Foraminifera in the Arabian Sea oxygen minimum zone and other oxygen-de"cient settings ...

30 Pages·1999·0.25 MB·English

Deep-SeaResearchII47(2000)25}54
Foraminifera in the Arabian Sea oxygen minimum
zone and other oxygen-de"cient settings:
taxonomic composition, diversity, and relation to
metazoan faunas
Andrew J. Gooday!,*, Joan M. Bernhard", Lisa A. Levin#,
Stephanie B. Suhr$
!SouthamptonOceanographyCentre,EmpressDock,EuropeanWay,SouthamptonSO143ZH,UK
"DepartmentofEnvironmentalHealthSciencesandMarineScienceProgram,UniversityofSouthCarolina,
Columbia,SC29208,USA
#MarineLifeResearchGroup,ScrippsInstitutionofOceanography,LaJolla,CA92093-0218,USA
$Institut fu(r Hydrobiologie und Fischereiwissenschaft, Universita(t Hamburg, Zeiseweg 9, 22679 Hamburg,
Germany
Received28August1998;receivedinrevisedform20January1999;accepted22January1999
Abstract
Previous work has shown that some foraminiferal species thrive in organically enriched,
oxygen-depletedenvironments.Here,wecompare&live’(stained)faunasinmulticorersamples
(0}1cm layer) obtained at two sites on the Oman margin, one located at 412m within the
oxygenminimumzone(OMZ)(O "0.13mll~1),theotherlocatedat3350m,wellbelowthe
2
main OMZ (O &3.00mll~1). While earlier studies have focused on the hard-shelled (pre-
2
dominantly calcareous) foraminifera, we consider complete stained assemblages, including
poorly known, soft-shelled, monothalamous forms. Densities at the 412-m site were much
higher (16,107 individuals.10cm~2 in the ’63-lm fraction) than at the 3350-m site (625
indiv.10cm~2).Speciesrichness(E(S )),diversity(H@,FishersAlphaindex)andevenness(J@)
100
weremuchlower,anddominance(R1D)washigher,at412mcomparedwith3350m.At412m,
small calcareous foraminifera predominated and soft-shelled allogromiids and sacamminids
wereaminorfaunalelement.At3350m,calcareousindividualsweremuchlesscommonand
allogromiidsandsaccamminidsformedasubstantialcomponentofthefauna.Therewerealso
strongcontrastsbetweentheforaminiferalmacrofauna(’300-lmfraction)atthesetwosites;
relativelysmallspeciesofBathysiphon,GlobobuliminaandLagenamminadominatedat412m,
very large, tubular, agglutinated species of Bathysiphon, Hyperammina, Rhabdammina and
Saccorhiza were important at 3350m. Our observations suggest that, because they contain
*Correspondingauthor.Tel.:[phone];fax:[phone].
E-mailaddress: [email protected](A.J.Gooday)
0967-0645/00/$-seefrontmatter ( 1999ElsevierScienceLtd.Allrightsreserved.
PII: S0967-0645(99)00099-5
26 A.J. Goodayetal./Deep-SeaResearchII47(2000)25}54
fewersoft-shelledandagglutinatedforaminifera,asmallerproportionofbathyal,low-oxygen
faunas is lost during fossilization compared to faunas from well-oxygenated environments.
Trends among foraminifera (’63lm fraction) in the Santa Barbara Basin (590 and 610m
depth;O "0.05and0.15mll~1respectively),andmacrofaunalforaminifera(’300lm)on
2
thePeru margin(300}1250mdepth; O "0.02}1.60mll~1), matchedthose observedon the
2
Omanmargin.Inparticular,soft-shelledmonothalamoustaxawererareandlargeagglutinated
taxawereabsentinthemostoxygen-depleted((0.20mll~1)stations.
Foraminifera often outnumber metazoans (both meiofaunal and macrofaunal) in bathyal
oxygen-depletedsettings.However,althoughphylogeneticallydistant,foraminiferaandmeta-
zoans exhibit similar population responses to oxygen depletion; species diversity decreases,
dominanceincreases,andtherelativeabundanceofthemajortaxachanges.Theforaminiferal
macrofauna(’300lm)were5timesmoreabundantthanthemetazoanmacrofaunaat412m
ontheOmanmarginbut 16timesmore abundantat the3350msite.Amongthemeiofauna
(63}300lm),thetrendwasreversed;foraminiferawere17timesmoreabundantthanmetazoan
taxaat412mbutonly1.4timesmoreabundantat3350m.Anabundanceoffoodcombined
with oxygen levels which are not depressed su$ciently to eliminate the more tolerant taxa,
probablyexplainswhyforaminiferaandmacrofaunalmetazoans#ourishedatthe412-msite,
perhapstothedetrimentofthemetazoanmeiofauna. ( 1999ElsevierScienceLtd.Allrights
reserved.
1. Introduction
Persistent oxygen depletion of bottom water and sediment pore water occurs in
certain regions, mainly associated with areas of upwelling or restricted circulation
(DiazandRosenberg,1995).Oxygenminimumzones(OMZs)arelong-termoceano-
graphicfeaturesthatleadtothedevelopmentofstrongoxygengradientswherethey
impingeonthesea#oor.Waterinsilledbasinsandfjordsalsomaybecomedepletedin
oxygenoverlongtime periods dueto poor circulationandenrichmentwith organic
matterderivedfromnaturalsourcesorpollution(Alve,1995a,b;PhlegerandSoutar,
1973). Other basins are periodically #ushed, sometimes seasonally, with oxygenated
waterleadingtoacycleofoxygendepletionandrenewal(e.g.BernhardandReimers,
1991).Riverinputsof"ne-grainedsedimentsrichinorganicmatteralsomayleadto
theperiodicdevelopmentoflow-oxygenoranoxicconditionsoncontinentalshelves
(Murrell and Fleeger, 1989), although the distribution of these regions is strongly
in#uencedbyhydrographicandotherconditions(vanderZwaanandJorissen,1991).
The importance of foraminifera in oxygen-de"cient benthic communities has
prompted reviews by Sen Gupta and Machain-Castillo (1993), Bernhard (1996) and
Bernhard and Sen Gupta (2000). Studies on low-oxygen foraminifera have focused
mainly on the following topics: taxonomic composition of assemblages, adaptive
morphology, life-history strategies (&opportunism’) and cellular ultrastructure, and
experimental studies involving foraminiferal responses to oxygen depletion, anoxia
and hydrogen sulphide. Most of these studies have concerned the fossilizable
(calcareousand multilocularagglutinated)componentof the fauna. With the excep-
tion of some recent experimental work (Bernhard and Alve, 1996; Moodley et al.,
1997,1998),littleisknownabouttheresponseofmorphologicallysimpleforaminifera
A.J. Goodayetal./Deep-SeaResearchII47(2000)25}54 27
(monothalamous allogromiids and saccamminids and tubular agglutinated taxa) to
oxygen depletion. Moreover, very few studies speci"cally address the structure and
diversity of complete (i.e. including all taxonomic components) rose Bengal-stained
(&live’)assemblages,orcompareforaminiferalandmetazoanabundanceanddiversity.
Wefocushereontheserarelyaddressedaspectsofbenthicforaminiferalfaunasin
low-oxygen habitats. We consider data from two contrasting sites on the Oman
margin.Onesiteislocatedat412mdepthinthecoreoftheOMZandispermanently
dysoxic. The other is located below the main OMZ at a depth of 3350m and is
reasonablywelloxygenated.Ourprincipleaimistoprovideadetaileddescriptionof
thestructureofcompletestainedforaminiferalassemblagesatthetwosites.Wethen
usethesefaunaldata(1) tocompareandcontrasttrendswithrespectto oxygenand
organic enrichment gradients among foraminifera and metazoans (both meiofauna
and macrofauna) and (2) to compare the diversity and taxonomic composition of
theOmanmarginassemblageswiththoseoffaunasfromtheSantaBarbaraBasinon
the California Borderland, where oxygen concentrations #uctuate according to the
hydrographicconditions (Bernhard and Reimers, 1989).
Aspects of the metazoan macrofauna from the shallower Oman margin site were
describedbyLevinetal.(1997).Metazoanmacrofaunafrombothsitesaretreatedby
Levin et al. (2000) and nematode abundances by Cook et al. (2000). Herring et al.
(1998) described the hydrography and midwater biology of the area on the basis of
observations made during August 1994. Gooday et al. (1997,1998) presented some
preliminary data on large agglutinated foraminifera and species diversity from the
deepest site (around 3400m) sampled during Discovery Cruise 211. The only other
previousstudiesofmodernbenthicforaminiferafromtheOmanmarginarethoseof
Stubbings(1939)andHermelinandShimmield(1990).Burmistrova(1969,1976,1977),
Zobel (1973), Gupta (1994), Jannink et al. (1998) and den Dulk et al. (1998) provide
accounts of Recent or Holocene foraminifera in other parts of the Arabian Sea.
2. Materials and methods
2.1. Sampling
The Oman margin samples were collected during R.R.S. Discovery Cruise 211
(October9 to November 11 1994) using a Barnett}Watsonmultiple corer equipped
withcoretubesof25.5cm2cross-sectionalarea.Assoonaspossibleafterrecovery,the
coresweretaken toacoldroom adjustedtotheambientbottom-watertemperature
(Table 1). A small amount of the &topmost’ sediment ((1ml) was removed using
a plastic pipette in order to sample the fauna living at the sediment-waterinterface.
Thecorewasthenslicedinto1cmthicklayersdownto10cmdepth.Allsampleswere
preserved in 10% formalin ("4% formaldehyde solution) bu!ered with sodium
borate(borax).Thetop1cmand&topmost’sedimentmaterialfromonecoretakenat
each site were examined for this study.
Inthelaboratory,thesedimentwassievedon300-,125-and63-lmscreens,stained
on the sieve for several hours using Rose Bengal, and sorted in water under a
28 A.J. Goodayetal./Deep-SeaResearchII47(2000)25}54
Table1
EnvironmentaldatafortheOmanmarginstudysites,summarisedfromLevinetal.(2000)
Property Station12692 Station12687
Depth(m) 412m 3350
Temperature(3C) 13}14 1.7
Oxygen(mll~1) 0.13 &3.00
%TOC 4.9 2.71
C:Nratio 8.5 9.4
Meangrainsize 42.2 42.8
%CaCO3 55.0 39.5
Surface(0}0.5cm)pigments(lgg~1) 770 185
binocularmicroscopeforstainedbenthicforaminifera.Inthe case ofthe 63}125-lm
fraction,theresiduewassplitusingaplanktonsplitteranda1/16thsubsamplesorted
completely for foraminifera. Some well-stained specimens were mounted dry on
micropaleontologicalslidestoprovidetaxonomicreferencematerial.However,most
were placed in glycerol in a glass cavity slide to enable the stained contents to be
examined in detail under a compound microscope. Species di!ered in their staining
characteristics.Someconsistentlystainedbrightlywhileothersincludedbothbrightly
stained and less well-stained specimens. In all cases, only the brightly stained tests
were regarded as &live’. Metazoan meiofauna also were removed from the sieve
residues and counted. Data for metazoan macrofauna were obtained from boxcore
samples collected nearby. Details of metazoan macrofaunal sampling and sample
processing methods are given in Levin et al. (2000).
Samples were taken in the Santa Barbara Basin (SBB) using a Soutar-type box
corerwhichwassubsampledusingacut-o!syringe,either1.9cm(StationSBB298E)
or2.6cm(StationSBB298S)diameter.Thetop1cmofeachsubcorewasremovedand
"xedin3%glutaraldehydein0.1Msodiumcacodylatebu!er.Thesamplewasplaced
in a graduated cylinder, agitated, and a subsample drawn o! with a plastic pipette.
Both the subsample and the original sample were allowed to settle in graduated
cylinders for 1h and their volumes recorded. The subsample was stained in Rose
Bengal for 12h, sieved over a 63-lm screen, and sorted wet for all stained benthic
foraminifera.Thevolumeofmaterialsortedwas1.215cm3(outofanoriginalvolume
of 2.84cm3) from Station SBB298E and 1.99cm3 (out of an original volume of
5.3cm3)fromStationSBB298S.AsubsamplefromStationSBB298Swasalsosieved
over a 125-lm screen.
Rarefaction curves and diversity indices (H@ log , J@log , Fisher alpha) were
10 10
calculated using the &BioDiversity’ programme (( Natural History Museum,
London/ScottishAssociation of Marine Sciences).
2.2. Study sites
The 412-m site (Discovery Station 12692d4; 19322.07@N, 58315.43@E) was situated
close to the core of the OMZ. The 3350-m site (Discovery Station 12687d8;
A.J. Goodayetal./Deep-SeaResearchII47(2000)25}54 29
18359.33@N,58359.09@E)wassituatedwellbelowthelowerOMZboundary,locatedat
approximately1000m (Levin et al., 2000). The horizontal distance between the two
sites was about 87km.
EnvironmentaldatafortheselocalitiesaresummarisedinTable1.Thesedimentat
412m was a soupy mud rich in organic matter, with little evidence of bioturbation
although without obvious laminations (Smith et al., 2000). Oxygen concentrations
weremuchloweratdepthsaround400m(0.13mll~1)thanat3350m(&3.00mll~1)
(Levin et al., 2000). The C:N ratios and mean sediment grain size were similar
betweensites.Asurfacelayerofphytodetrituswaspresentonmulticorersamplesfrom
thedeeperstation.PercentTOCandsurfacepigmentconcentrationswereallhigher
at 412m than at 3350m, suggesting more enriched conditions at the shallower site.
However, considering the bathymetric depth, both %TOC and pigment concentra-
tions were surprisingly high at 3350m (Gooday et al., 1998, Table 1 therein). This
probably re#ects the fact that phytoplankton pigment concentrations in the upper
water column are broadly similar above the two sites (Banse and McClain, 1986,
Fig.4).Thus,thegreaterbathymetricdepthanddistancefromlandofthedeepersite
(Lee et al., 1998), rather than contrasts in surface primary productivity, seem to be
responsibleforthedi!erencesinTOCandpigmentconcentrationsshowninTable1.
CoresfromthecentralSBB(Station298E;34313.48@N;120302.93@W;590mdepth)
consisted of "ne-grained mud with well preserved laminations and organic carbon
values of 5}8% (Reimers et al., 1990). Bottom-water oxygen concentrations were
approximately 0.05mll~1 and samples were devoid of animals larger than about
1cm. Station 298S o! Palos Verde (33346.69@N; 118332.49@W; 610m depth) was
characterisedby rather coarser-grained sediments (silty mud), bottom-water oxygen
concentrations of 0.15mll~1, and occasional ophiuroids. Oxygen concentrations
were measured using microwinkler analysis (Broenkow and Cline, 1969) in water
collected in Niskin bottles attached to the frame of the corer.
2.3. Terminology and taxonomic notes
Thereis noconsistent,universallyacceptedterminologyforbottom-wateroxygen
concentrations(TysonandPearson,1991).Here,weemploythetermsoxic(’1.00ml
O l~1), dysoxic (0.10}1.00ml O l~1) and microxic ((0.10ml O l~1; with or
2 2 2
without reducing conditions) (Bernhard and Sen Gupta, 2000). Oxygen depletion is
usedasageneraltermforconcentrations(1.00mll~1.Mostmacroinfaunalanimals
have some direct contact with the overlying bottom water. However, we recognise
that oxygen concentrations within the sediments will be lower than bottom-water
values and may also #uctuate spatially and temporally. The amount of oxygen
availabletomeiofaunalorganismslivingbelowthesediment/waterinterfaceisthere-
foreverydi$culttomeasureaccuratelyand,inanycase,willchangeastheorganisms
move around within the sediments (Moodleyet al., 1998).Hence, we use the above-
mentionedtermsto characterisebenthic habitats and not the microenvironmentsin
which individual foraminifers reside.
Thetwomostabundantspeciesat412mareproblematic.Thetop-rankedspeciesis
aslenderformresemblingBolivinaseminudaCushman1911(seeBarmawidjajaetal.,
30 A.J. Goodayetal./Deep-SeaResearchII47(2000)25}54
1992, p. 2, Figs. 1}4). The second-ranked species, while closely similar, has more
in#atedlaterchambersandresemblesBolivinainyataHeron-AllenandEarland1913
(see Todd and BroKnnimann, 1957, p. 8, Fig. 32}34). We recognise that B. seminuda
exhibits a continuous range of variation with B. dilatata Reuss. However, the form
thatweidentifyasB. inyata has moreclearlyin#ated chambersthan illustrationsof
B. dilatata given by Barmawidjajaet al. (1992) and Janninket al.(1998, p. 1, Fig.1).
WeregardB.inyataandB.seminudaasdistinctspeciesinourmaterial,althoughtheir
taxonomic status merits further study.
Species are classi"ed into higher taxa according to the system of Loeblich and
Tappan (1987). The sub-order Rotaliina includes buliminid and bolivinid genera.
3. Results
3.1. Abundance
Stained foraminiferaldensitiesin the upper 1cm of sedimentwere di!erent at the
twosites.Inthe’300-lmfraction(i.e.themacrofauna),foraminiferawere1.5times
more numerous at the 412-m site (80 individuals.10cm~2) compared to the 3350-m
site (52 indiv.10cm~2). The di!erential increased when "ner sieve fractions were
considered.Inthe’125-lmfraction(125}300plus’300lm),foraminiferawere7.5
times more numerous at 412m (2457 indiv. 10cm~2) than at 3350m (342 indiv.
10cm~2), while in the ’63-lm fraction (63}125 plus 125}300 plus ’300lm),
densities were about 25 times higher at the 412-m site (16,107 indiv. 10cm~2;
estimatedfroma1/16thsplit)thanatthe3350-msite(625indiv.10cm~2)(Table2).
These density values include foraminifera extracted from the &topmost sediment’
andassociatedphytodetritalaggregates.Thetopmostsediment(volume0.65ml)from
the412-mcorewasextremelyrichinstainedspecimens.Comparedtotheremainder
of the 0}1cm layer, it yielded 2.9 times (’125lm) and about 3.7 times (’63lm)
moreforaminifera,whennumberswerenormalisedtothesamevolume(Table2).At
3350m, there was no clear concentration of stained foraminifera in the ’125-lm
fractionoftopmostsediment,butsmallerspecimens(thoseinthe’63-lmfraction)
wereabout1.7timesmoreabundantinthesurfacematerialthaninthe0}1cmlayer.
Phytodetritalaggregates present in the 412-m core contained 33 stained individuals
("13indiv. 10cm2). Only three individuals were directly associated with the
phytodetrituspresentinthe3350-mcore.Inbothcasestheamountofphytodetritus
present was very small ((0.1ml).
3.2. Taxonomic composition
Hyaline calcareous foraminifera (rotaliids) dominated the stained fauna at
412m. Their dominance was particularly evident in the ’63-lm fraction where
85%of specimenswere rotaliids comparedto about 70%in the ’125-lmfraction
(Table3).Theonlyothertaxa(allbutonenon-calcareous)representing’1%ofthe
assemblage (’125- and ’63-lm fractions) were allogromiids, bathysiphonids
A.J. Goodayetal./Deep-SeaResearchII47(2000)25}54 31
nt
re %) %) %) %)
!e 2m %))%3 3 )%7 0
ndindi Total 26.75c 13(2.152(8.3277(44.342283(45.625 13(2.5229(43.241283(54.524
a
s
e
g
n
a
m)sizer hytodet. small
l P v. 1 0
0
0
3
}
l#3}125m125 (3350m) Topmostsediment &1ml 090(9.1)%220(22.2%)310680(68.6%)990 2(6.5)%11(35.5%)1318(58.1%)31
6 7
meiofaunal(2m). Station1268 0}1cm 325.75cm 13(2.2%)49(8.4)%269(45.9%)330256(43.7%)586 11(2.2)%218(44.1%)228265(53.6%)494
dc
l’nal(300m)anberspercore(25.5 Total 325.0cm 080(0.5)%2453(15.2%)253313,457(83.5%)16,107 4(0.42)%368(39.1%)372569(60.5%)940
2m)inmacrofauatedfromnum Phytodetritus v.small 13 0 lmfraction.
dforaminifera(individualper10c~2Numbersareindiv.10cm,calcul Station12695(412m) 0}1cmTopmostsediment 3324.35cm0.65cm 0065(0.45)600(1.09)%%5836(15.9%)6460(11.7%)24657059!12,235(83.9%)47,908(87.2%)14,70054,967 3(0.32)16(3.5)%%368(39.6%)16(3.5%)37132!558(60.1%)428(92.6%)929462 completesortof1/16thof63}125
nee. on
aipl d
Table2 Abundanceofstpartsofthesam Volume ForaminiferaAttachedlTotal300m>l125}300mlTotal125m>l63}125mlTotal63m> MetazoanslTotal300m>l125}300mlTotal125m>l63}125mlTotal63m> !Estimatebase
32 A.J. Goodayetal./Deep-SeaResearchII47(2000)25}54
arhe m
ult l
ocof 25 2 6 9 98
ultilplit ’1 6.2 0.90 000000021.12.3013.92.30.4052.60 209
ms
" h
6t
F1 m
A1/ l
nt’;Mona 298SSBB610m0.15 ’63 1.18 0.290 0.5900000018.01.7703.533.830.290.2970.20 339
meed
edibas m
pmostser)are 298ESBB590m0.05 l’63 0 1.390 0000000006.69000091.90 866
oy
"&tla
m m
Sc l
T1 5
SBB).92(0} TS’12 1.69 03.59 0.2110.8000002.1130.90.630.631.480.630.2149.00 473
(6
n2
asin1 m
Bo l
rabaramStati TS’63 0.39 0.220.48 0.031.9600.110003.1112.44.710.52000.0376.40 3566
ao
Bfr m
arginstationsandSantal’125mfractions)data 12692Omanmargin412m0.13 0}1cm0}1cmll’’63m125 0.882.77 0.090.230.093.79 0.010.081.095.55000.300.05(0.010.030.150.05000.013.596.0011.11.870.861.941.0100.01000084.970.700 33,0576487
m#
m
n
am l
ml 5
oO125 ’12 6.1 8.95.48 1.81.973.954.902.5701.982.42.22000.211.170.351.173.97 61
oupsatthetwl63m(i.e.63} 12687Omanmargin3350m!3.00 l’63m 22.61 23.215.93 7.1211.062.124.811.310.501.1216.423.500.060.310.061.870.694.182.29 16048
gr’
r
oe
majTh
ofra.
Table3Percentageabundanceagglutinatedforaminifel63}125mfraction. Station Bathymetricdepth~1O(ml.l)2 Sizefraction AllogromiinaAstrorhizacea:Saccamminidae:Soft-shelled‚agenamminaPsammosphaeridae:SpheresanddomesBathysiphonidaeOthertubesHyperamminaceaKomokiacean-likeAmmodiscaceaAgglutinatedchainsHormosinaceaTrochamminaceaSpiroplectamminaceaTextulariaceaOtherMAFMiliolinaLageninaRotaliinaOthertaxa Totalspecimens
A.J. Goodayetal./Deep-SeaResearchII47(2000)25}54 33
(Bathysiphon spp.), rigid saccamminids (Lagenammina spp.), hormosinaceans (almost
entirely Leptohalysis spp.), trochamminaceans, spiroplectamminaceans, textularia-
ceans and lagenids (mainly Lenticulina). The topmost sediment at this site yielded
a disproportionately large number of trochamminaceans and bathysiphonids com-
pared to the 0}1cm layer (Table 3).
In contrast, hyaline calcareous foraminifera (lagenids and rotaliids), trocham-
minaceans and spiroplectamminaceans were rare in the 125- and ’63-lm frac-
tions at the 3350-m site while all other taxa were more abundant than at 412m.
Allogromiids,soft-shelled saccamminids and hormosinaceans (mainly Reophax spp.)
were most frequent and rigid agglutinated spheres and domes, bathysiphonid and
other agglutinated tubes, rigid saccamminids (Lagenammina spp.) and hippo-
crepinaceans(tinyHyperamminaspp.)werealsocommon,againinbothsizefractions.
At the 412-m site, more than half of the 10 most abundant species in both the
’63-and’125-lmfractions(totalsample,i.e.0}1cmlayerandtopmostsediment
combined) were rotaliids (Table 4). The top-ranked species was Bolivina seminuda.
Species abundances in the topmost sediment (’63lm) were rather di!erent from
thosein the total sample,suggestingthat somespecies are concentratedclose to the
sediment}water interface. In particular, Bolivina inyata was ranked "rst in the top-
most sediment and a tiny agglutinated species, Morulaeplecta sp., was substantially
moreabundant(3.89%)inthislayerthaninthe0}1cmlayerasawhole(0.93%).At
the 3350-m site, only one calcareous species (Epistominella exigua) appeared among
thetopten(’63-lmfractiononly)(Table4).Mostoftheremainingspeciesbelonged
toa variety ofagglutinated taxa.A species of thehormosinaceangenus Leptohalysis
sp. was ranked "rst in both residues.
3.3. Diversity and dominance
Sixty-four unfragmented species were recognised in the ’125-lm fraction at
412m. About 5000 specimens removed from the 63}125-lm fraction (topmost sedi-
ment and 1/16th split of 0}1cm sample) yielded only six additional species. The
3400-m site was more speciose with 158 unfragmented species recognised in the
’125-lm fraction and 208 in the ’63-lm fraction. Species richness measured by
rarefaction (E(S )) was 23.9 at 412m and 58.7 at 3350m (’125-lm fraction).
100
Rarefaction curves (Fig. 1) con"rm these strong di!erences. Dominance (R1D) was
substantially higher at 412m (27.0%) compared to 3350m (6.8%). It is even higher
(44.1%) in the ’63-lm fraction of the topmost sediment from the shallower site.
The 412-m assemblage (’125-lm fraction; 0}1cm layer) yielded a Shannon}
Wiener Information index (H@) of 1.228, Fisher alpha index of 9.28, and Shannon
eveness measure (J@) of 0.69 (Table 5). We have not incorporated data from the
63}125-lm residue of this sample into diversity calculation since only a subsample
was sorted. However, diversity measures for the ’63-lm fraction of the topmost
sedimentwereconsistentlylowerthanforthe0}1cmlayer(Table5).Corresponding
values for the 3350-m assemblage (’125-lm fraction) were as follows: H@"1.939;
Fisher Alpha index"60.97; J@"0.87.
34 A.J. Goodayetal./Deep-SeaResearchII47(2000)25}54
Table4
Percentageabundanceoftoptenspeciesindi!erentsizefractionattwositesontheOmanmargin.‘Totala
refers to 0}1cm layer and topmost sediment combined. TS"topmost sediment, SWS"Soft-walled
saccamminid. Major taxa are identi"ed as follows: A"Allogromiida; He"Hemisphaeramminidae;
Ho"Hormosinacea; L"Lageniina; P"Psammosphaeridae; R"Rotaliina; Sp"Spiroplectam-
minacea; Sr"Saccamminidae rigid-walled; Ss"Saccamminidae soft-walled; Te"Textulariacea;
Tr"Trochamminiacea. Trochammina sp. 1 (412-m site) may be conspeci"c with Trochammina pacixca
CushmanofBernhardetal.(1997)and„rochamminasp.ofKaminskietal.(1995).
12695dd2(412m)
TS’63lm Total’125lm Total’63lm
B.in-ata R 38.1 Bolivinaseminuda R.24.2 Bolivinaseminuda R 43.8
Bolivinaseminuda R 18.4 B.in-ata R 8.47 B.in-ata R 20.6
„rochamminasp.1 Tr 9.27 „rochamminasp.1 Tr 7.96 Buliminasp. R 7.36
?Morulaeplectasp. Sp 3.89 Buliminasp. R 5.22 „rochamminasp.1 Tr4.79
Buliminasp. R 3.83 Suggrundasp. R 5.22 ?Fursenkoinasp. R 4.51
‚eptohalysissp.1 Ho2.66 Nonionellasp. R 5.08 Chilost.ovoidea R 2.52
?Fursenkoinasp. R 2.44 Bolivinasp.1 R 4.14 Nonionellasp. R 1.57
‚enticulinan N 2.03 ‚eptohalysissp.1 Ho3.02 ?„extularia Te1.57
Suggrundasp. R 1.95 Bathysiphonsp.2 A 2.94 ‚eptohalysissp.1 Ho1.06
;vigerinaperegrina R 1.84 Chilostomellaovoidea R 2.47 Spiroplectammina Sp0.88
12687dd8(3350m)
Total’125lm Total’63lm
‚eptohalysissp.2 Ho 6.79 ‚eptohalysissp.2 Ho5.30
‚agenamminasp.1 Sl 4.70 ‚agenamminasp.1 Sl 4.34
Reophaxsp.2 Ho 3.52 SWSsp.1 Ss 4.27
SWSsp.1 Ss 3.26 Allogromiidsp.1 A 4.27
Indet.psammosph. P 3.00 0Nodellum’sp. A 2.72
Agglut.tube Indet. 2.74 Reophaxsp.1 Ho2.28
R.a!.scorpiurus Ho 2.74 Reophaxsp.2 Ho1.99
Reophaxsp.3 Ho 2.61 Allogromiidsp.2 A 1.99
Crithioninasp. He 2.48 Epistominellaexigua R 1.84
R.a!.pilulifera Ho 2.09 R.a!.scorpiurus Ho1.77
3.4. Population size structure
The foraminiferal macrofauna (’300lm) made up a smaller proportion of all
foraminifera at 412m than at 3350m. Only 0.5% of the total fauna (’63lm) was
retainedona300-lmsieveat412mcomparedwith8.3%atthe3350-msite(Table2).
Maximumtestdimensionscon"rmthisshift(Fig.2).Althoughtestshadasimilarsize
rangeatthe twosites, 92.9% were(500lm at 412mcomparedwith66.3%atthe
deeper site. On the other hand, maximum dimensions peaked around 120lm at
3350mbut around160lmat412m,re#ectingthemoreelongatetestmorphologies
of many foraminifera within the OMZ.