Meiobenthos in the Sub-equatorial Pacific Abyss

The meiobenthos (also called the benthic meiofauna) is a heterogenous group of benthic organisms, both protists and metazoans. Initially distinguished among the benthic organisms on account of their size [as organisms retained on 0.063 (0.032)–1.00 (0.500 or 0.250) mm mesh size sieves], the grouping has become recognised as a distinct ecological category, important by its major contribution to benthic metabolism and secondary production. While marine ecological research usually addresses entire meiobenthic communities considered as assemblages of interacting components represented by high-rank taxonomic units called the major taxa (phyla, orders, families), there is a general awareness of an immense taxonomic richness (diversity) those taxa represent. Whenever detailed taxonomic studies on the meiobenthos have been carried out, a great number of new species, genera and higher-rank taxa has been described. However, the knowledge of this diversity, particularly in the deep sea, is still greatly limited. Ecological research on the meiobenthos revealed the grouping to be a sensitive indicator of environmental changes. Consequently, the meiobenthos is being increasingly frequently used in monitoring and evaluating impacts of factors that disturb the natural state of sedimentary environment. The reliability of such evaluations may be enhanced by refining the resolution of taxonomic analyses and by coupling them with information on functional traits of the meiobenthic taxa present in an assemblage. While such approach is gaining popularity in research on coastal areas, it is still very rare in the deep sea, although the meiobenthos-related variables have been used in evaluating impacts in the deep sea.

The deep seafloor, i.e. seabed areas at depths exceeding 800–1,300 m, cover about 88 % of the world ocean’s bottom. The most extensive areas represent the 3,000–6,000 depth range and include abyssal plains (depths > 4,000 m) the largest of which is the abyssal plain of the Pacific Ocean. The water column overlying it consists of a number of layers differing in their major characteristics. The most characteristic layers include that encompassing the oxygen minimum zone (OMZ, 100–1,000 m depth range in the Pacific) and the near-bottom layer, directly impinging on the seafloor. Once considered extremely stable, the near-bottom layer is now known to be prone to hydrodynamic effects such as tides and currents. The latter are generally weak, but periods of intensified current activity are not infrequent. The water column effects influencing the abyssal seafloor include also the transmission of the wind-generated surface physical energy down to the bottom (the “benthic storms”) on the one hand and sedimentation of surface-produced organic matter on the other. Both the benthic storms and organic matter deposition are known to be periodically, or episodically, intensified, thus contributing to natural environmental variability in the abyss. The Pacific abyssal plain sedimentary cover is mostly biogenic in origin. A characteristic part of the Pacific abyss is a huge (about 2 million km²) polymetallic nodule field within the NE sub-equatorial seafloor area experiencing low sedimentation rates and constrained by the Clarion and Clipperton fractures (the Clarion-Clipperton Fracture Zone, CCFZ). CCFZ extends sub-latitudinally along about 4,200 km, its surface inclining slightly westwards with depths in the east-west direction from about 4,000 to about 5,400 m. The seafloor, although generally flat, does show (particularly in the eastern part) distinct topographic features which are volcanic in origin. The relatively thin (50–200 m) sedimentary cover is formed by recent biogenic sediments (siliceous ooze). The major characteristic of the area is the presence of polymetallic nodule deposits. The nodules, occurring at abundances frequently exceeding 10 kg/m², are mostly exposed on the sediment surface, but some are also embedded or buried in the sediment. The nodules, in addition to the occasionally occurring larger hard-rock fragments of cobalt-rich ferromanganese crusts, add to the deep-sea habitat heterogeneity and themselves constitute both a unique deep-sea habitat, of a great interest to marine ecologists, and an important mineral resource, of a great appeal to the marine mining community the size of which has been recently growing considerably.

The metazoan meiobenthos of the sub-equatorial North-Eastern Pacific abyss, CCFZ included, groups a fairly high number of higher taxa (including nematodes, harpacticoid copepods, ostracods, kinorhynchs, tardigrades, gastrotrichs, halacaroid mites, loriciferans, meiobenthos-sized polychaetes), the free-living nematodes and harpacticoid copepods being dominant. Both groups are very diverse in terms of taxon (genus, species) richness, the genus-level lists from the studies conducted so far containing 10–246 and 34–62 genera of nematodes and harpacticoids, respectively. Most genera (e.g. Acantholaimus among the nematodes and Pontostratoites among the harpacticoids) seem to occur throughout CCFZ, although the dominant genera change depending on the location (e.g. the Terschellingia nematodes were found to dominate in the eastern part of CCFZ and Acantholaimus in most other locations sampled). Most of the individuals found in samples represent as yet unknown, undescribed species. Meiobenthic abundances were found to vary over a range on the order of 10¹–10² ind./10 cm², i.e. an order or two lower than the densities recorded in shallower and shelf waters. The variability in the abundance is thought to results from sediment- and habitat type-dependence (nodule-bearing versus nodule-free bottom, the nodules supporting characteristic faunas of their own) as well as from small-scale patchiness induced by natural factors (near-bottom water dynamics, activity of megafauna, presence of biogenic structures such as large protozoans the Xenophyophorea and the Komokiacea). The CCFZ meiobenthos was found to respond to episodic inputs of organic material in the form of phytodetritus sedimentation, some nematode (desmoscolecids) and harpacticoid (argestids) taxa being particularly responsive and increasing their abundance.

Effects of disturbance in the marine environment are assessed, in situ or in the laboratory, based on various indicators, including measures of change in benthic community attributes (abundance, biomass, composition, diversity). The benthic organisms used usually represent the macrobenthos, but the meiobenthos is increasing frequently recommended for such assessments. In anticipation of the polymetallic nodule extraction from the abyssal Pacific areas, a number of field experiments were conducted in which seafloor alteration resembling that accompanying nodule mining, or effects similar to those expected from mining activities, was simulated using various devices: a test miner (the 1975–1980, with a 2006 follow-up, DOMES experiment in CCFZ), a plough-harrow (the 1989–1996 DISCOL experiment in the Peru Basin, S Pacific), and a Benthic Disturber (Benthic Impact Experiments or BIEs: the 1991–1993 US-Russian Joint BIE, the 1994–1997 JET, and the 1995–2000 IOM BIE, all in CCFZ). The severity of impact was assessed by analysing, more or less comprehensively, changes in meiobenthic community-related variables which included qualitative (taxonomic composition, with a finer resolution in nematodes and harpacticoid copepods) and quantitative (abundances of total meiobenthos and of the key taxa, relative abundances) characteristics. Attempts were also made to assess the degree of recovery from the disturbance by re-sampling the disturbed areas at various time intervals post-disturbance. Meiobenthic communities were observed to be affected by the disturbance, reduced abundances immediately post-disturbance being the major community-level manifestation of impact. Effects observed during follow-up studies differed considerably ; although, in most cases, the overall community recovery was recorded, sometimes as early as several months after the disturbance, the composition of both nematode and harpacticoid taxocoenes was altered. The causes underlying the alteration are difficult to be unequivocally explained. The patch mosaic effects which could have been at play could have been accompanied by effects of some natural phenomena such as episodes of phytodetritus sedimentation known to affect deep-sea meiobenthic communities.