Latitudinal and temporal variability in the community structure and ...

K. Guilini et al. / Progress in Oceanography 110 (2013) 80–92 81and experimentally simulated depositions of phytodetritus is onlyminimal or absent compared to bacteria, foraminiferans and severalmacrofaunal taxa (e.g. Gooday, 1988; Graf, 1992; Pfannkuche,1993; Soltwedel, 1997; Galéron et al., 2001; Guilini et al., 2010 andreferences therein). The only rapid responses found to an episodicfood supply in the deep sea were an increase in mean nematodesize at the German BIOTRANS site (April–July; Soltwedel et al.,1996) and nematode migration towards the sediment surface inthe Polar Front (December–January; Veit-Köhler et al., 2011). Doubledmeiofaunal abundance (mainly nematodes), from summer toautumn in sediments from the bathyal Mediterranean (de Bovéeet al., 1990) and the Northeast Atlantic along the Hebridean margin(Mitchell et al., 1997) indicate, however, a response on a longertime scale. Therefore, it is speculated that deep-sea nematodesare feeding additionally or alternatively on rather unlimited, otherfood sources than phytodetritus (e.g. bacteria, ciliates, flagellates,foraminiferans, fungi, DOC) and that they may be characterizedby generally slower rates of colonization, respiration and somaticgrowth (Guilini et al., 2010, 2011). Rigid evidence that any of thesefood sources are of substantial importance to maintain the metabolicdemands of nematodes, or respiration measurements underin situ conditions are, however, lacking.This study describes and combines both structural and functionalaspects of deep-sea nematode assemblages along a latitudinaltransect following the Prime Meridian across the SO (49–70°S).Besides a spatial scale, a temporal scale was considered with regardsto the short-term response of nematode assemblages to aseasonal deposition of particulate organic matter (POM). Therefore,a station located in the Polar Front (52°2 0 S, 0°1 0 W), where a phytoplanktonbloom had settled during the course of the expedition,was revisited after 52 days to perform repeated measurements.Overall, structural aspects comprise standing stocks and communitycomposition, while functional aspects refer to feeding ecology,which is investigated by means of fatty acid (FA) compositions.Nematode FA compositions are determined to provide a time-integratedview on the organisms’ diets at the community level. The FAmarker concept relies on the fact that certain FA are incorporatedinto consumers in a conservative manner (e.g. Dalsgaard et al.,2003; Lee et al., 2006). By comparing nematode FA with what isknown from literature on surface water POM, other benthic organisms,bulk sediment and bottom-water POM properties, we aim toprovide insights as to the resources used by deep-sea nematodecommunities across the Southern Ocean. Although this techniqueis widely spread in marine ecological studies (see review by Kellyand Scheibling (2012)), it was only applied three times on marinefree-living nematodes (Leduc, 2009; Leduc and Probert, 2009; VanGaever et al., 2009).The following hypotheses were tested:(1) Structural and functional characteristics of deep-sea nematodeassemblages do not differ along a latitudinal gradientacross the SO.(2) Deep-sea nematodes at the Polar Front do not respond to aseasonal pulse of organic matter to the seafloor.2. Materials and methods2.1. Study site and sampling procedureSamples were collected during the ANDEEP-SYSTCO expeditionon board of the RV Polarstern (ANT XXIV/2, 28.11.2007–04.02.2008). Sediment samples were taken at depths from1935 to 5323 m, at six sites along a north–south gradient, inclose proximity of the Prime Meridian between 49°S and 70°S;across the Polar Front, the Weddell Sea and the Lazarev Sea(Fig. 1, Table 1). The stations are further indicated with theirabbreviations (PF: Polar Front, sPF: south Polar Front, cWS: centralWeddell Sea, MR: Maud Rise, LS: Lazarev Sea). All stationsoccur in different water masses and environments with differenttopographical, sedimentary and productivity conditions. The PF,sPF and cWS station are situated on the abyssal sea floor, MRis a seamount and LS is located at the continental slope. The stationlocated at a southern position in the Polar Front (sPF, at52°S) was visited twice; once when a phytoplankton bloomwas detected in the euphotic water layer based on increasedfluorescence levels (06.12.2007; Herrmann and Bathmann,2010) and 52 days later when the remains of the bloom had settledto the sea floor (26–27.01.2008). These remains were visibleas a greenish fluff layer on the sediment surface (Veit-Köhleret al., 2011). Sachs et al. (2009) calculated the labile portion ofthe organic carbon flux (LC org ) based on in situ and ex situ measurementsof O 2 profiles in surface sediments (Table 1).All meiobenthic samples were taken with a multiple corer(MUC) sampling device equipped with 12 plexiglass cores (innerdiameter: 9.4 cm, equivalent to 69.4 cm 2 ). The MUC was deployedone to three times per station, depending on ship time availabilityand sea state. The processed samples were thus a combination oftrue replicates and pseudo-replicates, since different cores fromthe same MUC deployment do not meet the criteria of randomsampling (Hurlbert, 1984). An overview of the processed samplesis given in Table 1. The single MUC deployment at the Polar Frontstation was only partially successful. It was visually determinedthat some cores were disturbed. The most undisturbed cores werereserved for FA analysis, in order to ensure enough organismscould be collected. From all stations, the sediment cores destinedfor meiofauna community analysis were sliced into 1-cm fractionsdown to 5 cm and together with the supernatant water preservedin a borax-buffered 4% formaldehyde-seawater solution. Furthermore,a minimum of four cores per deployment were used to scoopoff the upper 5 cm of sediment. This material was immediatelysieved in the lab with filtered seawater (32 lm mesh) over stackedsieves (1 mm, 500 lm, 100 lm and 32 lm) to prevent clogging.The retained 500 lm, 100 lm and 32 lm fractions were pooledand stored at 80 °C. Back at the laboratory, the nematodes wereextracted for fatty acid analyses.2.2. Nematode parametersSediment samples destined for identifying the nematode communitywere rinsed with tap water over a 32 lm mesh sieve (noupper sieve size used). The fraction retained on the 32 lm sievewas three times centrifugated with the colloidal silica polymerLevasil Ò (H.C. Stark, 200/40%, q = 1.17, for 6 min at 4000 rpm)and kaolin (McIntyre and Warwick, 1984) to extract all organisms.After staining with Rose Bengal, all metazoan organisms weremanually sorted to higher taxon level (following Higgins and Thiel,1988) and counted under a Leica MZ 12.5 stereomicroscope (8–100 magnification). Where possible, about 50–100 nematodeswere picked out randomly with a fine needle from each centimetersediment layer. They were gradually transferred to glycerine (Seinhorst,1959) before being mounted on glass slides. Nematodeswere identified to genus level under a compound microscope(1000 magnification). Adults were distinguished from juvenilesbased on the development of a vulva and uterus in females and agonad and spicules in males. Based on mouth morphology, all identifiedindividuals were classified into four feeding type groupsaccording to Wieser (1953): selective deposit feeders (1A), nonselectivedeposit feeders (1B), epistratum feeders (2A), and predators/scavengers(2B). By using a Leica DMR compound microscopeand Leica LAS 3.3 imaging software, nematode length (L, filiformtail excluded) and maximal width (W) were measured. Nematode