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

More documents

Recommendations

Info

$Wetenschap - \Nijverheid- Handel$

K. Guilini et al. / Progress in Oceanography 110 (2013) 80–92 87Table 6Results from main and pairwise one-factor multivariate and univariate PERMANOVA and PERMDISP analyses testing for differences in nematode community composition, andnematode density, relative abundance, biomass, mean individual biomass, and juvenile/adult ratio (J/A), respectively, over the top 5 cm of the sediment between the sPF and sPF(2nd visit) station. Significant differences are indicated in bold.Test Factor NematodecommunityNematodedensityNematode relativeabundanceNematodebiomassNematode meanindividual biomassPERMANOVA Time df 1 1 1 1 1 1MS 2169.6 1.3 10 6b 3.4 10 12 559.2 a 7.9 10 4 0.13Pseudo-F 1.49 4.58 Denominator is 0 9.35 0.10 7.65Sediment depth df 4 4 4 4 4 4MS 3865.3 3781300 2439.8 1159.5 0.03 0.27Pseudo-F 3.66 b 33.61 b 53.43 b 27.48 b 3.14 a 3.57 aRe(Time) df 4 10 10 4 4 2MS 1456 287860 5.3 10 13 59.8 0.008 0.28Pseudo-F 1.38 a 2.56 a Negative 1.42 0.91 4.27 aTime x Sediment depth df 4 4 4 4 4 4MS 929.8 188480 192.0 202.0 0.002 0.054Pseudo-F 0.88 1.68 4.20 a 4.79 a 0.26 0.84Residuals df 16 40 40 16 16 8MS 1055.1 112510 45.66 42.19 0.009 0.07Total df 29 59 59 29 29 29PERMDISP Sediment depth F 0.49 0.05 0.41 8.89 a 0.007 1.97df 1 1 1 1 1 1 4df 2 8 8 8 8 8 25Pairwise test 0–1 cm: sPF, sPF (2nd visit) t 1.12 0.20 2.40 a 1.551–2 cm: sPF, sPF (2nd visit) t 0.91 2.15 1.05 0.322–3 cm: sPF, sPF (2nd visit) t 1.26 2.22 2.02 0.053–4 cm: sPF, sPF (2nd visit) t 0.92 1.28 0.13 0.164–5 cm: sPF, sPF (2nd visit) t 0.90 2.10 1.37 1.37a 0.001 the broad latitudinal range covered along the ANDEEP-SYSTCO transect (49–70° S, i.e. around 2400 km), relatively littlevariation is found between the deep-sea nematode communitiesin terms of generic composition. Although the MDS plot (Fig. 2)gives an indication of a subdivision into four communities, thelow percentage of exclusive genera at each station, together withthe dominance of the same four genera between all stations andmeiofauna-inherent, high small-scale variability within the stationscreates considerable overlap between most of the stations.It is more likely that identification to species level would reveala clearer separation in subcommunities, since a high degree ofspecies turnover between SO stations was found before (Vermeerenet al., 2004; Fonseca et al., 2006; Ingels et al., 2006; De Meselet al., 2006). The most remarkable community composition differenceamong the ANDEEP-SYSTCO stations is however the communityat the seamount Maud Rise (MR) which differs from all butthe abyssal south Polar Front (sPF) station. Similarly, the compositionof several macrofaunal groups at this station showed aclearly different composition from other formerly sampled stationsin the deep Weddell Sea (Brandt et al., 2011). Moreover,similar to the nematode assemblages, the composition of polychaetesat this seamount showed highest affinity to the sPF stations(Wilmsen and Schüller, 2011). The unique hydrographyregime at the seamount MR creates conditions that result in a differentfrequency, quantity and quality of fresh food compared toother Weddell Sea areas (Abelmann and Gersonde, 1991; Weferand Fischer, 1991). However, since MR is under influence fromseasonal sea-ice coverage, resulting in a lower organic carbon fluxwith a different composition compared to the Polar Front region(Wefer and Fischer, 1991), the ecological conditions that determinethe similarity between the nematode communities of theserelatively distant areas are unknown. The fauna at other seamounthabitats is, however, apparently often similar to neighboringareas that fall within organisms’ preferred depth ranges (Clarket al., 2010).The composition of the deep-sea nematode assemblages consideredat genus level along the ANDEEP-SYSTCO transect is comparablewith deep-sea nematode communities worldwide. Threeout of four genera that dominate at all ANDEEP-SYSTCO stations(Acantholaimus, Halalaimus, and Desmoscolex) are known as dominantgenera (>2% relative abundance) at deep-sea (i.e. slope to hadalzone) areas worldwide (Vanreusel et al., 2010). Our resultstherefore confirm that there is no distinct Antarctic nematodecommunity at genus level (Vanhove et al., 1999, 2004; Sebastianet al., 2007). In the study of Vanhove et al. (1999) nematode (generic)structural and trophic diversity decreased from the shelfbreak and upper slope (EG(100) = 40, H 1 = 3.85–3.45) towardsdown slope (EG(100) = 30, H 1 = 2.94). In comparison, the LazarevSea (LS) station at a comparable depth as the downslope station inthe Eastern Weddell Sea, has a higher structural and trophic diversity(EG(100) = 39, H 1 = 3.5). The rarefaction index for the LSslope station also agreed well with the index calculated for slopesworldwide (EG(100) = 40; Vanreusel et al., 2010.) Similarly to whatwas found on a worldwide scale, the rarefaction index reducedwhen the less diverse abyssal communities were taken into account(EG(100) = 34.5 versus 35.5 in Vanreusel et al. (2010)). TheLS station seemed to be the area of highest polychaete diversitytoo (Wilmsen and Schüller, 2011). Two factors that are generallybelieved to increase diversity at slopes compared to the abyss areenhanced food input and the reduced stability of the continentalslope environment due to the higher eventuality of landslides(Wilmsen and Schüller, 2011 and references therein).Epistratum feeders (2A) sensu Wieser (1953) dominated at moststations (LS, MR, sPF). Vanhove et al. (1995, 1999, 2004) and Sebastianet al. (2007) observed a dominance of epistratum feeders inSouthern Ocean slope and abyssal sediments where the highest inputof organic matter was expected based on highest nematodedensities. They explained this as a functional adaptation of nematodecommunities to short-term events of fresh food supply. However,no such link emerges when our feeding type compositions are
88 K. Guilini et al. / Progress in Oceanography 110 (2013) 80–92compared with the labile portion of the carbon flux at each AN-DEEP-SYSTCO station, calculated by Sachs et al. (2009) (Table 1).4.2. Nematode fatty acid compositionsMost food web studies that investigate FA compositions of Antarcticmarine organisms focus on plankton (reviewed by Dalsgaardet al. (2003)). Although benthos is the richest element in the marinefood web in terms of numbers of species, their roles and interactionsare poorly known (Griffiths, 2010). Würzberg et al.(2011a,b) were the first to analyze the FA composition of membersof different Antarctic, benthic, polychaete families and orders ofperacarid crustaceans, in addition to their potential food sources,including sediment, bottom-water POM and foraminiferans. Thiswas done on samples collected at the same ANDEEP-SYSTCO stationsas the nematode samples for this study originated from.Moreover, the analyzed sediments originated from pseudo-replicatesamples, i.e. from the same multiple corer drops as the samplescollected to study the nematodes; thus allowing comparison.An indication for selective feeding behavior by nematodes isfound when comparing nematode FA patterns with FA data obtainedfor sediment and bottom-water POM from the stations sPF(2nd visit), cWS, MR and LS by Würzberg et al. (2011b). SFA(16:0, 18:0) and MUFA (16:1x7, 18:1x9, 18:1x7) dominated sedimentand bottom-water POM, while typical indicators of freshplanktonic detritus (PUFA; e.g. 20:5x3, 22:6x3) that occur in highproportions in nematodes (34–45%), were only detected in minorproportions in the sediment and bottom-water POM (9.4–28.7%and 9.1–19.2%, respectively, Table 7; Würzberg et al., 2011b). Thehigh portion of PUFAs in nematodes confirms that marine nematodescan be a high quality food source for predators (Leduc,2009). Nevertheless, the FA content of nematodes is relativelylow compared to e.g. marine zooplankton, i.e. minimum and maximumdiffering by one order of magnitude (Lee et al., 2006). Thisgenerally low FA content of the nematodes, together with the predominanceof typical phospholipids of biomembranes (20:5x3,22:6x3 and 16:0; Graeve et al., 1997) indicates that they do notaccumulate lipids for energy storage. This is in contrast with thestudy of Danovaro et al. (1999) who suggested that energy storageis a survival strategy of deep-sea nematodes based on their elevatedlipid concentrations compared to coastal nematodes. No increasein fatty acid content with depth appears along the ANDEEP-SYSTCO transect and the average fatty acid content in the Antarcticdeep-sea nematodes is considerably lower than in the Mediterraneandeep-sea nematodes (2% versus 17% of nematode dry weight,respectively). Whether the overall low fatty acid content of nematodesin this study is the result of low food availability or a constantfeeding behavior throughout the year is addressed furtherin the discussion based on the response to the seasonal food input.Overall, although FA profiles of polar lipids change with dietarycomposition (Peters et al., 2006), caution has to be taken wheninterpreting FA patterns of organisms with low lipid reserves. Sinceselective ingestion of fresh planktonic detritus cannot be distinguishedfrom strong retention of these highly unsaturated FA inmetabolic processes (Brett and Müller-Navarra, 1997) their proportionscould be enhanced (Würzberg et al., 2011b).A significant difference in nematode FA composition betweenthe southernmost slope station (LS) and all other stations wasfound. Furthermore, FA proportions of nematodes at the seamountMR differed from those at abyssal sPF (2nd visit) and PF stations.Since the pattern in FA compositions among stations is only weaklycorrelated with the pattern in nematode community compositions,the occurrence of distinct genera or distinct nematode feedinggroups (sensu Wieser, 1953) alone cannot explain the variance inFA compositions of the communities. We expect that the observedvariance in FA compositions can rather be explained by the combinationof variance in the FA composition of available food sourcesand the occurrence of distinct species with species-specific foodpreferences or metabolic characteristics resulting in different FAcompositions. The interpretation of FA as tracers of different foodsources is, however, complicated by many factors. Hydrodynamicprocesses in the first place, influence temperature, light and nutrientavailability, three key factors affecting the FA pattern of the localplanktonic primary producers (Dalsgaard et al., 2003). Waterdepth may also play a role, since lipid compositions of planktonand detritus can be altered considerably during sedimentationFig. 4. Nematode relative abundance (%) and total biomass (lg dwt per 10 cm 2 ) per centimeter from the south Polar Front station, before (black bars) and after (gray bars) thebloom settled. The averages and standard deviation of triplicates are plotted.Table 7Relative concentrations of saturated FA (SFA), monounsaturated FA (MUFA) and polyunsaturated (PUFA) in nematodes, sediment and bottom-water POM at the different ANDEEP-SYSTCO stations, based on data obtained in this study and the study of Würzberg et al. (2011b), respectively.PF sPF sPF (2nd visit) cWS MR LSNem Nem Nem Sed POM Nem Sed POM Nem Sed POM Nem Sed POMP SFA 20.8 32.1 25.0 35.1 47.9 21.3 25 58.9 20.6 31.2 62 29.9 30.9 45.4P MUFA 36.1 27.6 29.7 20.2 19 15.3 50.2 15.4 25.4 40.9 15.3 34.7 24.8 28.2P PUFA 43.1 40.4 45.3 11 11.3 34.2
Page 1 and 2: Progress in Oceanography 110 (2013)
Page 3 and 4: 82 K. Guilini et al. / Progress in
Page 7: 86 K. Guilini et al. / Progress in
Page 13: 92 K. Guilini et al. / Progress in

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?