Meiobenthic Nematoda

Long term changes in nematode assemblages inhabiting deep-sea fields of polymetallic nodules

Long term changes in nematode assemblages from deep-sea nodule fields

The Clarion-Clipperton Fracture Zone (CCFZ, Tropical NE Pacific) is one of areas perspective for future commercial mining of polymetallic nodules. Nodules contain such valuable metals as Ni, Co, Cu, and deep-sea nodule mining can begin soon. Before commercial mining begins, studies must be conducted to evaluate the possible impact on the sea floor and living communities. First, we should estimate the proportion of biological diversity that will be destroyed by deep-sea mining.

In June 1986, IFREMER initiated a complex survey of a polymetallic nodule formation province located around 14°N 130°W in the CCFZ (NIXO-47 cruise). The data on nematode density and composition of nematode assemblages were reported by Renaud-Mornant & Gourbault (1990). Further, in 2004, the second research cruise NODINAUT took place there, and meiobenthic samples were taken from the same locality. We studied nematodes from these samples and found that nematode assemblages in 2004 were absolutely distinct from ones in 1986 (Miljutina et al., 2010). The differences in composition between the samples might be caused by some long-term environmental changes (by order of decades), such as the supply of organic matter to the seabed, or by high geological activity in the area.

In 2012, new meiobenthic samples were collected from the same locality during the research cruise BIONOD. We study nematode assemblages from these samples and compare them with previous data. The aim is to evaluate long-term changes in nematode communities inhabiting this area of CCFZ in order to distinguish them from possible changes caused by future commercial nodule mining.

Recovery of nematode communities after experimental nodule mining

We investigate also nematode assemblages inhabiting the 26-year-old track created by experimental deep-sea mining of polymetallic nodules, and two adjacent, undisturbed areas, one containing nodules and one without nodules. The aim was to compare density, assemblage structure, and diversity indices in order to assess the process of recovery of the nematode assemblage inhabiting the disturbed site. This experimental dredging was conducted in 1978 by the Ocean Minerals Company (USA) in the area of a French mining claim in the Clarion-Clipperton Fracture Zone (Tropical Eastern Pacific) at a depth of about 5000 m.

The nematode assemblage had not returned its initial state 26 years after the experimental dredging: the total nematode density and biomass within the dredging track were significantly lower than outside the track; the biodiversity indices showed significantly lower nematode diversity within the track; and the structure of the nematode assemblage within the track differed significantly from those in the two undisturbed sites outside the track. It was found that such parameters as the rate of sediment restoration (which depends on local hydrological conditions) and the degree and character of the disturbance appeared to be of considerable importance for the recovery rate of the deep-sea nematode assemblages and their ability to recolonize disturbed areas. Such rates of recolonization and recovery may vary widely in different deep-sea regions and should be taken into consideration when planning deep-sea mining.