Our knowledge of the biodiversity of the Southern Ocean (SO) deep benthos is scarce. In this review, we describe the general biodiversity patterns of meio-, macro- and megafaunal taxa, based on historical and recent expeditions, and against the background of the geological events and phylogenetic relationships that have influenced the biodiversity and evolution of the investigated taxa. The relationship of the fauna to environmental parameters, such as water depth, sediment type, food availability and carbonate solubility, as well as species interrelationships, probably have shaped present-day biodiversity patterns as much as evolution. However, different taxa exhibit different large-scale biodiversity and biogeographic patterns. Moreover, there is rarely any clear relationship of biodiversity pattern with depth, latitude or environmental parameters, such as sediment composition or grain size. Similarities and differences between the SO biodiversity and biodiversity of global oceans are outlined. The high percentage (often more than 90%) of new species in almost all taxa, as well as the high degree of endemism of many groups, may reflect undersampling of the area, and it is likely to decrease as more information is gathered about SO deep-sea biodiversity by future expeditions. Indeed, among certain taxa such as the Foraminifera, close links at the species level are already apparent between deep Weddell Sea faunas and those from similar depths in the North Atlantic and Arctic. With regard to the vertical zonation from the shelf edge into deep water, biodiversity patterns among some taxa in the SO might differ from those in other deep-sea areas, due to the deep Antarctic shelf and the evolution of eurybathy in many species, as well as to deep-water production that can fuel the SO deep sea with freshly produced organic matter derived not only from phytoplankton, but also from ice algae.

The importance of biodiversity for the functioning of ecosystems is still very unclear. Many hypotheses, mainly based on terrestrial studies, have been formulated, focussing on the plant diversity-productivity relationship. In this study, diversity-related and species-specific effects of bacterivorous nematodes on the decomposition rate of cordgrass detritus Spartina anglica and on the associated microbial community were investigated using laboratory microcosm experiments. Four bacterivorous nematode species (Diplolaimelloides meyli, Diplolaimelloides oschei, Diplolaimella dievengatensis and Panagrolaimus paetzoldi) were added either separately or in combinations of 2 or 3 species to the microcosms. In contrast with previous reports, no stimulation of the decomposition process was observed in the presence of nematodes. Still, clear differences in process rates were found between nematode treatments. P. paetzoldi, reaching considerably higher densities than the other nematode species, suppressed bacterial activity and diversity, probably due to overgrazing. This was, however, not translated into slower decomposition. Within treatments exclusively containing monhysterid nematode species (D. meyli, D. oschei and D. dievengatensis), differences in microbial activity and decomposition rates were found, but again no link was observed between activity and decomposition. Our data did not support any hypothesis predicting enhancement of process rates with an (initial) increase in numbers of nematode species, or redundancy among the studied species. Rather, we obtained support for an idiosyncratic diversity model, as differences in the effect of bacterivore nematode species and species combinations could not be predicted at the start of the experiment. This could be explained by the inhibitory interactions between nematode species.

Many aquatic nematodes secrete mucus while moving, and prominent microbial growth on nematode mucus tracks has been observed. This has been interpreted as a mutualistic interaction in which nematodes may feed on the micro-organisms that colonise their tracks (i.e. the mucus-trap hypothesis). Because of recent evidence that nematodes can affect bacterial community composition, we tested whether bacterial communities growing on nematode mucus differ from extant communities. We characterised the bacterial epigrowth of tracks produced on agar by 2 estuarine nematode species (the facultative predator Adoncholaimus fuscus and the bacterivore Geomonhystera disjuncta) and compared it to that of artificial tracks and to the bacterial inocula. The experiment lasted 8 d, with bacterial community analyses (using fatty acid methyl ester [FAME] analysis) after 2, 4, 6 and 8 d. Although our experimental design promoted a low-diversity bacterial community, multidimensional scaling generally separated communities on nematode tracks from inocula, artificial track communities typically being intermediate and highly variable. In a total of 6 bacterial inocula spotted with A. fuscus, only 1 bacterial strain was recorded on nematode tracks, compared to 6 on artificial tracks and 7 in the inocula. In addition, colony morphology of this particular bacteria, Pseudoalteromonas tetraodonis, was less diverse on nematode tracks than on artificial tracks or inocula. Treatments with G. disjuncta yielded similar yet less consistent and less pronounced results. Our results suggest that nematode mucus may affect colonisation and succession patterns of bacteria. This may have important implications for food-web interactions and ecosystem functions involving both bacteria and nematodes.

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The present study explored the selective feeding properties of Antarctic and Arctic deep-sea nematodes within an experimental setup. Nematodes are assumed to play an important role in the carbon flux within the polar bathyal food webs, but knowledge about their natural diets is limited. For the first time, deep-sea multicore sediment samples from both polar regions were incubated aboard research vessels with either 13 C-labelled bacteria or diatoms to determine whether the nematode community prefers freshly settled phytodetritus to a bacterial food source. The cores were collected at 2112 to 2400 m water depth and incubated onboard for 1, 3 and 6 d in the Arctic (Hausgarten) and for 1, 7 and 14 d in Antarctica (Kapp Norvegia). Natural carbon isotope signals of nematodes and organic sedimentary carbon showed a clear average offset (+ 3.2 ‰). The contribution of bacteria to the diet of nematodes explained this 13 C offset and observed natural 13 C isotopic signatures. The nematodes showed a clear, relatively rapid (maximum at 6 to 7 d) and significant selective response to the pulse of 13 C enriched bacteria in surface sediments of both regions. This indicated that bacteria were preferred over fresh phytoplankton as a carbon source for both Arctic and Antarctic deep-sea nematode communities. The results also suggest that bacteria may provide a path through which unused detritus may enter the traditional metazoan food web by microbial reworking of organic matter. At the same time, uptake rates of nematode communities were minimal, which suggests the contribution of nematodes to benthic mineralisation of freshly deposited organic matter may be limited in deep polar seas. KEY WORDS: Deep-sea nematodes · Bacteria · Phytoplankton · Polar regions Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 406: [121][122][123][124][125][126][127][128][129][130][131][132][133] 2010 and bacterial activity (Sullivan et al. 1990). Benthic bacterial populations in Antarctic sediments can reach very high abundances (Novitsky 1987, Karl & Novitsky 1988) constituting another potentially abundant food supply for the benthos. Both diatoms and bacteria have been found to be an important food source for the free-living nematodes in shallow-water benthic environments (Moens & Vincx 1997). Information on nematode food sources in the Arctic and Antarctic is limited and for the deep sea there is a general lack of knowledge about which specific foods sustain the nematode communities. However, considering the importance of diatoms in the Antarctic and Arctic euphotic waters and their subsequent export to the deep-sea floor, along with the high pelagic and benthic bacterial productivity in these regions, both diatoms and bacteria may constitute important food sources for nematodes.Nematodes are the most abundant metazoan component of the meiobenthos (32 to 1000 Mm) in the deep sea and their numerical importance increases with water depth, as is reflected in their often high standing stocks (...