MONITORING MEGABENTHIC COMMUNITIES IN ABYSSAL MANGANESE-NODULE SITES OF THE EAST PACIFIC-OCEAN IN ASSOCIATION WITH COMMERCIAL DEEP-SEA MINING

1. Marine mining of manganese nodules will affect the deep sea benthos in a harmful and longterm way. The near complete removal of manganese nodules will results in a change of the megabenthic community from one that is rich in both diversity and density to a soft bottom community with greatly reduced diversity. 2. Pre-pilot mining tests and pilot mining operations, which have to demonstrate the feasibility of deep-sea mining, should be accompanied by environmental impact studies before commercial mining begins. 3. The photographic assessment of megabenthic communities represents one cost-effective possibility of monitoring large-scale impacts on the deep sea floor. A method based on the analysis of video and photomaterial was used to obtain basic data on community structure and density of the abyssal megabenthos of undisturbed manganese nodule sites in the North (Clarion-Clipperton Fracture Zone) and South (Peru Basin) Pacific Ocean. 4. A comparison of the results with data from the literature demonstrates that the different methods used result in different density values. 5. Total density varied between less than 300 to more than 1600 individuals/10 000 m2. 6. There is a clear need for standardization of methods. A system which combines video and photographic capabilities is useful. It will give the best results when the distance of the camera to the sea floor is between 2 and 3 m, and the area covered by one photograph is within 3 to 5 m2.