If manganese nodules were to be mined in the future it would be a severe intrusion into the deep-sea biological environment because the harvesting machines would plough up large areas of the sea floor. It is very difficult to assess precisely how and to what extent the deep-sea ecosystem would be impacted, because so far only small areas have been scientifically investigated. The few existing studies, however, clearly show that there is more life in the deep basins than was previously believed.
Many of the organisms live buried within the deep-sea sediments, especially in the upper 15 centimetres of the sea floor. The initial impression of a barren desert is deceptive. A large number of organisms also live in the open water. The deep-sea organisms are divided into different categories based on their size. For the differentiation of small species, the size of the sieve openings used to filter the animals out of the bottom or water samples is a useful criterion. The 4 following categories are generally used:

MICROFAUNA: This consists of organisms that are smaller than the openings of a very fine sieve of 0.03 millimetres. It comprises almost exclusively microorganisms.

MEIOFAUNA: This includes, for example, the copepods and nematodes (small worms), as well as foraminifera, a group of singlecelled animals that live in calcareous shells. These organisms are retained on sieves with openings of 0.03 to 0.06 millimetres.

MACROFAUNA: This group includes animals that are caught on sieves with openings of 0.3 to 0.5 millimetres. Large numbers of macrofaunal organisms live in the sediments, especially bristle worms, but also crabs and mussels.

MEGAFAUNA: This includes animals that can be seen with the naked eye on underwater videos or photographs, for example, fish, sponges, sea cucumbers and starfish. These organisms are from 2 to over 100 centimetres in size.

A special feature of the Pacific manganese nodule areas is the presence of unusually large species of foraminifera. In contrast to their miniscule cousins, the Xenophyophora are up to 10 centimetres in size and are thus included in the megafauna. Xenophyophores live on top of the sediment and, like sea cucumbers, leave behind feeding tracks several metres long.
It is largely unknown how large the proportion of endemic species living in manganese nodule areas is. Marine biologists from various research institutes are presently evaluating bottom samples obtained on expeditions. Many endemic species have already been discovered.
Additionally, it is presumed that the species compositions in and on deep-sea sediments change every 1000 to 3000 kilometres, which means it would change within a manganese nodule area. The reason for this is that the nutrient conditions in different marine regions vary slightly, because nutrient levels are partially dependent on transport by near-surface water currents. When more nutrients are contained in the water, then algae can produce more biomass, which subsequently rains down to the bottom. Different organisms predominate depending on the supply of carbon. Compared to the nutrient-rich coastal areas, the differences in the amount of carbon between the various deep-sea regions are relatively small. Nevertheless, they apparently cause differences in species compositions. Marine biologists therefore insist that mining be regulated to the extent that the different species assemblages, and thus the character of the deep-sea areas in question, are at least in part preserved and that a successful recolonization is possible. These factors, as well as the protection of endemic species, should be considered in the mining regulations of the ISA.

fig. 2.17 > Various animal species, including sea cucumbers, deep-water prawns, fish and brittle stars, have been found in the CCZ. © BGR