Potential Environmental Mining in Impact of Manganese-Nodule the Deep Sea

Abstract

ABSTRACT Deep-sea mining operations will result in the removal and redistribution of sediments and benthic organisms and, where airlift or hydraulic lift systems are employed, in the discharge of nutrient-rich, sediment-laden bottom water into the surface layer. The data prescribed here focus on two aspects of the environmental impact problem of manganese nodule mining: the establishment of baseline environmental conditions prior to dredging and the potential effects of the surface discharge of bottom water on phytoplankton populations. Data on the chemical and biological oceanography of prospective manganese-nodule mining areas are extremely sparse. In general, benthic biomass is low, ranging between 0.01 and 0.05 gm/sq m. Phytoplankton productivity and standing crop in the photic zone are also low (less than 150 mg C/sq m/D and 30 mg cgla/sq m, respectively), probably due to slow rates of dissolved inorganic nitrogen flux into the photic zone above the thermocline. The results of enrichment experiments conducted in a Mn-nodule area of the western North Atlantic indicate that the surface discharge of bottom water will stimulate phytoplankton productivity and may affect phytoplankton species composition. However, these changes will be small and primarily dependent on the rate at which the effluent is diluted by mixing with surface water. The greater the rate of dilution by mixing, the more rapid will be the phytoplankton growth response, but the smaller will be the actual yield per unit volume. INTRODUCTION The mining of ferromanganese deposits from the deep sea will affect benthic and pelagic environments depending, to a great extent, on the methods of retrieval and separation employed. Three mining systems have been proposed, each of which will remove different combinations of water sediment and nodules from the ocean floor. 45 The continuous-line bucket (CLB) system utilizes open-mesh metal dredge buckets to gather and retrieve nodules. The buckets are attached in series by a continuous loop of cable running from the mining vessel to the ocean floor. The system is designed to bring only nodules to the surface. The two remaining systems utilize a bottom collecting device for nodules and vertical transport through a pipe by hydraulic (HP) or airlift (ALP) pumping. Both of these systems will transport large quantities of bottom water and sediments to the surface, where they will be discharged. The major environmental effects of these mining systems can be grouped into three categories:redistribution of bottom sediments on or near the bottom,redistribution or destruction of benthic organisms and their habitat andintroduction of sediments and/or associated bottom water at or near the surface. The major environmental effects of CLB mining will probably be confined to Categories 1 and 2, while both the HP and ALP systems will, following nodule separation, discharge nutrient rich, sediment-laden bottom water into the surface layer where it will probably remain.