FY 1980

Environmental investigations during manganese nodule mining tests in the north equatorial Pacific in November 1978

Ozturgut, E., J.W. Lavelle, O. Steffin, and S.A. Swift

NOAA Tech. Memo. ERL MESA-48, Boulder, CO, 50 pp (1980)

The results of environmental studies conducted during the monitoring of a pilot-scale manganese nodule mining operation by Deepsea Ventures, Inc./Ocean Mining Associates are presented. These studies included observations of the chemical and physical structure of the upper 300 m of the water column, nephelometry and particulate sampling, light profiles, primary production experiments, and macrozooplankton studies. These observations were made prior to mining operations in the ambient water and during the 18 hours of mining. The characteristics of the mining discharge were found to be: flow rate 80–95 P/s; total particulate concentration 3–9 g/P; temperature 4–5°C; bulk density 1.029–1.032 g/cm; Fe 6% by weight; Mn 2.76% by weight; and Ca 0.93% by weight. The plume generated by the mining discharge was visible 2 km from the mining ship, with a width of 300–400 m at that distance. However, the plume was detectable by nephelometry at a distance of 7–8 km, which corresponds to a plume age of about 12 hours. Nephelometer records indicate that the near-surface concentration of particulates in the plume decreased from 900 mg/P, at a distance of 70 m from the mining ship, to about 70 mg/P at 8 km. Background particulate concentrations in the area were 30 mg/P. The plume was observed to be confined to the upper 40 m in three casts made at plume ages of one, two, and three hours. In one-hour plume water, the total light attenuation coefficient in the upper 25-m layer increased fourfold to 0.14 m from the mean ambient value of 0.034 m due to shading by the particulates. Based on this reduction in light intensity and on ambient in situ and on-deck primarily production measurements, a reduction of about 40% in primary production along the plume axis is estimated in a one-hour-old plume. This is a very limited area, and overall reduction in primary production within the total plume would be much less. The mining discharge did not have any other effect on primary production according to on-board experiments lasting less than one day. Similarly, the results of field observations and experiments conducted for periods of one to two days suggest that mining discharge has no significant effect on macrozooplankton at the concentration levels observed during this pilot mining test.