benthic disturbance periods respectively. Watermass (Temperature-Salinity: T-S) structure was characterised with the presence of North Indian Deep Waters having a salinity maximum of 34.75 psu around 45.7 σ4 isopycnal. A near-depth uniformity in temperature and salinity in the abyssal water column was due to Antarctic Bottom Watermass with 45.95 σ4. The light transmission profiles showed very negligible changes (1 to 2%) from pre- to post- disturbance periods suggesting that the bottom waters were not affected by the sediment plumes. The intrusion depths of surface-discharged mining effluents were assessed and the anticipated mining effects in surface layers are discussed.

Introduction

Several pioneering investors of seabed mining of polymetallic nodules have been involved in conducting environmental impact studies for the areas designated to them by the International Seabed Authority. Notable among these studies are the Japanese Deep Sea Impact Experiment (JET) of Metal Mining Agency of Japan (Kajitani, 1997); Joint US Russia" s Benthic Impact Experiment -BIE (Ozturgut et aI, 1997 and Trueblood et aI, 1997); Inter Ocean Metal of Poland" s BIE (Kotilinski and Tkatchenko, 1997); Korea" s Deep Ocean Study-KODOS (Hyun etai, 1997); NAVABA (Natural Variability of Baseline) study of China" s Ocean Mineral Resources R&D Association-COMRA (Feng, 1997) and Indian Deep Sea Experiment-INDEX (Desa and INDEX group, 1997). The mining of manganese nodules will affect both benthic and pelagic layers of the water column depending upon the mining methodology adopted. Earlier studies of Amos et al (1973) have shown the effects of high concentrations of nutrients of the bottom water, when added to surface waters, in enhancing phytoplankton growth during laboratory impact assessment experiments.

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