Hydrochemical properties of the water column were measured at the Indian Experiment (INDEX) site in the Central Indian Ocean Basin, as part of baseline studies for the environmental impact assessment of benthic disturbance. The surface mixed layer was very thin (~40m) and was characterized by high oxygen, high pH and low nutrients with nitrate limiting productivity. Below this layer, three distinct layers could be identified, based on their hydrochemical properties: The Salinity Maximum layer which appeared at 125-175m depth was associated with shallow oxygen minimum (1.25ml/l) and weak maxima in nutrients; The Oxygen Maximum Layer in the depth range 250-500m was associated with high oxygen concentrations (4.5ml/l), high pH (>7.8) and weak minima in nutrients; The Salinity Minimum Layer corresponding to the Antarctic Intermediate Water was seen as a narrow band, about 400m thick, in the depth range 8001200m. The bottom water was characterized by high oxygen and high nutrients. An evaluation of possible effects of surface discharge of mining tines- a slurry consisting of fine nodule fragments, bottom water and sediments … suggests that the discharge, with an expected solid content of 50g/I, will induce increased primary production, through nitrate enrichment in nitrogen limited surface waters, of the order of 1.343 x 10-2g fixed carbon per litre of discharge, while the organic C associated with the sediment will result in a net oxygen demand of 306ml per litre of discharge. Based on the above figures and the baseline characteristics of the water column, a dilution factor of 0.98 has been calculated for the surface discharge to have non-discernible effects.
An experiment, simulating the benthic disturbance likely to be generated during deep-sea mining of nodules, was initiated in 1995, as part of the Indian Experiment aimed at studying the" effects of deep-sea mining on the environment.