Barron, J., Larsen, B., et al., 1991Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 119

35. COMPARISONS OF DIATOM BIOCOENOSES WITH HOLOCENE SEDIMENT ASSEMBLAGESIN PRYDZ BAY, ANTARCTICA1

Dean A. Stockwell,2 Sung-Ho Kang,3 and Greta A. Fryxell3

ABSTRACT

Quantitative and qualitative examinations of the planktonic diatom population of Prydz Bay, East Antarctica, wereconducted during Ocean Drilling Program Leg 119. Water column production was intense, as evidenced by high concen-trations of chlorophyll a (ranging from 40.0 to 576.5 mg Chi a m~2) and by high diatom abundances (ranging from 3.76× 1010 to 2.9 × 1011 cells m~2). Water column biomass was primarily dominated by three ice-related pennates: Nitz-schia closterium, N. cylindrus, and N. curta. Together, these three diatoms accounted for 64%-96% of the water col-umn assemblage.

Sediment diatom assemblages mirrored this intense water column production, with valve numbers ranging from 3.4× 107 to 6.8 × 109 valves g~l sediment. The genus Nitzschia typically formed 70% of the diatom thanatocoenose. N.curta and N. cylindrus together accounted for 34.6%-78.3% of the total assemblage. The heavily silicified restingspores of Chaetoceros were also an important component of these sediments (accounting for 8%-34% of the total as-semblage).

INTRODUCTION

Sediments of the Antarctic continental shelf have been char-acterized by Kozlova (1966), Edwards and Goodell (1969), andLisitzin (1962, 1972) as being terrigenous and glacially derived.These terrigenous, iceberg sediments consisting of silt-clayey de-posits surround Antarctica extending seaward from the shore-line for 500-640 km (Kozlova, 1966). Within such sediments,diatom remnants may constitute 30%-60% of the sedimentarymatter. This type of clayey deposit, greatly enriched in diatomfrustules, has been found to characterize Prydz Bay, Antarctica(Lisitzin, 1960; Kozlova, 1966). Diatom concentrations are re-portedly high within these regional oozes, approaching 1 × 108

frustules g~1 sediment (Kozlova, 1966).Numerous investigations conducted since 1844 (Kozlova, 1966;

Fenner et al., 1976; Schrader, 1976) have established that Ant-arctic phytoplankton assemblages overlying these sediments liveunder a unique set of environmental conditions. Phytoplanktonpopulations are classically diatom-dominated, although Phaeocy-stis blooms commonly occur (Garrison et al., 1987; Fryxell andKendrick, 1988). Combining consistently low water tempera-tures and abundant nutrients with small to negligible verticaldensity gradients, phytoplankton productivity for inshore wa-ters appear unusually high (El-Sayed, 1970; Tilzer et al., 1985).These living phytoplankton assemblages are themselves charac-terized by wide regional variations in both species compositionand biomass (Tilzer et al., 1985).

During Leg 119 of the Ocean Drilling Program, water col-umn samples were collected in conjunction with the drilling offive sites in Prydz Bay, Antarctica. The study was supplemen-tary to the investigations done on fossil diatoms within the sedi-ment cores. Figure 1 illustrates the location of the Prydz Baydrill sites and station locations. Table 1 shows the position ofthe stations, sampling dates, water depth, sea surface tempera-

1 Barron, J., Larsen, B., et al., 1991. Proc. ODP, Sci. Results, 119: CollegeStation, TX (Ocean Drilling Program).

2 Marine Science Institute, P.O. Box 1267, The University of Texas at Austin,Port Aransas, TX 78373-1267, U.S.A.

3 Department of Oceanography, Texas A&M University, College Station, TX77843-3146, U.S.A.

ture, and water column chlorophyll a concentrations for eachstation occupied. The objective of this study was to comparewater column diatom populations with assemblages found inthe surface sediments.

METHOD OF STUDYWater column sampling was conducted from the service ves-

sel, Maersk Master, in the vicinity of the five ODP drill sites(Sites 739-743) occupied from 18 January to 3 February 1988.Hydrographic measures of light penetration, chlorophyll a con-centrations, water temperature, salinity, and density were madewith respect to depth at the 14 stations occupied (see Barron,Larsen, et al., 1989). These stations were chosen at random on adaily basis as the service vessel had time available for sampling.Quantitative phytoplankton cell counts were routinely madefrom discrete water column samples and integrated over the 200m water column (Kang, 1989). A detailed analysis of distribu-tion of the most abundant phytoplankton within Prydz Bayduring this study is presented by Kang and Fryxell (this volume).Vertical and net hauls (35 µm) were made to a depth of 200 m inconjunction with each hydrocast to supplement species identifi-cations. Additionally, vertical hauls of open and closing nets (35µm) were made over the following depth intervals: 0-50 m; 50-100 m; and 100-200 m. The natural autofluorescence of chloro-phyll was excited by UV epifluorescent microscopy on boardship {Maersk Master) in order to assess living phytoplanktoncells.

Quantitative sediment samples were processed as describedby Stockwell (this volume). Sediment samples were oven driedfor 24 hr at 60°C. The dry weight of each sample was measured.The samples were acid cleaned and repeatedly washed with de-ionized water. Slide preparations followed the procedures ofSchrader (1974) and Schrader and Gersonde (1978) with modifi-cations as suggested by Laws (1983). Sedimentation concentra-tions were adjusted to represent known amounts of sedimentper slide. A microscope slide of each surface sediment samplefurnished data on species composition, relative species abun-dances, and on the number of frustules per gram of sediment.Hyrax mounts of mud-line samples were prepared on board shipor in the laboratory from preserved material using methods forplanktonic samples (Simonsen, 1974). Relative proportions weredetermined for each drill site based on the examination of ap-

667

D. A. STOCKWELL, S.-H. KANG, G. A. FRYXELL

M<>0 < v yAntarctic Circle

Four Ladies?. ^(g)742 Bank U

-Ufc.' .

• • . •. •

iiliiill70 u E 75UE βO°E

Figure 1. Prydz Bay, East Antarctica site locations occupied during Ocean Drilling Program, Leg 119. Bathymetric contoursshown in meters.

proximately 300 cells recovered from near-surface sediments.Sample locations are as follows: Hole 739C, mud line; Hole740A-1R-1, 1-2 cm, Hole 740B-1R-1, 1-2 cm; Hole 741A, mudline and Hole 741A-1R-1, 5-6 cm; Hole 742A, mud line andHole 742A-1R-1, 4-6 cm; and Hole 743A, mud line. Countswere made on a Nikon Model SE light microscope at 650 ×,with counting procedures described by Stockwell and Hargraves(1986).

During Leg 119 several floating sediment trap arrays were de-ployed as described in Biggs et al. (1989). From each trap, ap-proximately 20 zooplankton fecal pellets were sacrificed for im-mediate microscopic examination. Using combined light andUV epifluorescent microscopy, individual fecal pellets were ex-amined for recognizable diatom frustules and the natural auto-fluorescence of living phytoplankton cells.

RESULTS

The species composition and frequencies of diatoms foundwithin the Prydz Bay surface sediments are shown in Table 2.Diatom concentrations were substantial and site-averaged valuesranged from 3.4 × iθ7 to 6.8 × 109 frustules/g sediment. Largefluctuations in absolute diatom numbers, however, were ob-served between adjacent sites as well as within each core.

The diatom assemblage in the surface sediment reflects themore heavily silicified component of the plankton community.All five drill sites were dominated by the ribbon-colony-formingNitzschia species. Chaetoceros resting spores, Chaetoceros se-tae, and fragments of Corethron criophilum were also quitecommon. For all sites, Chaetoceros resting spores accounted fora mean value of 7%. Nitzschia curta outnumbered Nitzschia cy-

668

DIATOM BIOCOENOSES

Table 1. Prydz Bay sample location summary. Drill site, sample station, sam-ple date, station position, site water depth, sea surface temperature, and inte-grated chlorophyll a concentrations.

Station

Outer Prydz Bay

119-739-1

119-739-2

119-739-3

119-739-4

Inner Prydz Bay

119-740-5

119-740-6

119-740-7

119-740-8

Inner Prydz Bay

119-741-9

119-741-10

Outer Prydz Bay

119-742-11

119-742-12

Outer Prydz Bay

119-743-13

119-743-14

Date(1988)

18 Jan

19 Jan

21 Jan

22 Jan

24 Jan

25 Jan

26 Jan

27 Jan

28 Jan

29 Jan

31 Jan

1 Feb

2 Feb

3 Feb

Position(Lat./Long.)

67°15'S75°OO'E67°17'S75°04'E67°17'S75°01'E67°17'S75°05'E

68°42'S76°40'E68°39'S76°35'E68°41'S76°41'E68°42'S76°39'E

68°24'S76°21'E68°24'S76°20'E

67°32'S75°3O'E67°34'S75°30'E

66°52'S74°38'E66°57'S74°42'E

Waterdepth(m)

412

816

561

410

987

Sea surfacetemperature

(°C)

0.1

1.6

1.4

0.6

0.4

Chlorophyll a(mgChlflm~2)

171.75

410.25

576.50

78.50

40.00

lindrus, but together they contributed from 34.6% to 78.3% ofthe total diatom assemblage (Table 2). Nitzschia kerguelensiswas also a common constituent of the sediment assemblage at6%. As reported by Krebs (1977) the major loss of diversityfrom water column to sediments occurred in genera havinglightly silicified species. This was particularly evident of Nitz-schia closterium, Nitzschia subcurvata, Chaetoceros neglectus,Chaetoceros dichaeta, Chaetoceros ßexuosus, and Tropidoneissp. cf. vanheurckii. A total of 10 species occupied approxi-mately 92% of the total sediment diatom assemblage.

Comparison of the relative abundance of diatom species forPrydz Bay phytoplankton samples with those from the sedimentare presented in Tables 3 and 4. Integrated water column diatomabundances (whole water samples) ranged from 3.76 × 1010

cells/m2 at Site 743 to 2.9 × 1011 cells/m2 at Site 739 (Kang andFryxell, this volume). Such differences may be attributed to dif-ferences in seasonal peak abundances. During this sampling pe-riod, Nitzschia closterium and Nitzschia cylindrus were the twodominant species ranking 1 and 2 with mean concentrations of7.6 × 1010 and 7.0 × 1010 valves/m2, respectively (Table 3). Netsamples at each site were primarily dominated by large cells ofCorethron criophilum.

Relative abundances at each drill site demonstrated the pre-dominance of the small pennate diatoms (ca. <20 µm) Nitz-schia cylindrus, Nitzschia closterium, and Nitzschia curta (Ta-

ble 5) for water column samples. Together, these three speciesoccupied from 64% to 96% of the water column diatom assem-blage. For the three offshore stations (Sites 739, 742, and 743),these three pennates were typically > 90% of the total assem-blage (Table 4). For the sediments, Nitzschia curta and Nitz-schia cylindrus were the two major contributors, with Nitzschiaclosterium completely absent. The larger-celled diatoms wereprimarily comprised of Chaetoceros dichaeta, Chaetoceros neg-lectus, Corethron criophilum, and Tropidoneis sp. cf. vanheur-ckii. These larger cells were also relatively more apparent at thetwo near-shore sites (Sites 740 and 741). This greater numericalabundance of larger cells was also coincidental with increasedchlorophyll a concentrations. Integrated chlorophyll a valuesranged from 40.0 (Site 743) to 576.5 mg Chi a m2 (Site 741).

Examination of vertical and horizontal net samples yielded aqualitative aspect of the phytoplankton diatom assemblage. Therelatively large mesh size of the nets selectively discriminatedagainst the small dominating pennate population observed inthe whole water samples. Corethron criophilum and Chaetoce-ros dichaeta were the notable dominants of this net material atall five sites. Both vegetative and auxospore cells of Corethroncriophilum were observed (Fig. 2). Many of these larger cellswere seen that were empty or did not show autofluorescence ofactive chlorophyll. Figure 2 illustrates the distribution of Cor-ethron cells with respect to three depth intervals for Sites 740,

669

D. A. STOCKWELL, S.-H. KANG, G. A. FRYXELL

Table 2. Relative species abundances and overall means found in Prydz Baysediment samples.

Species

Actinocyclus actinochilusAsteromphalus hyalinusA. parvulusAzpeitia tabular isChaetoceros atlanticusC. bulbosusC. criophilusC. dichaetaC. sporesCorethron criophilusDactyliosolen antarcticaEucampia antarctica var. rectaNavicula directaNitzschia antulataN. curtaN. cylindrusN. kerguelenisN. lineolaN. obliquecostataN. panduriformisN. ritscheriN. separandaN. spp.N. sublinearisN. turgiduloidesPorosira glacialis (rsp)P. psudodenticulataRhizosolenia alataR. hebetata f. semispinaR. hebetata f. bedensStellarima microtriasSynedra sp.Thalassionema sp.T. frenguelliopsisT. gracilisT. gracilis var. expectaT. gravidaT. lentiginosaT. oliveranaT. perpusillaThalassiosira (spore)T. tumida, linearT. ritscheri, fasciculatedT. trifultaTrichotoxon reinboldiiPennateCentric

Others

Distephanus speculumChrysophyte cystTotal

Valves counted in sediment

740

000.1600000.323.590.150.15002.80

68.138.910.3201.5802.180000.620.7700.160.1600.310000.7600000.155.250000.470.320

0.161.99

99.37

645

Sites741

00.280000.1400.144.0400.160.300.302.40

65.997.770.7103.2301.030.4500.310.160.7300000.5900.140.161.140.410.140.28004.85000.160.440.320.32

0.142.84

99.97

680

742

0.68000.1500.130.260.135.450.130.850.1302.51

55.737.492.0606.2002.040.5101.8800.580.150.1300.150.680.15001.740.2600.540.1305.540.15000.550.300.30

0.262.40

100.27

729

739

00.160000006.210.160.160.6301.86

54.837.153.420.313.7304.180.470.311.090.470.16000.7800.310002.180.6300.620.1605.7400.1600.470.620.16

0.312.49

99.86

644

743

0.280.160.160.140.16000

15.5700.710.4401.45

30.585.79

23.550.161.350.144.141.9700.7400.16000.32000002.390.1401.06004.980.16000.600.550

0.321.86

99.99

674

741, and 742. Net materials from 50 m to the surface appearedas a rich green soup at all three sites. Most of the Corethroncells observed (67<Vo-78%) were healthy (autofluorescing) vege-tative cells. Empty or nonfluorescing cells made up an addi-tional 21%-30'Vo of the population within this depth interval.Auxospores were rarely observed in this upper layer. Relativeproportions of auxospores increased with depth (5%-15%), be-ing most abundant in the 50-100 m depth interval. However, vi-able or autofluorescing auxospores rarely exceeded 2°7o of thetotal population within these samples. At Site 740, the most in-shore station, the relative proportions of full to empty cellschanged dramatically with increasing depth. Below 50 m, emptycells formed more than 60% of the population. The trend of in-creasing empty cells with increasing depth was also evident atSites 741 and 742. However, empty cells exceeded 60% of the

population here only at greater depths (i.e., 100-200 m depthinterval).

Short-term sediment trap deployments also demonstrated arapid transport of abundant krill-like fecal pellets from surfacelayers (Barron, Larsen, et al., 1989). Microscopic examinationof these fecal pellets revealed much breakage of valves and littlecellular auto fluorescence. Notable exceptions were the ingestedbut intact and brightly fluorescing spores of Chaetoceros neglec-tus. Fecal pellets from Sites 741 and 742 were also characterizedby less cellular breakage and the more numerous occurrences ofautofluorescing cells.

Numerous empty diatom frustules were also observed to oc-cur in the whole water samples. The proportions of full toempty cells are suggestive of a population^ state of health orgrazing pressures.. Distributions of these cells are discussed by

670

DIATOM BIOCOENOSES

Table 3. Ranking of mean species abundances for watercolumn and mean ranking of species in relative abun-dances for the sediments of Prydz Bay, Antarctica.

Table 4. Ten most common species in plankton and sediment.RA = relative abundance.

Species

Actinocyclus actinochilusAsteromphalus hyalinusA. parvulusAzpeitia tabularisChaetoceros atlanticumC. bulbosusC. convolutusC. criophilusC. dichaeta*C. ßexuosusC. neglectus*C. sp. cf. wighamü (veg)*C. sp. (rsp)* +Corethron criophilum*Dactyliosolen antarcticaD. tenuijunctusEucampia antarcticaLeptocylindrus mediterraneusNavicula directaNavicula sp.Nitzschia angulata +N. closterium*N. curta* +N. cylindrus*+N. heimii*N. kerguelensis+N. lecointeiN. lineolaN. obliquecostata +N. panduriformisN. prolongatoidesN. pseudonanaN. ritscheri* +N. separandaN. spp.N. subcurvataN. sublinearisN. sublineataN. turgiduloidesOdontella weissflogiiPorosira glaciaiis (rsp)P. pseudodenticulataRhizosolenia alataR. hebetata f. semispinaR. hebetata f. bidensStellarima microtriasSynedra sp.Thalassionema sp.Thalassiosira frenguelliopsisT. gracilis +T. gracilis var. expectaT. gravidaT. lentiginosaT. oliveranaT. perpusillaT. ritscheri, fasciculatedThalassiosira sp.Thalassiosira sporeT. trifultaT. tumida, linear +Thalassiothrix longissimaTrichotoxon reinboldiiTtopidoneis antarcticusT. beligicaseT. glaciaiisT. sp. cf. vanheurcküPennateCentric

OthersDistephanus speculumChrysophyte cyst +

Mean ranking

Plankton

29

32

243927376

193

1074

311835302628

1528

21

17

2022

9383011

131530

341633384138383810213816383838141138

5251440362312

Sediments

252730303830

3727

33018

20

9

12

4

296

38

713

12

22

14383022

19

1726

248

Planktonspecies

Nitzschia closteriumN. cylindrusChaetoceros neglectusCorethron criophilumN. curtaC. dichaetaC. sp. (rsp)N. heimiiN. ritscheriC. sp. cf. wighamü (veg)Others

RA(%)

42.7939.684.583.322.851.890.880.750.600.482.18

Sedimentspecies

Nitzschia curtaN. cylindrusChaetoceros sp. (rsp)N. kerguelensisThalassiosira tumidaN. obliquecostataN. ritscheriN. angulataT. gracilisT. sp. (rsp)Others

RA(<7o)

55.107.426.996.015.693.421.712.201.661.157.65

Note: * indicates 10 most abundant water column species. +denotes 10 most abundant sediment species. Resting sporesdenoted as (rsp) and vegetative cells designated as (veg).

Note: rsp indicates resting spore; veg indicates vegetative cell.

Kang and Fryxell (this volume). Initial microscopic observationsof the numerous grazers and the abundant fecal pellets withinthese five sites substantiate that grazing pressures upon the phy-toplankton were indeed heavy. Additionally, microscopic exami-nation of living materials at Site 739 revealed that many chainsof Chaetoceros, Corethron, and Rhizosolenia were nonfluores-cent or empty. Previously unreported, this localized populationof Corethron criophilum appeared to be diseased (Fryxell, pers.observation).

DISCUSSIONSatellite passive-microwave observations of Antarctic sea ice

by Zwally et al. (1983) show that the mean monthly sea-ice con-centrations are generally greater than 70% from April to De-cember in Prydz Bay (Middleton and Humphries, 1989). Essen-tially, open water predominates only in January and early Feb-ruary. Air temperatures are generally below freezing even insummer, with freezing processes being active for most of theyear (Middleton and Humphries, 1989). Satellite imagery, fur-thermore, suggests a slightly longer ice-free period for the innerpart of Prydz Bay compared to outer Prydz Bay for the 1987-88window. Considering these facts, our 3-week occupation ofPrydz Bay represented a major portion of the active phyto-plankton growing season for 1988. The general water columncirculation pattern for Prydz Bay may be described as a closedcyclonic gyre (Smith et al., 1984). Such containment would al-low local underlying sediments to receive the bulk of regionalprimary production from one rotating water mass.

The extremely high concentrations observed in phytoplank-ton biomass, represented both in cell numbers and in chloro-phyll a concentrations, appear substantially elevated when com-pared to the Southern Ocean as a whole. Our estimates of 1010

cells/m2 are slightly higher than those of Kozlova (1966) and Li-gowski (1983) for Prydz Bay, both on the order of 108 cells/m2.Bloom proportions in conjunction with a recently receding iceedge and increasing water column stability are suggestive of theaustral spring blooms along marginal ice zones (Garrison et al.,1987, Tilzer et al., 1985). The primary agents of this bloomwere the three nanoplanktonic pennates Nitzschia cylindrus,Nitzschia closterium, and Nitzschia curta. Although quite com-mon in Antarctic waters, N. cylindrus and N. closterium havebeen reported to dominate water column phytoplankton assem-blages only by Fryxell and Kendrick (1988) and Garrison et al.(1987). Kozlova (1962) previously observed the dominance ofNitzschia curta in surface waters of Prydz Bay. All three speciesare also, however, common constituents of sea-ice communities.Nitzschia subcurvata, another ice-related species, was also acommon water column inhabitant.

Garrison et al. (1987) have demonstrated strong similaritiesbetween ice and planktonic algal assemblages in the vicinity of

671

D. A. STOCKWELL, S.-H. KANG, G. A. FRYXELL

Table 5. Standing crop of planktonic diatoms and the relative contributions of the dominant diatomspecies. Mean total diatom abundances given as × 109 cells m 2. Relative abundances of diatoms in thewater column and in the sediments.

Site

739740741742743

Meantotal

287.87261.75180.10117.9437.64

N. closterium(«% of total)

Water

65.332.821.552.810.4

N. cylindrus(°/o of total)

Sediment

0.00.00.00.00.0

N. curta(°/o of total)

Water

28.048.539.337.077.1

Large-celleddiatomsSediment

7.158.917.747.495.79

Water

2.406.503.446.023.43

Sediment

59.8368.2265.4855.7330.58

("% of total)

2.713.733.72.61.0

Leg 119 Site 740100

100

0-50 50-100 100-200Depth Interval (m)

Leg 119 Site 741

I Veg. emptyEü Veg. auto.^ Aux. empty0 Aux. auto.

0-50 50-100 100-200Depth Interval (m)

Leg 119 742

0-50 50-100 100-200Depth interval (m)

Figure 2. Distribution of the relative proportions of Corethron criophi-lum life stages (vegetative cells vs. auxospores), shown with respect todepth. Condition of cells, as determined by autofluorescence, indicatefull or empty cells.

recently receded ice edge systems. In addition, mechanisms forbloom initiation within these marginal ice zones have been dis-cussed by Smith and Nelson (1985). Our occupation of PrydzBay occurred following the disappearance of local pack ice. Theintense bloom conditions encountered exemplify the short grow-ing season characteristic for this embayment. The phytoplank-ton community, as a whole, appeared dominated by ice-related,nannoplanktonic diatoms, although local variations in bothbiomass and in species composition were observed.

El-Sayed and Turner (1977) reported chlorophyll a concen-trations for Antarctic waters to average 17.4 mg/m2 (range0.23-337.7 mg/m2) when integrated over the euphotic zone.Holm-Hansen et al., (1977) found integrated chlorophyll a con-centrations in the upper 200 m to average 10.45 mg/m2 (range0.73-30.8 mg/m2). These values suggest that spring Prydz Bayphytoplankton biomass was substantially elevated when com-pared to the Southern Ocean as a whole over much of the ice-free season. Additionally, grazing pressures on this populationwere heavy as evidenced by numerically abundant diatom-ladenfecal pellets (pers. observation).

Diatom frustules in the clayey sediments of Prydz Bay wereabundant, as previously reported (Kozlova, 1966). Our valuesare more directly comparable to the concentration range re-ported by Kozlova (1966) for the clayey-diatom oozes of PrydzBay (13.6-111 × 106 frustules/g). These values are slightlyhigher than the range of frustules found for typical Antarcticiceberg sediments of the continental shelf (0.55-50 × 105 frus-tules/g) as reported by Kozlova (1962) and Krebs (1977). The ge-nus Nitzschia typically formed 70% of the diatom thanato-coenose within these sediments, but the species dominance dif-fered from that seen in the plankton. The coastal or neriticspecies, N. curta, was the dominant organism within Prydz Baysediments. Kozlova (1966) described N. curta as a diatom notedfor its mass occurrences in the plankton of coastal regions andhaving a good capacity for preservation. The lightly silicified N.closterium, dominating the water column populations, was notobserved in sediment samples. Another substantial contributorto the sediment assemblage was N. cylindrus. Two of the threedominant water column species, therefore, were able to surviveselective dissolution and sedimentation processes to also domi-nate the sediments. It should be noted, however, that the relativecontributions of N. cylindrus to the total sediment assemblageappeared very much reduced when compared to the planktoniccontributions. Subsequent analysis of sediments downcore alsoindicated that the heavily silicified resting spores of Chaetoceroscan account for approximately 8%-34% of Prydz Bay sedimentassemblages. The genus Chaetoceros is a common planktonicform that has been poorly represented by vegetative formswithin the sediment.

The diatom concentrations in the sediments of Prydz Bay re-flected the short-lived but intense productivity patterns of theoverlying surface waters. Additionally, diatom numbers withinthis system appear high because there is little influx of terrige-

672

DIATOM BIOCOENOSES

nous materials to dilute concentrations (Kozlova, 1966). Duringthe process of sedimentation, water column diatom assemblagesundergo selective dissolution. Selective mechanical and chemi-cal destruction of frustules reduces absolute diatom concentra-tions within these sediments.

Species composition, likewise, undergoes a distinct, selectivealteration. Common within water column assemblages, the lightlysilicified species such as Nitzschia closterium, Chaetoceros neg-lectus, Chaetoceros ßexuosus, and Tropidoneis sp. are not ob-served in the sediments. Mechanical breakage of Corethroncriophilum and Chaetoceros dichaeta reduced their appearanceswithin the sediments. Krebs (1977) found this selective preserva-tion to favor the more heavily silicified benthic diatoms of Ar-thur Harbor, Antarctica. Benthic species were not commoncomponents of the Prydz Bay materials. The deep water natureof our sampling sites precluded their occurrence in favor of amore planktonic regime. The benthic species Cocconeis impera-trix, however, did make an appearance in the sediments closerinshore (Sites 740 and 741).

In general, the observed diatom composition of Prydz Baysediments had some similarities to that observed over a 3-weekperiod for the water column assemblages. Abundant remains ofwell-preserved diatoms were deposited beneath areas of recur-rent high productivity. Both of the assemblages were typicallydominated by ice-related pennates less than 20 µm in size. An-other important sedimentary component, the robust Chaetoce-ros resting spores, were also usually less than 20 µm. An in-crease in larger-celled diatoms at Sites 740 and 741 may indicateconditions of increased ice-free periods. Further information isneeded before correlations to ice coverage may be extended tosediment samples. Although selective preservation of frustuleswas apparent, such removals did not seem to greatly affect dia-tom diversity within the sediments. Apparently, the sedimentsof Prydz Bay reflect in the diatom assemblage the intense butshort-lived growing season in a semi-enclosed gyre that makes itan ideal area for comparison.

CONCLUSIONSLocal variations in biomass and in species composition were

observed for the Prydz Bay water column during our samplingperiod. Although a closed gyre, the effects of water column sta-bility and ice coverage need to be examined in further detail withrespect to species distributions and abundances. The diatom con-centrations within the underlying near-surface sediments also re-flected the short-lived but intense productivity pulses of theplankton. Mirroring the water column, sediment assemblageswere characterized by a grouping of small, ice-related forms. To-tal diatom abundances decreased moving out of Prydz Bay,both in water column and sediment assemblages. AlthoughNitzschia curta dominated these near-surface sediment assem-blages, selective dissolution removed or reduced in numbers 5 ofthe 10 most common species within the plankton. The impor-tance of water column dominants, such as Nitzschia closterium,N. cylindrus, and Corethron criophilum, were greatly reducedwith sedimentation. Quantification of the processes regulatingselective preservation of diatom assemblages over the course ofgeological time are essential for future resolution of paleocli-mate and further paleo-ecological interpretations.

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Date of initial receipt: 29 June 1990Date of acceptance: 5 July 1990Ms 119B-205

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