Hydrobiologia 269/270 : 235-242, 1993 .H. van Dam (ed .), Twelfth International Diatom Symposium .© 1993 Kluwer Academic Publishers. Printed in Belgium .

Diatoms in surface sediments of the Gotland Basin in the Baltic Sea

Tuulikki GronlundGeological Survey of Finland, Betonimiehenkuja 4, SF-02150 Espoo, Finland

Key words : diatom flora, ebridians, marine sediments, eutrophication, Baltic Sea

Abstract

A sediment core, 55 cm long, from station F81 in the Gotland Basin of the Baltic Sea was analysed fordiatoms and ebridians . Chrysophyte stomatocysts found in the core were also counted but not identi-fied . The aim was to trace environmental changes, e .g. eutrophication and salinity variations. There isevidence that eutrophication has been increasing in the Baltic Sea in recent decades .

Brackish-marine plankton diatoms dominate the entire core and reflect the local planktonic taxa ratherwell. The dominant taxon is the polyhalobous Actinocyclus octonarius . The main biostratigraphical changewithin the core analysed takes place at a depth of about 22 cm, where the abundance of diatoms, andespecially of Chaetoceros spp ., Thalassiosira hyberborea var . pelagica and T. baltica start to increase . Thismay reflect eutrophication which can be estimated to have started c . 200 years ago .

Introduction

Numerous studies have been published on recentand subrecent Baltic Sea diatoms (e .g. Simonsen,1962; Snoeijs, 1988), but only a few on planktonicdiatom flora (Edler et al., 1984; Risberg, 1990 ;Miller & Risberg, 1990) . Molder (1962) and Hall-fors & Niemi (1975) have investigated diatomsfrom the area of station F8 1 . Molder analyseddiatoms from water above a depth of 15 m andfrom the uppermost layer of the sediment . Heclassified the diatoms into four groups by salin-ity: 47 % of the flora are marine and 32 % brack-ish varietes . Hallfors & Niemi (1975) analyseddiatoms from the uppermost layer of the sedi-ment, and suggested that this layer contained ma-terial deposited during one growing season. Theydivided the diatom flora into different ecologicalgroups by salinity and habitat (planktonic andlittoral) and concluded that the sedimentation ofthe diatoms was mainly local . The diatom species

235

diversity was then 1 .73 at station F81 (H,Shannon-Wiener index) .

Miller & Risberg (1990) analysed siliceous mi-crofossils from two short sediment cores, P-18and SNV-3, taken from the western part of theGotland Sea, northwest of station F81 . Accord-ing to them, the micro-algal stratigraphy may re-flect an acceleration in the eutrophication rateabout 20 years ago . A core taken northwest ofstation F81 was also studied by Thulin et al.(1986, 1992), but they did not retrieve the top20 cm .

The aim of this study was to trace any envi-ronmental changes (e.g . eutrophication and sa-linity) in the subfossil diatom flora in the BalticSea proper, as there is evidence that eutrophica-tion has started to affect the Baltic Sea (Rosen-berg, 1984; Elmgren, 1989; Miller & Risberg,1990 ; Baltic Marine Environment ProtectionCommission, 1990 pp. 77-151) .

Materials and methods

In 1990, a short sediment core (F81, 55 cm) was taken from the research vessel Aranda from a depth of 240 m in the Gotland Basin (Fig. 1) using the crust-freezing technique (Saarnisto, 1975). The sampler was modified from Renberg (1981). The siliceous microfossil content of the core, mainly diatoms but also ebridians and chrysophyte cysts, was analysed.

The sediment core consists entirely of clay- gyttja with a loss-on ignition of about 10%. The sediment core between depths of 55 and 30 cm consists of carbonaceous laminated sediment, in- dicating a lack of bioturbation. The sediment from the 30 cm level upwards is a dark-grey, mixed mud displaying flamelike patterns and disturb- ances without any distinctive laminations.

The rate of accumulation of the sediment has been about 1 mm/year according to soot ball, 2 1 0 ~ b and 13'Cs determinations (Salonen et al., 19%).

The subsamples for diatom analysis were taken with a knife at 2-cm intervals in the top 24 cm, and at irregular intervals at lower depths. After drying and weighting they were bleached in dilute H,O, for 24 h at 50 "C, and then subjected to repeated suspension and decantation. After mounting in Hyrax (RI = 1.65), at least 500 dia- tom valves were identified in each subsample. The diatom concentration values of the core were obtained by adding Lycopodium tablets to a specific volume of sediment (Stockmarr, 1971, 1973). Three tablets, each containing 11 000 300 spores, were added to each weighed subsample. Lycopodium spores were calculated per 2 500 identified diatom frustules (Eronen, 1976).

Indentification of the diatoms was hindered by extensive dissolution of silica from the diatom walls. The dissolution was particularly extensive in the centric diatom flora (Fig. 2). The number of badly fragmented unidentified centric diatoms was counted in relation to 1 5 0 0 identified dia- toms in each subsample. Their concentration was also assessed by means of Lycopodium tablets.

The diatoms were divided according to their salinity requirements and main life habitat into three groups: brackish-marine planktonic, brack- ish-marine littoral, freshwater planktonic and lit- toral.

The diatom floras of Hustedt (1930), Cleve- Euler (1% I), M6lder & Tynni (1967, 1968-1972) and the papers of Hasle & Lange (1989) and

Fig. I. The location of station F81 in the Baltic Sea and the other locations mentioned: 1-2 the locations of the study sites of Miller & Risberg (1990), 3 the location of the study site of Thulin etal . (1986, 1992).

Fig. 2. Partly dissolved centric diatom frustules. Scale bar = 10pm.

Miller & Risberg (1990) have mainly been usedboth for species identification and as a source ofecological information .

Results

At depth level 0-2 cm the diatom species diver-sity is H = 2.218 (Shannon Wiener index, accord-ing to Pielou, 1966). This diversity is greater thanthat presented by Hallfors & Niemi 1975, thismainly owing to the greater number of taxa andthe difference in certain dominant species (Acti-nocyclus octonarius Ehrenb ., Skeletonema costatum(Grey.) Cleve, Achnanthes taeniata Grun.) .

A total of 31 diatom genera and 84 taxa wereidentified in 19 subsamples taken from depthlevels of 0-55 cm (Gronlund, 1992) .

The abundance curve of the identified diatomshas a one small peak and two larger ones (Fig . 3) .The abundance, which is rather low in the bottompart of the core, increases from a depth of 30 cm,briefly exceeding 60 000 at a depth of 25 cm andbeing about 70000 frustules g - ' in sediment at 5cm.

The abundance curve of the unidentified cen-tric diatoms is also shown in Fig . 3. The abun-dances of both unidentified centric diatoms andidentified diatoms are nearly the same in the sub-samples from a depth of 40 to 30 cm. Because theCentrales diatoms clearly dominate (89-100%)the entire core, it can be concluded that the dis-solution of the diatoms is quite extensive at justthese levels .

The dissolution problem has been discussed byseveral authors, including Lewin (1961), Meri-lainen (1973), Eggimann et al. (1980) and Flower(1993). Dissolution depends on many factorssuch as the morphology of the basin, the qualityof the water, and the quality and silica content ofthe frustules. The dissolution of frustules, whichis more common in highly alkaline than in acidwaters, also depends on the ambient water tem-perature, and is more common in warm water(Lewin, 1961). The rate of dissolution is quitehigh in sea water owing to the accelerating effectof NaCl (Lewin, 1961) . Because of the undersatu-

237

Fig. 3 . Calculated number of diatoms g -1 sediment (solidline = identified diatoms, dashed line = unidentified centricdiatoms) . The unidentified centric diatoms were counted inrelation to >_ 500 identified diatoms .

ration of silica, the likelihood of dissolution inwater increases with the sinking time .

Changes in species composition

Planktonic brackish-marine diatoms are clearlydominant throughout the core (Fig . 4). The oc-currence of these species is shown in Fig . 5, andphotos of some taxa are reproduced in Fig. 6 . Thedominant taxon is Actinocyclus octonarius, a poly-halobous, cosmopolitan species common in ner-itic plankton (Simonsen, 1962) . A . octonarius var .crassus (W . Smith) Hendey and var . tenellus

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percentages of ecological groups of identified diatoms, uni-

dentified centric diatoms, Ebria tripartita and chrysophyte

cysts .

A DIATOMS

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(Breb .) Hendey were also found, but more

sparsely. These three taxa, all of which were found

in the topmost subsamples accounting for 30-

40% of all diatoms, were almost totally lacking

(as A . ehrenbergii) in the materials of Molder

(1962) and Hallfors & Niemi (1975) . Two of them,

however, were found (as A . ehrenbergii and var .

grassa) in the core from the western Gotland basin

studied by Miller & Risberg (1990), starting to

increase clearly about 30 years ago .

The marine planktonic Rhizosolenia calcar-avis

Schultze prefers warm water (Hustedt, 1930) and

is also common, particularly in the lower part of

the core. It is at its maximum at 43-30 cm level

but thereafter decreases, being rather rare in the

top part of the core. The abundance of the iden-

tified diatom flora was at its minimum at 40-30

cm level, probably due to the rather high disso-

Fig . 5 . Relative abundance of brackish-marine diatom taxa in station F81 .

Fig . 6 . Fig. A . Thalassiosira hyperborea var . pelagica, Fig . B . Rhizosolenia calcar-avis, Fig. C . Thalassiosira hyperborea var . lacunosa,

Fig. D. Terpsinoe americana, Fig . E . Actinocyclus octonarius, Fig. F . Thalassiosira baltica . Scale bar= 10 µm .

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lution (cf. Fig . 3) . Being the most resistant partsof the usually delicately silicious frustule, only thespiny apices of Rhizosolenia frustules have beenpreserved in the sediment at those levels (cf. alsothe curve of the partly dissolved centric frustulesin Fig . 4). Consequently, Rhizosolenia calcar-avismay be overrepresented in the core, in its lowerpart in particular. The species, which is commonin the laminated sediments of the cores from P-18studied by Miller & Risberg (1990), and was alsofound at the 20 cm level in the core taken north-west of station F81 by Thulin et al . (1986; 1992),was not mentioned in the studies of Molder (1962)or Hallfors & Niemi (1975) from station F81 .

Chaetoceros species are quite common, espe-cially at a depth of about 10 cm in the presentcore. They occur mainly as resting spores . Thereis a mass occurrence of Chaetoceros spp. restingspores in the topmost sediments at station P-18studied by Miller & Risberg, 1990 . According tothem, this may reflect either increased nutrientaccess due to outwash and upwelling or a dete-rioration in the living conditions of Chaetocerosspp. vegetative cells . Chaetoceros spp. restingspores are common in the material of Hallfors &Niemi (1975) but are lacking in that of Molder(1962) .

Thalassiosira hyperborea var . pelagica (Cleve-Euler) Hasle and var . lacunosa (Berg) Hasle arefound mainly in the topmost part of the core, thelatter in greater abundance than the former .Under different names, these two species are alsomentioned in diatom floras of the Baltic Sea pre-sented earlier (Cleve-Euler, 1951 ; Molder, 1962 ;Molder & Tynni, 1968; Hallfors & Niemi, 1975 ;Hakansson & Locker, 1981 ; Huttunen & Niemi,1986 ; Miller & Risberg, 1990) . Niemi (1971) hadobserved T. hyperborea var . pelagica (as Coscino-discus lacustris) in highly eutrophic water aroundHelsinki and in eutrophic waters elsewhere in theFinnish archipelago, above all in late summer .According to Niemi (1971), T. hyperborea var .pelagica seems to favour high nutrient concentra-tions. These two Thalassiosira species are lessabundant in the materials of Molder (1962) andof Hfillfors & Niemi (1975) . Thalassiosira hyper-borea var . lacunosa is common in the core from

station P-18 but it decreases clearly in the top-most part (Miller & Risberg, 1990) . Thalassiosirabaltica (Grun.) Ostenf., a species associated withbrackish water and plankton, is quite common inpresent Thalassiosira species material . It is alsocommon in Molder's flora and in the cores stud-ied by Miller & Risberg (1990), but it is onlybriefly mentioned by Hallfors & Niemi (1975) .According to Niemi (1972), the nutrient condi-tions of warm, eutrophic archipelago watersprobably favour T. baltica and T. hyperborea var .pelagica .

Other brackish-marine planktonic diatoms inthe core studied are Coscinodiscus asteromphalusEhrenb., C. nodulifer Schmidt and Thalassionemanitzschioides Grun. The main brackish-marine lit-toral diatoms are Diploneis didyma (Ehrenb .)Cleve, Grammatophora oceanica Ehrenb. andSynedra tabulata (Agardh) Kiitz .

A typical oceanic species, Terpsinoe americana(Bail.) Ralfs, was also found in the uppermostsubsample. It appears to have grown in the Bal-tic Litorina Sea during the Holocene climatic op-timum (Alhonen et al., 1984; Risberg 1986) . Ac-cording to Witkowski (1991), the surfacesediments of Puck Bay, Poland, suggest thatTerpsinoe americana may be a relict component ofAtlantic diatom flora .

One ebridian species, Ebria tripartita (Schum.)Lemm ., is quite abundant . Ebridians are ratherwidespread unicellular marine phytoplankton thatare extremely abundant in nutrient-rich cool wa-ters at high latitudes (Lipps, 1979) . Ebria tripartitawas found in every subsample . Its abundance washighest, 8%, at a depth of 16-20 cm . It was alsoabundant in the top part of core P-18 studied byMiller & Risberg (1990), a finding which theytentatively attribute to a stage of high nutrientaccess combined with upwelling of cold water .There is no record of a marked increase in theabundance of this taxon in the present core .

Freshwater components

Some of the samples also contained freshwatertaxa (up to 8 %), the most important being plank-

tonic Aulacoseira species (mainly A . islandica(0 . Mull.) Simonsen), Stephanodiscus astraea(Ehrenb.) Grun. and littoral Epithemia turgida(Ehrenb.) Kiltz .

Chrysophyte stomatocysts found in the studiedcore were also counted but not identified . Beingwidely distributed primarily in freshwater envi-ronments, they were found in all the subsamplesexcept those from a depth level of 53 cm . Thestomatocysts account for 0 .3-9.6 % of the diatomfrustules identified, and their occurrence corre-lates well with that of freshwater diatoms foundin the core (cf. Fig . 3) .

Comparison with earlier findings from station F81

There are some differences between the diatomflora now analysed from the topmost part of coreF81 and those studied earlier by Molder (1962),and by Hallfors & Niemi (1975) from the surfacesediment at the same station . The common taxonin the present material, Actinocyclus octonarius, israre in both above materials . Chaetoceros specieswhich are common at least in the topmost part ofthe present core, are lacking in Molder's materialbut were found by Hallfors & Niemi . Skeletonemacostatum (Grey.) Cleve and Achnanthes taeniataGrun., which are common in the material of Hall-fors & Niemi (1975), are very sparse in Molder'sand the present materials . The differences may bepartly attributed to differences in the dissolutionrate of the diatom frustules . Skeletonema costatumin particular is a very weakly silicified species .

Conclusion

The diatoms found in the core studied are mainlymarine and planktonic species, indicating localsedimentation . Although rather little is knownabout the ecological demands of diatoms andabout the factors affecting Baltic water, the con-tent of microfossils at station F81 points to in-creasing eutrophication from about 20-30 cm up-wards . A similar trend was reported by Hallfors& Niemi (1975) and Miller & Risberg (1990) .

24 1

Sedimentation rate and tentative 210Pb and 137Csdeterminations suggest that eutrophication becansome 200 years ago .

Acknowledgements

I thank Professor Veli-Pekka Salonen and DrIlppo Vuorinen for supplying the sediment core .I also thank Dr Elina Leskinen, who helped withthe determination of diatom species diversity, toEila Paavilainen, who assisted me in the labora-tory, Satu Moberg, who made the final drawings,and Gillian Hakli, who revised the English .

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