CNova Hedwigia, Beiheft 143, p

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1 CNova Hedwigia, Beiheft 143, p Stuttgart, July 2014 Ancylus Lake stage in the Gulf of Gdańsk (southern Baltic Sea) based on diatom taphocoenoses Małgorzata Witak* and Dorota Jankowska Institute of Oceanography, University of Gdańsk Piłsudskiego 46, Gdynia, Poland * Corresponding author: ocemaw@univ.gda.pl With 7 figures and 2 tables Abstract: The aim of this study was to reconstruct the palaeoenvironmental changes in the south-western part of the Gulf of Gdańsk that took place during the last freshwater stage of the Baltic Sea development, i.e., the Ancylus Lake stage. Diatom assemblages from cores taken from both the shallower and deeper parts of the gulf reveal Early Holocene evolution of the basin, which is discussed with respect to bathymetry, ph, trophic and saprobic status. Diatom taphocoenoses of the oldest sediments deposited in close proximity to the palaeo-shore indicate that very shallow, strongly alkaline waters rich in dissolved nutrients with rather moderate content of organic matter existed in Preboreal/Boreal chronozone. The occurrence of acidophilous forms document peat bog formation in proximity to the Ancylus Lage. At that time, palaeohydrology of the present-day Hel Peninsula area could have been influenced by the riverine waters. Due to climatic amelioration in Boreal chronozone, the impact of Vistula River inflow increased and nutrient load periodically increased in the whole area. Changes in climatic conditions are also evidenced by the disappearance of cold-water nordic-alpine taxa. The diatom assemblage preserved in the upper part of Boreal sediments (core 6/2001) could be a signal of a decreasing water level as a result of the Ancylus Lake regression. Key words: Ancylus Lake, paleoecology, diatoms, Baltic Sea, Gulf of Gdańsk Introduction The postglacial evolution of the Gulf of Gdańsk and the coastal lagoons of the Baltic Sea was dependent on global, regional and local factors. These relationships are usually recorded in marine and lacustrine deposits. The retreat of the Late Weichselian ice sheet caused environmental changes on a global scale, controlled by oceanic circulation variability, climatic fluctuations and eustatic sea-level change (e.g., Berglund et al. 2005, Harff et al. 2007). Regional factors, including glacio-isostatic land uplift/sinking and groundwater level changes, played an important role in shaping the coastal zone (Mojski 2000). Furthermore, palaeohydrology of lagoons could have been controlled by input from riverine waters (Starkel 2001). The postglacial history of the Gulf of Gdańsk, which has a wide connection with open water, shows a strong relationship with the concurrent paleoecological state of the Baltic Sea. After the final drainage of the Baltic Ice Lake the strait near Mt. Bilingen in central Sweden was opened. This event started the second stage of the Baltic Sea development Yoldia Sea, which lasted from 10.3 to C ka BP. The connection between the Yoldia Sea and the ocean was severed due to rapid glacio-isostatic uplift of Scandinavia. The rate of this uplift exceeded the eustatic ocean level rise and thus the next stage in the Baltic Sea development the Ancylus Lake was initiated. According to Björck (1995) the Ancylus Lake stage lasted from 9.5 to C ka BP, however, Uścinowicz (2003) suggests that it ended 600 years earlier. The name of the stage is derived from the snail Ancylus fl uviatilis Müller, known from the littoral and supralitoral zones of the northern and eastern coast of the Baltic Sea. The Ancylus Lake stage started with a transgression in the southern Baltic that resulted from the isolation of Lake Vänern from the 2014 J. Cramer in der Gebr. Borntraeger Verlagsbuchhandlung, Stuttgart, Germany DOI / /2014/ /2014/023 $ 5.00

2 450 M. Witak & D. Jankowska Fig. 1. Location of sediment cores studied. A dotted line shows shorelines during the Ancylus Lake stage ca C ka BP, palaeochannels of rivers and palaeodeltas are also drawn; based on Uścinowicz (2003). sea (Björck 1995). During this phase ( C ka BP) the Ancylus Lake drained through narrow and shallow straits in central Sweden, the Göta Älv outlet and the Otteid/Steinselva outlet (Berglund & Björck 1994). Between C ka BP, the water level rose in the southern Baltic from ca. 40 to 25 m below the present sea level, at a rate of about mm/a (Uścinowicz 2003). As a consequence, the shoreline migrated south by ca. 10 km in the southern part of the Bornholm Basin and by ca. 5 km in the Gdańsk Basin. During the maximum transgression of Ancylus Lake, the presently shallow part of the coastal zone, including the Puck Lagoon and Vistula Lagoon was exposed (Fig. 1). The sandy barrier of Hel Peninsula was not yet established. The Ancylus transgression ended abruptly with a sudden water level drop at C ka BP (Berglund & Björck 1994, Jensen et al. 1999). A new drainage pathway was established at that time through the Darss Sill area in the southern Baltic and northwards through the Store Belt Strait (Björck 1995). The regression phase of the Ancylus Lake lasted in the southern Baltic Sea until C ka BP (Uścinowicz 2003). At that time the water level decreased from ca. 25 to 28 m below the present level. Glacio-isostatic movements played an important role in the Early Holocene evolution of the southern Baltic. After the postglacial uplift phase, the residual phase initiated at ca. 11 ka BP, and lasted to ca C ka BP. Due to a brake effect caused by hydro- and sediment-isostasy the area was rising during that time at a slow rate ( C ka

3 Ancylus Lake stage in the Gulf of Gdańsk (southern Baltic Sea) 451 BP about 5.5 mm/a), and the uplift extent amounted to 15 m. Forebulge migration across the southern Baltic area took place during the following period (ca C ka BP). Clayey sedimentation persisted in the deep basins during the Ancylus Lake stage. The water level fluctuations of that period are recorded in the clayey sediments of the Central Baltic (Huckriede et al. 1996). However, the events of the Ancylus Lake are also recorded in its coastal zone (e.g. Berglund et al. 2005, Lampe 2005). According to Uścinowicz (2003), cliffs were evolving at that time in some parts of the Polish coast. As a result of water lowering, sand spit structures emerged in the western and southern parts of Gulf of Gdańsk. The Vistula River discharged into the southern part of the Gulf of Gdańsk, where its delta developed (Kramarska et al. 1995). This process was very intense. Larger volumes of riverine waters caused the aggradations of the Vistula channel as a result of increasing air temperatures and humidity in the Boreal chronozone (Mojski 1988, Starkel 2001). First diatom studies in the Gulf of Gdańsk dated from the 1920s (Witak 2013). The results of biostratigraphic investigations of long sediment cores, including Early Holocene sequences, have been reported from several regions. Schulz (1926) was the first to use fossil diatoms to reconstruct palaeogeography and palaeoecology of the Ancylus Lake. He published the first list of freshwater forms typical for this stage that occurred in sediments of Gdańsk vicinity, the Vistula Delta Plain and Hel Peninsula. Diatom and pollen data from Early Holocene deposits recovered in deep boreholes at Hel and Jurata served as a basis for distinguishing the Ancylus section (Sandegren 1935, 1938). The fossil diatom assemblages preserved in muddy and silty sediments of Hel Peninsula corresponded to the regressive phase of the Ancylus Lake (Bohr & Sokół 1972). The whole cycle of the basin evolution including the earlier phase of transgression was also described based on the Jastarnia (Bogaczewicz-Adamczak 1982) and Jurata borehole sections (Bogaczewicz-Adamczak & Żukowska 1990). The transgressive and regressive phases were also recorded in diatom assemblages of the Gdańsk Deep region (Davydova et al. 1970, Pieczka & Zaborowska 1989, Witkowski 1994, Yuspina & Savukyniene 2003). Data on past bathymetry are derived from diatom assemblage variations in cores taken from the Gulf of Gdańsk (Zaborowska & Zachowicz 1982, Abelmann 1985). Witkowski (1994) documented the most common diatoms of the Ancylus Lake stage. The impact of the Vistula River inflow on the Gulf of Gdańsk in Early Holocene was also discussed in Stachura-Suchoples (2006). Although numerous biostratigraphic studies to date focused on Ancylus Lake stage within the Gulf of Gdańsk area, our understanding of the environmental conditions in Preboreal and Boreal chronozones is still incomplete. The goal of this paper is to document the environmental changes recorded by diatom assemblages in three cores taken from both shallower and deeper parts of the Gulf of Gdańsk. Special emphasis is placed on ph, trophic and saprobic status. Preliminary results on Early Holocene diatom flora preserved in these cores (Krzymińska et al. 2005, Witak et al. 2006) are the basis for the discussion presented here. The Middle and Late Holocene evolution of the study area is discussed in detail elsewhere (Witak 2010). Materials and methods The lower parts of three sediment cores taken from the south-western part of the Gulf of Gdańsk, between Hel Peninsula and Gdańsk Gdynia south-western region (Fig. 1) were examined in this study. Core 4/2001 was collected in the shallower zone of the gulf at a water depth of 29.8 m. Cores 3/2001 and 6/2001 were taken in the deeper part of the gulf, off the tip of Hel Peninsula, at water depths of 54.5 m and 62.0 m, respectively. Cores were taken using a vibro corer onboard r/v Dr Lubecki. For the purposes of this study, the cores were obtained from the Polish Geological Institute, Branch of Marine Geology, Gdańsk, where they are curated. Core logs and sediment lithology are tabulated in Table 1. Stratigraphy for all cores is based on pollen analysis, discussed in detail by Krzymińska et al. (2005). Pollen data indicate that the sediments of the lower parts of cores 4/2001 ( cm) and 6/2001 ( cm) were deposited close to the Preboreal/Boreal boundary. The whole interval of core 3/2001 ( cm) and higher parts of cores 4/2001 ( cm) and 6/2001 ( cm) correspond to the Boreal chronozone.

4 452 M. Witak & D. Jankowska Table 1. Main parameters and lithology of the studied cores. No core φ λ 3/ o 33,640 N 18 o 44,913 E 4/ o 31,083 N 18 o 41,349 E 6/ o 33,282 N 18 o 47,938 E Core length [cm] Early Holocene section [cm] Lithology sandy mud, light olive-grey (5Y 6/1), with interbeddings of fine sand, at the depth of 298 cm piece of wood, HCl+; fine sand with an admixture of mud, grey (N3), at the depth of 345 cm plant remains, at the depth of 370 cm pieces of wood, HCl+; silt, grey (N5), an admixture of sand, at the depth of 290 cm piece of wood, HCl ; silt, grey (N5), HCl ; medium sand, light-grey (N6), at the depth of cm interbeddings of clay, HCl ; silt, grey (N5), HCl++; gyttja, admixture of sand, grey (N4), HCl ; fine sand, grey (N5), HCl ; irregular lenses of grey silty sand (N4), HCl+; with black organic interbeddings, HCl ; silty sand, layers of organic matter, HCl ; irregular lenses of grey silty sand (N4), HCl+; with black organic interbeddings, HCl ; silty sand with layers of coarse sand, grey (N4), HCl ; medium sand, interbeddings of coarse sand, grains of gravel, grey (N4), HCl ; layers of silty sand, yellowish grey (SGY 4/1) and clay, brown, HCl+++; For this study, cores were subsampled at 5 cm intervals. Subsamples were treated following the method of Battarbee (1986). Sediment (ca. 2 cm 3 ) was treated with 10% HCl to remove CaCO 3 and washed several times with distilled water. Organic matter was oxidized using 30% H 2 O 2. Permanent slides were mounted using Naphrax diatom mountant (refractive index n D = 1.73). Light microscope examination was performed using a Nikon microscope, by means of a 100 oil immersion objective. The counting method of Schrader & Gersonde (1978) was used, and between 500 and 700 valves was counted in each sample in order to estimate the percentages of individual taxa. The identification of diatoms was aided with the following literature: Hustedt ( ), Cleve-Euler ( ), Krammer & Lange-Bertalot (1986, 1988, 1991a, 1991b), Lange-Bertalot & Krammer (1987), Lange-Bertalot & Metzeltin (1996), Krammer (2000, 2002, 2003), Lange-Bertalot (2001), Håkansson (2002). Diatoms were divided into groups according to their biotope requirements (Round 1981), with respect to their salinity requirements (Kolbe 1927), as well as trophic system (Neumann 1932), and with respect to saprobic requirements (Kolkwitz & Marsson 1908). The percentages of all ecological groups were calculated in all cores. Autecologic data, listed in Table 2 along with other pertinent parameters, were obtained from Hustedt ( ), Cleve-Euler ( ), Simonsen (1962), Krammer & Lange-Bertalot (1986, 1988, 1991a, 1991b), Denys (1991), Vos & de Wolf (1993), Van Dam et al. (1994), Hofmann (1994), Krammer (2000, 2002, 2003), Lange-Bertalot (2001).

5 Ancylus Lake stage in the Gulf of Gdańsk (southern Baltic Sea) 453 Table 2. Ecological preferences of the dominant diatom taxa recorded in the sedi ments of Ancylus Lake in the SW Gulf of Gdańsk. habitat: pl planktic; b benthic; salinity: e euhalobous; m mesohalobous; oh oligohalobous halophilous; oi oligohalobous indifferent; ohb oligohalobous halophobous; ph: alb alkalibionthic; alph alkaphilous; ind indifferent; acph acidophilous; acb acidobionthic; trophy: eu eutraphenthic; meso mesotraphenthic; oligo oligotraphenthic; dys dystraphenthic; saprobity: poly polysaprobous; α-meso α-mesosaprobous; β-meso β-mesosaprobous; oligo oligosaprobous; xeno xenosaprobous habitat salinity ph trophy saprob. Achnanthes lemmermannii Hustedt b oh acph eu-meso xeno Amphora fogediana Krammer b oi oligo oligo A, inariensis Krammer b oi oligo A. ovalis Kützing b oi alph eu α-meso A. pediculus (Kützing) Grunow b oi alph eu-meso α-meso A. thumensis (Mayer) Cleve-Euler b oi ind oligo oligo A. granulata (Ehrenberg) Simonsen pl oi alph eu β-meso A. islandica (O. Müller) Simonsen pl oi alph meso-oligo β-meso A. italica (Ehrenberg) Simonsen pl oi alph eu-meso β-meso Cavinula cocconeiformis (Gregory ex Greville) Mann & Stickle b oi alph eu β-meso C. jaernfeldtii (Hustedt) Mann et Stickle b oi acph oligo oligo C. scutiformis (Grunow ex A.Schmidt) D.G.Mann & A.J.Stickle b oi Cocconeis neodiminuta Krammer b oi alph eu-dys β-meso C. neothumensis Krammer b oi alph eu-meso oligo C. placentula Ehrenberg var. euglypta (Ehrenberg) Grunow b oi alph eu-meso β-meso C. placentula var. lineata (Ehrenberg) Van Heurck b oi alph eu-meso β-meso C. placentula var. placentula Ehrenberg b oi alph eu-meso β-meso C. placentula var. pseudolineata Geitler b oi alph eu-meso β-meso Cymbellafalsa diluviana (Krasske) Lange- Bertalot & Metzeltin b oi oligo Diploneis domblittensis (Grunow) Cleve b oi oligo oligo D. oblongella (Naegele) Cleve-Euler b oi alph eu-meso oligo Epithemia adnata (Kützing) Brébisson b oi alb-alph eu-meso β-meso E. frickei Krammer b oi alph eu β-meso E. sorex Kützing b oh alb-alph eu β-meso Eunotia bilunaris (Ehrenberg) Mills b oi acph meso-oligo β-meso Eunotia paludosa Grunow b oi acb oligo oligo Fragilaria heidenii Østrup b oi alph eu F. infl ata var. istvanffyii (Pantocsek) Hustedt b oi alph F. martyi (Héribaud) Lange-Bertalot b oi alph eu-meso oligo F. parasitica (W. Smith) Grunow b oi eu-meso β-meso

6 454 M. Witak & D. Jankowska F. polonica Witak et Lange-Bertalot b oi Geissleria schoenfeldii (Hustedt) Lange- Bertalot et Metzeltin b oi alph eu-dys β-mesooligo Gomphonema minutum (Agatdh) Agardh b oi ind eu β-meso Gyrosigma acuminatum (Kützing) Rabenhorst b oi alph eu α-meso Karayevia bottnica (Cleve) Lange-Bertalot b oh alph eu-meso β-meso K. clevei (Grunow) Round et Bukhtiyarova b oi alph eu-meso β-meso N. cari Ehrenberg b oi alph eu α-meso N. cryptotenella Lange-Bertalot b oi alph meso oligo N. jentzschii Grunow b oi N. rotunda Hustedt b oi alph oligo N. subminuscula Manguin b oi alph eu poly N. subrotundata Hustedt b oi alph oligo Placoneis clementis (Grunow) Cox b oi alph eu-meso β-meso P. elginensis (Ehrenberg) Heinzerling b oi alph eu-dys β-meso Planothidium calcar (Cleve) Round & L.Bukhtiyarova b oi oligo P. joursacense (Heribaud) Lange-Bertalot b oi alph meso-oligo P. lanceolatum (Brébisson) Round et Bukhtiyarova b oi alph eu-meso α-meso P. oestrupii Round & Bukhtiyarova b oi ind meso oligo Pseudostaurosira brevistriata (Grunow) Williams et Round b alph eu-meso oligo Rhopalodia gibba (Ehrenberg) O. Müller b oi alph eu β-meso Staurosira binodis (Ehrenberg) Lange Bertalot b oi alph eu-meso β-meso S. construens (Ehrenberg) Williams et Round b oi alph eu-meso β-meso S. venter (Ehrenberg) Cleve et Möller b oi alph eu-meso β-meso Staurosirella lapponica (Grunow) Williams et Round b oi alph eu-meso S. pinnata (Ehrenberg) Williams et Round b oi alph eu β-meso Stephanodiscus hantzschii Grunow pl oi alb-alph eu poly S. medius Håkansson pl oi eu S. neoastraea Håkansson et Hickel pl oi alb eu β-meso Tabellaria fl occulosa (Roth) Kützing b ohb acph oligo-dys oligo

7 Ancylus Lake stage in the Gulf of Gdańsk (southern Baltic Sea) 455 Results Core 3/2001. Pollen analysis revealed that sandy and muddy sediments between cm correspond to the Boreal chronozone. In this part of the core diatoms were represented by well-preserved and species-rich assemblages. A total of 106 species, subspecies and varieties representing 30 genera were identified in this core interval. The diatom assemblage was dominated by benthic forms (Fig. 3, 2). However, the abundance of planktic forms increased upsection and amounted to 15% in the upper part. This trend corresponded mainly to the increasing abundance of Aulacoseira spp., including A. granulata, A. islandica, A. italica and sporadically A. distans. Other planktic taxa, i.e., Stephanodiscus hantzschii, S. medius and S. neoastraea occurred in lower abundance, and were observed near the top of this interval as well. The striking feature of this diatom assemblage was the high abundance of freshwater species (about 95%). The most abundant oligohalobous indifferent forms were Amphora inariensis, A. pediculus, Fragilaria martyi, Karayevia clevei, Pseudostaurosira brevistriata, Staurosirella lapponica and S. pinnata. Their content ranged from 10% to 25%. Pseudostaurosira brevistriata reached its maximum abundance of 45% at the depth of 235 cm. In addition, in the upper part of the core Navicula rotunda, N. subminuscula, N. subrotundata, Staurosira binodis, S. construens and S. venter reached higher abundances. The group of the so-called nordic-alpine taxa preferring cold freshwater basins was represented by benthic taxa, Amphora fogediana, A. thumensis, Cavinula cocconeiformis, C. jaernfeldtii, C. scutelloides, Cymbellafalsa diluviana, Navicula jentzschii, Planothidium calcar, P. joursancense and P. oestrupii. Noteworthy is higher content of Tabellaria fl occulosa in the upper part of the core an oligohalobous halophobous species. The abundance of oligohalobous halophilous forms was very low and did not exceed 4%. This group was represented by Achnanthes lemmermannii, Karayevia bottnica and Placoneis clementis. Marine and brackish-water species, regarded as allochtonous components in this freshwater taphocoenosis, were recorded only occasionally. Core 4/2001. In Preboreal/Boreal fine sands and gyttjas the diatom assemblage was poorly preserved. Numerous valves were broken, which often prevented identification. The floristic diversity was rather low, with 79 species and 31 genera, but the abundance of some species was very high. Over 90% of this diatom assemblage consisted of freshwater benthic forms (Fig. 4, 5). The major components were Epithemia adnata (max. 58%), Eunotia bilunaris (max. 22%) and Rhopalodia gibba (max. 18%). They were accompanied by Amphora pediculus, Cocconeis neodiminuta, C. neothumensis, C. placentula, Epithemia frickei, Eunotia paludosa, Fragilaria martyi, Gomphonema minutum, Karayevia clevei, Pseudostaurosira brevistriata, Staurosira spp., Staurosirella lapponica and S. pinnata. In the gyttja interval ( cm), the abundance of the hitherto dominant taxa was distinctly decreased. In the upper part of this unit, the major components were Amphora pediculus, Cocconeis neodiminuta, Fragilaria heidenii, F. martyi, Gomphonema minutum, Pseudostaurosira brevistriata, Staurosira binodis, Staurosira construens, S. venter and Staurosirella pinnata. In Preboreal/Boreal sediments, the nordic-alpine diatoms were also observed and their abundance amounted to ca. 6%. The content of Tabellaria fl occulosa ranged from 2% to 5% in the upper part. In Boreal sediments the diatom taphocoenoses were quite abundant and well preserved. A total of 183 species representing 47 genera were identified in this part of the core. Except for Staurosira spp., the major components of the underlying gyttja assemblage occurred abundantly in the Boreal sediments ( cm). Other benthic species, i.e., Cocconeis neothumensis, C. placentula and its varieties, Epithemia sorex, Gyrosigma attenuatum and Karayevia clevei were also observed in silty sediments. A characteristic feature of the diatom assemblage preserved in overlying deposits medium grained sands ( cm) and silts ( cm) is the variable abundance of freshwater planktic taxa, ranging from 10% to 40%. Three peaks at the depths of 330, 300 and 280 cm correspond to increased abundances of Aulacoseira granulata, A. italica, Stephanodiscus medius and S. neoastraea. Benthic forms were represented by, e.g., Amphora

8 456 M. Witak & D. Jankowska Fig. 2. The diatom record of core 3/2001 distribution of selected taxa; Olig. hl oligohalobous halophilous, Olig. hb oligohalobous halophobous, * nordic alpine species included Amphora fogediana, A. thumensis, Cavinula cocconeiformis, C. jaernfeldtii, C. scutelloides, Cymbellafalsa diluviana, Navicula jentzschii, Planothidium calcar, P. joursancense and P. oestrupii.

9 Ancylus Lake stage in the Gulf of Gdańsk (southern Baltic Sea) 457 Fig. 3. The diatom record of core 3/2001 distribution of ecological groups.

10 458 M. Witak & D. Jankowska Fig. 4. The diatom record of core 4/2001 distribution of chosen taxa; PB Preboreal chronozone, Bo Boreal chronozone.

11 Ancylus Lake stage in the Gulf of Gdańsk (southern Baltic Sea) 459 Fig. 5. The diatom record of core 4/2001 distribution of ecological groups.

12 460 M. Witak & D. Jankowska Fig. 6. The diatom record of core 6/2001 distribution of chosen taxa.

13 Ancylus Lake stage in the Gulf of Gdańsk (southern Baltic Sea) 461 Fig. 7. The diatom record of core 6/2001 distribution of ecological groups.

14 462 M. Witak & D. Jankowska pediculus, Fragilaria martyi, Karayevia clevei, Pseudostaurosira brevistriata, Staurosira binodis, S. construens and Staurosirella pinnata. The major component of the oligohalobous halophobous group was Tabellaria fl occulosa, attaining the highest abundance of 8%. In the whole part of the core studied, euhalobous and mesohalobous species were noticed sporadically. Core 6/2001. The diatom taphocoenoses of the Preboreal/Boreal silty sands were well preserved, abundant and represented by a diverse assemblage (Fig. 6, 7), including 123 species within 38 genera. The group of freshwater benthic taxa was dominant; however, at core depths of 315, 300 and 286 cm, maxima of planktic diatoms were observed (ca. 20%, 30% and 40%, respectively). In these intervals Aulacoseira spp. played an important role in the assemblage with A. islandica as a dominant species. Other planktic diatoms, i.e., Stephanodiscus medius and S. neoastraea occurred rarely. The major benthic forms were Amphora pediculus and Fragilaria infl ata var. istvanffyi. These were accompanied by Fragilaria martyi, Pseudostaurosira brevistriata, Staurosira venter and Staurosirella pinnata. In the middle part of the silty sand interval, Cocconeis placentula and its varieties were observed frequently. The frequency of oligohalobous halophilous species did not exceed 5%. In Boreal sediments, the diatom assemblage comprised 140 species repre senting 45 genera, and preservation was good. The characteristic feature of the diatom assemblage within the medium-grained sands ( cm) was a distinct increase in the abundance of benthic forms represented by, e.g., Cocconeis neodiminuta and C. neothumensis. Planktic taxa (Aulacoseira spp., Stephanodiscus spp.) occurred rarely. In the overlying silty sands ( cm) Amphora pediculus, Fragilaria infl ata var. istvanffyi, Fragilaria martyi, Pseudostaurosira brevistriata, Staurosira venter and Staurosirella pinnata dominated. At the depth of 231 cm Epithemia adnata, Eunotia bilunaris and Rhopalodia gibba reached the highest abundance, ca. 52%, 9% and 13%, respectively. In the upper part of the Boreal deposits Diploneis domblittensis, D. oblongella and Epithemia frickei were more abundant. The frequency of planktic forms, with Aulacoseira spp., Stephanodiscus medius and S. neoastraea ranged from 1% to 30%. The only representative of oligohalobous halophobous species, Tabellaria fl occulosa was observed permanently in silty sands. Discussion Diatom analyses supported by pollen stratigraphy and lithology enabled a reconstruction of palaeoenvironmental changes in SW Gulf of Gdańsk during the Ancylus Lake stage, the last freshwater stage of the Baltic Sea development. The diatom assemblages in the sediment cores indicate that the evolution of the study area was dependant on global (climate), regional (water level fluctuations) and local (riverine inflow) factors. Spatial and temporal palaeohydrological changes including depth, ph, trophy, saprobity and temperature are recorded in the studied lacustrine sediments. The oldest sediments deposited close to Preboreal/Boreal boundary with do m i nant Pinus and Cyperaceae pollen were found in the bottom parts part of cores 4/2001 and 6/2001 (Krzymińska et al. 2005). Their location relative to the position of the southern shoreline of the Ancylus Lake indicates that sedimentation took place in the littoral zone (core 6/2001), including the shallowest part, very close to the shore (core 4/2001). The strong predominance of freshwater benthic diatoms in the fine sands and gyttjas of core 4/2001 indicate the existence of a very shallow freshwater basin. The diatom assemblages including an abundance of alkaliphilous (Fragilaria spp., Pseudostaurosira brevistriata, Rhopalodia gibba, Staurosira spp., Staurosirella spp.) and alkalibiontic taxa (Epithemia adnata) indicate ph values exceeding 7. Similar diatom assemblage occurred in the Early Holocene sediments of the westernmost part of the Gulf of Gdańsk, i.e., in the Puck Lagoon area (Witak 2002). In Preboreal and Boreal chronozones, very shallow, small alkaline lake, isolated from

15 Ancylus Lake stage in the Gulf of Gdańsk (southern Baltic Sea) 463 the Ancylus Lake existed there. Acidophilous forms (e.g. Eunotia bilunaris, Tabellaria fl occulosa) and acidobiontic taxa (e.g. E. paludosa) constitute allochthonous components of the diatom assemblages preserved in core 4/2001. Their presence is probably linked to washover from nearby peats. In the Early Holocene, peat bogs occurred in the neighbouring terrestrial environment (e.g. Tomczak 1995, Witak 2002). The dominance of eu-mesotraphentic diatoms (e.g. Epithemia adnata, Fragilaria martyi, Pseudostaurosira brevistriata, Staurosira spp., Staurosirella spp.) accompanied by eutraphentic forms (e.g. Fragilaria heidenii, Gomphonema minutum, Rhopalodia gibba, Staurosirella pinnata) indicates that the waters were rich in dissolved nutrients. At that time the diatom assemblage of silty and sandy sediments of the lower part of core 6/2001 originated in a deeper basin zone. This is documented by the occurrence of autochthonous planktic forms, i.e., Stephanodiscus medius and S. neoastraea. The latter form, named Stephanodiscus astrea (Ehrenberg) Grunow, according to Sandegren (1938) was one of the indicators of Ancylus deposits of Hel Peninsula. Similar diatom assemblage, dominated by benthic freshwater taxa with open lake plankton represented by Stephanodiscus spp. was reported from the Early Holocene silty sediments of Hel Peninsula by Bohr & Sokół (1972), Bogaczewicz-Adamczak (1982), and Bogaczewicz-Adamczak & Żukowska (1990). Increased abundance of freshwater plankton was reported in off-shore lacustrine clays of the Gdańsk Deep (Zaborowska & Zachowicz 1982, Pieczka & Zaborowska 1989, Witkowski 1994). However, occurrence of allochthonous species, e.g., Aulacoseira islandica, A. granulata and A. italica indicate that the palaeohydrology of this area was impacted by the extent of the riverine water influx caused by the proximity of the Vistula River outlet. According to Witkowski (1994) these taxa are abundant in the Vistula River and their distribution conforms to the direction of Vistula water spreading within the Gulf of Gdańsk. This was corroborated by diatom studies in the Vistula Lagoon (Witak 2010). The mass occurrence of Aulacoseira spp. was directly related to Nogat and Bauda rivers discharging into the Vistula Lagoon. The frequency of these diatoms may indicate that the influence of the Vistula River on environmental conditions at that time was rather limited, but apparently variable. Aulacoseira spp. was also reported as indicators of the Vistula River influence on palaeohydrology in the Ancylus Lake stage by Stachura-Suchoples (2006). The changes in the riverine water discharge evidently impacted the trophic status. The stronger riverine influx documented by peaks of Aulacoseira spp. resulted in periodic increase of dissolved nutrients. In eutrophic waters, abundant Amphora spp. and Cocconeis spp. could develop (Denys 1991). In Preboreal/Boreal taphocoenoses of both cores, the so-called nordic-alpine diatoms were observed with a total frequency of ca. 3 5%. They were represented by, e.g., Amphora fogediana, A. thumensis, Cavinula cocconeiformis, C. jaernfeldtii, C. scutelloides, Cymbellafalsa diluviana, Navicula jentzschii, Planothidium calcar, P. joursancense and P. oestrupii. All of these species occur in well-oxygenated lakes, where they can develop in a wide ph-range (Krammer & Lange-Bertalot 1986, Hofmann 1994). They were observed in both fossil and modern lacustrine sediments in Scandinavia (Cleve-Euler ). Some of them, i.e., Amphora thumensis, C. jaernefeltii, N. jentzschii, P. calcar P. oestrupii, were observed in postglacial lacustrine sediments in the Kuujjuaq area, Northern Québec (Pienitz et al. 1991). Amphora fogediana, Navicula jaernefeltii and N. scutiformis develop in freshwater basins in the northern part of Europe and North America (Krammer & Lange-Bertalot 1986). Achnanthes oestrupii, A. fogediana and Planothidium calcar were observed in modern sediments of Bear Island (Metzeltin & Witkowski 1996). All of these species occurred with higher frequency in Late Glacial lacustrine sediments of Rzucewo Deep, within the Puck Lagoon (Witak 2002). Their presence in the study area could be a signal of relatively low water temperature. This hypothesis is corroborated by the diatom assemblages preserved in the younger sediments of all studied cores, where the content of the whole cold-water group decreased to 1 2%. There is no doubt that this phenomenon is directly linked to climatic amelioration of the Boreal chronozone. Pollen spectra dominated by Betula, Corylus and Alnus, accompanied

16 464 M. Witak & D. Jankowska by Salix, Ulmus and Quercus (Krzymińska et al. 2005) indicate broadleaf-forest development under warmer climatic conditions. An increase in air temperature and humidity in the Boreal chronozone resulted in an enhanced riverine influx into the basin (Starkel 2001). Larger volumes of Vistula waters, carrying an abundance of Aulacoseira spp. supplied dissolved nutrients and organic matter. This phenomenon is pronounced in the diatom record of core 6/2001, and the upper part of core 3/2001. In the case of core 4/2001, the higher frequency of riverine plankton may also be related to the Kacza river inflow. The increase of trophy and saprobity is recorded in all the studied cores by the abundance of Amphora pediculus, Karayevia clevei, Staurosira binodis, S. construens, S. venter, Staurosirella lapponica and S. pinnata. Other representatives of eutraphentic or eu-mesotraphentic and α- or β-mesosaprobous taxa, i.e., Amphora copulata, Cocconeis placentula, Fragilaria heidenii, F. parasitica, Navicula cari, Plano thidium lanceolatum were also noted, but with lower frequency. The pronounced decrease of riverine water inflow recorded by the low content of Aulacoseira spp. could have been caused by limitation of nutrient and organic matter supply. In this case, eutraphentic taxa were replaced by mesotraphentic forms and oligosaprobous diatoms developed. Such environmental conditions were recorded in the medium-grained sands of core 6/2001, with the high abundance of Cocconeis neodiminuta and C. neothumensis. Numerous taxa mentioned above belong to the alkaliphilous group. The strong predominance of this group in all studied cores, with a frequency usually exceeding 80%, clearly indicates that ph was rather stable and exceeded 7 in the Boreal chronozone. The drastic drop in the alkaliphilous group abundance observed in silty sands of core 6/2001 at the depth of 231 cm is related to mass occurrence of the alkalibiontic species Epithemia adnata (52%). The second component of this diatom taphocoenosis was the alkaliphilous form Rhopalodia gibba (13%). Both species were observed in high abundance in the underlying Preboreal/Boreal sediments of the shallowest coastal part of Ancylus Lake (core 4/2001). That could be a signal of decreasing water level due to regression of Ancylus Lake in the Boreal chronozone. This stage lasted in the southern Baltic Sea from ca 9.2 to 8.6 ka BP and the water level decreased from ca. 25 to 28 m below the present level (Uścinowicz 2003). As a consequence, the shoreline migrated northward and sediments of the upper part of core 6/2001 might have been deposited in the very shallow zone of the basin. This interpretation is corroborated by the low frequency of the open lake plankton, e.g., Stephanodiscus spp., and the higher content of Diploneis domblittensis sensu lato, and Epithemia frickei observed in the upper part of Boreal sediments in core 6/2001. Diploneis spp. and Epithemia spp. were also reported from sediments recovered at Hel Peninsula, and corresponded to the regressive stage of Ancylus Lake (Bohr & Sokoł 1972). The abundance of the acidophilous species Eunotia bilunaris (9%) accompanied by Tabellaria fl occulosa could be linked to the peat bog development in the close vicinity of the basin. It is likely that the southern area (core 4/2001) was located outside the Ancylus Lake range and emerged. The lack of evidence for emergence of the area located south of the shoreline probably results from erosional processes caused by transgression during the next stage of the Baltic Sea development the Mastogloia Sea. Due to the same reason, regressive sediments in core 3/2001 were probably also eroded. Lack of Mastogloia sediments below the Littorina sandy deposits of cores 4/2001 and 3/2001 was discussed in detail by Witak (2010). The evidence for water level drop corresponding to the regressive phase of Ancylus Lake in core 6/2001 can be associated with the location of the core in the highest water depth in this study (62 m). Acknowledgements This study was partly financed by the Council of Science, Ministry of Science and Higher Education research grant No 6 PO4D

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