Oceanological and Hydrobiological Studies. Holocene diatom biostratigraphy of the SW Gulf of Gdańsk, Southern Baltic Sea (part II)

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1 Oceanological and Hydrobiological Studies International Journal of Oceanography and Hydrobiology Vol. XXXVI, No.3 Institute of Oceanography ISSN X (3-20) 2007 University of Gdańsk eissn DOI /v Original research paper Received: Accepted: December 21, 2006 May 23, 2007 Holocene diatom biostratigraphy of the SW Gulf of Gdańsk, Southern Baltic Sea (part II) Małgorzata Witak 1, Julita Dunder Institute of Oceanography, University of Gdańsk Department of Marine Geology Al. Piłsudskiego 46, Gdynia, Poland Key words: diatoms, paleoecology, Holocene, Gulf of Gdańsk Abstract The diatom records of two sediment cores taken in the SW part of the Gulf of Gdańsk indicate paleoenvironmental variability in the last three stages of formation of the Baltic Sea: the Mastogloia, the Littorina and the Post-Littorina. Diatom community compositions of the cores show the influence of several factors on past environmental conditions of the study area: (1) regional salinity changes, (2) changes of the Vistula River inflow, and (3) sedimentary processes connected with the Hel Peninsula development. 1 Corresponding author: ocemaw@univ.gda.pl Copyright by Institute of Oceanography, University of Gdańsk, Poland

2 4 M. Witak, J. Dunder INTRODUCTION The results presented comprise a biostratigraphical study of diatoms in two sedimentary cores taken in the outer region of Puck Bay area, and which constitute continuation of studies of the diatom record of three further cores (Fig. 1). Pollen analyses have show that these cores represent the last 9 ka (Witak et al. 2006). Paleoecological changes of the study area, from the freshwater, alkaline, eutrophic basin of the Early Holocene through the development of the marine condition in the Atlantic and Early Subboreal chronozones linked with the Littorina transgression, to the brackish water stage of the Post-Littorina Sea, have been reconstructed from diatom taphocoenose correlations. Anthropogenic assemblages with small planktonic pollution tolerant taxa have also been described from the sub-bottom sediments. Analyses of the new cores drilled between Gdańsk and Hel (core 1/2001) and between Gdynia Oksywie and Hel (core 5/2001) have enabled (1) the differentiation of diatom assemblage zones in the cores, (2) the comparison and correlation of paleoecological units, (3) the reconstruction of environmental changes recorded in the sediments, and (4) discussion of the spatial shift associated with the last stages of the Baltic Sea evolution. An outline of the evolution of the study area The evolution of the coastal zone of the Southern Baltic Sea, formed as a result of the decay of the last Scandinavian ice-sheet, has been the subject of numerous previous studies (e.g. Pawłowski 1922, Rosa 1963, 1968, 1991, Rudowski 1979, Tobolski 1989, Tomczak 1990, 1995, Kramarska et al. 1995, Mojski 1995, 2000, Rotnicki 1999, Rotnicki et al. 1999, Uścinowicz 1999, 2003, Zawadzka - Kahlau 1999, Borówka et al. 2005). In the study area, located in the vicinity of Puck Bay, in the SW part of the Gulf of Gdańsk, several drastic changes of environmental status are recorded in Late Glacial and Holocene sediments. The postglacial history of the development of this region, which has a wide connection with open water, shows a strong genetic relationship with the concurrent paleoecological state of the Baltic Sea. The paleohydrological regime of the study area is impacted by water masses flowing in from the Gdańsk Basin and the inflow of the Vistula river (Mojski 1988). A deglaciation of the Southern Baltic Sea began c. 14 ka BP with the retreat of an ice-sheet from the Gardno end-moraines (Uścinowicz 1999). However, during the Gardno Phase (c ka BP), the Gdańsk Basin and the Gulf of Gdańsk were still occupied by the ice-sheet and the Vistula River probably flowed to the north (Mojski 1988). During the next phase of deglaciation, called the Słupsk Bank Phase (c ka BP), an ice-sheet formed a lobe in the Copyright by Institute of Oceanography, University of Gdańsk, Poland

3 Holocene diatom biostratigraphy of the Southern Baltic Sea 5 Gdańsk Basin, whilst in the western part of the Gulf of Gdańsk a large icedammed lake extending to Darłowo was formed. At that time laminated lacustrine sediments, mostly silty clays and clayey and sandy silts, were formed in the Puck Bay area (Uścinowicz 2003). The Vistula discharged into a lake, initiating the beginning of delta development (Mojski 1990). The retreat of the ice-sheet front from the Słupsk Bank Phase end-moraines caused further enlargement of the proglacial lake. As a result of the ice-sheet meltdown in the Słupsk Furrow, lakes in the Gdańsk Basin and the Bornholm Basin became connected and, in consequence, c ka BP the Baltic Ice Lake was formed (Mojski 2000). Fig. 1. Location of the analyzed cores; 1 cores analyzed in this study, 2 cores analyzed in Witak et al (2006).

4 6 M. Witak, J. Dunder Results of many geological investigations have shown that in the Late Glacial and Early Holocene the shoreline of the Polish coastal zone migrated at different rates and in different directions (Rosa 1994, Rotnicki 1999, Tomczak 1995, Uścinowicz 2003). Uścinowicz (2006) suggests the period between ka BP was characterized by large and abrupt changes of water level in the Southern Baltic Sea. Interactions between glacio-isostatic movements and glacio-eustatic increases in the ocean level caused the opening and closing the connection between the Baltic and open ocean. Due to these processes, three transgressions progressions and three regressions took place at that time. During the transgressions progressions of the Baltic Ice Lake (c and ka BP) Puck Bay region was partly flooded (up to 20 m below present sea level). During the first and last very rapid drainages of the basin, the shoreline probably retreated by about 5 km in the Gulf of Gdańsk, and the study area was land. Subsequently, due to a water level rise at the end of the Younger Dryas and Preboreal, the shoreline of the area migrated back again. The Yoldia Sea ( ka BP) water level rose to about 40 m below present sea level and in the SE part of the Gulf of Gdańsk a shallow lagoon developed (Kramarska et al. 1995). Subsequently, at the beginning of the Ancylus Lake stage ( ka BP), the water level increased to about 25 m below present sea level (Mojski 2000), thus the larger part of today s Puck Bay was submerged. Finally, c. 9.2 ka BP a small regression occurred, which probably caused the emergence of sand spit structures in the western and southern parts of the Gdańsk Basin (Uścinowicz 2003). At that time, the Vistula flowed along the western part of the today s delta to the southern part of the Gulf of Gdańsk (Mojski 1988). Due to eustatic ocean level rise following climatic amelioration, the Danish Straits opened up and saline waters flowed into the Baltic. According to Tomczak (1995) this marine transgression began at the beginning of the Atlantic chronozone (c. 8.0 ka BP). However, Uścinowicz (2003) suggests that this process started in the Late Boreal ( ka BP). During the Mastogloia Sea stage ( ka BP) the water level increased by about 13 m and during the period ka BP the Littorina Sea level rose from about 15 m below present sea level to about 2.5 m below it. Overall in the Middle Holocene the coastline of the Gulf of Gdańsk moved southwards by about 5 km, and by the end of Atlantic chronozone approached its present location (Uścinowicz 2003). Mojski (2000) argues that the old shoreline of the maximum extent of the Littorina transgression is recorded by the cliff between Gdańsk and Sopot. Its location indicates that the sea even ingressed over the present shoreline by about km in Atlantic chronozone. The Vistula River flowed to the Gulf of Gdańsk, although due to the migration of the coastline the outlet moved southwards to its present location. Right branches, however, fell into the primeval Vistula Lagoon. At the same time, the Hel Peninsula was growing Copyright by Institute of Oceanography, University of Gdańsk, Poland

5 Holocene diatom biostratigraphy of the Southern Baltic Sea 7 intensely and at the end of the Atlantic reached at least Jurata (Tomczak 1995). This sandy barrier was narrow and unstable, often broken during numerous, heavy storms. C ka BP the terrestrial area of the western part of Puck Bay was flooded and a brackish/marine lagoon developed. This shallow basin occurred in the outer part of the Puck Bay area, and had a higher salinity than at present until the Early Subboreal chronozone. In the last stage of the evolution of the Baltic, the Post-Littorina Sea, the salinity gradually decreased and the conditions transformed from marine to brackish in the study area. According to Uścinowicz (2003), during the Subboreal chronozone the sea level rose from about 2.5 m to 0.6 m lower than present. The Hel Peninsula developed towards to the south-east as a result of longshore sand transport, due to marine erosion of the adjoining Swarzewo Holm to the west. Simultaneously, the Puck Lagoon extended to Władysławowo vicinity and cliffs developed on slopes of the Puck Morainic Upland (Uścinowicz & Miotk-Szpiganowicz 2003). Cliffs located in the southwestern part of the Gulf of Gdańsk moved landwards. In the last 2.5 ka the sea level increased very slowly by about 0.6 m (Uścinowicz 2003). However, some evidence of sea level change around ka BP were found in several places along the Polish coastal zone (e.g., Tomczak 1995, Tobolski et al. 1997, Latałowa 1999, Pomian et al. 2000). In the Subatlantic chronozone the Hel Peninsula continued to elongate with a low rate of accretion (Mojski 2000). Because of the erosion of cliffs near Władysławowo, the north-west part of peninsula migrated south-west, whereas the south-west part broadened greatly. At that time, frequent storms broken the barrier and overflow fans were formed in Puck Lagoon (Kramarska et al. 1995). The Vistula flowed into the Vistula Lagoon, which as Uścinowicz (2003) suggests, was largest at the beginning of Subatlantic. Due to natural processes of riverine accumulation the Vistula Delta plain developed, with many branches and streams flowing over the delta area. Since the Middle Ages the hydrography of the region has been impacted by human activity (Cyberski 1995, Starkel 2001). Deforestation, and construction of flood dams and polders have greatly intensified erosion and denudation processes and increased the supply of sedimentary material into the Vistula Lagoon and Gulf of Gdańsk (Majewski 1969). Changes in the location of the Vistula River mouth as a result of floods in the last millennium are discussed in detail by Cyberski (1995) and Cyberski et al. (2006). As a result of the building of an artificial channel near Świbno, and closing the branches (the Dead Vistula and the Szkarpawa), in 1895 the flow of the Vistula is entirely directed into the sea.

6 8 M. Witak, J. Dunder MATERIALS AND METHODS Two sediment cores were retrieved by means of a vibro corer during a cruise of r/v Dr Lubecki organized by the Polish Geological Institute, Branch of Marine Geology, Gdańsk, in 2001 (Fig. 1, Table 1). Core 5/2001 was taken from the outer part of Puck Bay at a water depth of 45 m, whereas core 1/2001 was collected outside Puck Bay at a water depth of 32 m. Samples were taken at regular 6 cm intervals to a depth of 3.15 m (core 1/2001) and 3.60 m (core 5/2001) and were prepared for diatom analysis according to the method described by Battarbee (1986). The prepared samples were dried onto coverslips and mounted with Naphrax (refractive index n D = 1.73). Quantitative analyses were carried out with a light microscope at a magnification of The counting convention of Schrader and Gersonde (1978) was used, and approximately 500 diatom valves, excluding Chaetoceros spp. resting spores, were counted at each level. The diatom floras and articles used for identifying the diatoms, and as a source of ecological information, are listed in Witak et al. (2006). The diatoms were classified with respect to habitat preferences (Round 1981) and salinity tolerance (Kolbe 1927). The percentage content of individual taxa as well as all ecological groups were estimated in each sample. Based on these details diatom assemblage zones (DAZ) were defined in both cores. Table 1 Main parameters of the studied cores Core no. φ λ Core length [m] Water depth [m] 1/ o 31,501 N 018 o 44,661 E / o 35,417 N 018 o 40,836 E RESULTS Diatomological analyses indicate that diatoms were generally abundant and preserved in a good state. However, an admixture of broken frustules was noted in all samples. In core 5/2001 diatoms occurred in each sample, whereas in core 1/2001 at a depth of 56 cm and between 39 and 0 cm hiatuses were observed in the record. Moreover, the results show considerable differences in numbers of identified taxa between the cores, with a higher diversity observed in the sample taken from the outer part of the Puck Bay (core 5/2001). In core 5/2001 a total of 141 diatom taxa belonging to 44 genera were identified, whilst in core 1/ genera and 90 species, including varieties and forms, were observed (Table 2). Floristic changes observed in the vertical structures of both cores allowed several zones to be distinguished (Fig. 2, 3). Copyright by Institute of Oceanography, University of Gdańsk, Poland

7 Holocene diatom biostratigraphy of the Southern Baltic Sea 9 Table 2 Number of diatom taxa of all ecological groups in cores Life form Halobous group No core Benthic Planktonic Euhalob. Mesohalob. Oligohalobous halophilous indifferent halophobous 1/ / Core 1/ I Aulacoseira spp. Thalassionema nitzschioides Rhizosolenia hebatata f. hebatata Pseudosolenia calcar-avis DAZ ( cm). The diatom taphocenose was dominated by freshwater plankton, mostly represented by Aulacoseira spp. which ranged from 45 to 85% of the total abundance. Marine planktonic taxa were common, including Thalassionema nitzschioides, Rhizosolenia hebatata f. hebatata, Pseudosolenia calcar-avis and Thalassiosira levanderii. Amongst the benthic taxa oligohalobous-indifferent strains, such as Pseudostaurosira brevistriata, Diploneis boldtiana and D. domblittensis, were observed in all samples. Euhalobous forms were also observed, primarily Diploneis bombus, D. smithii, and Opephora schwarzii, whilst of the mesohalobous taxa Nitzschia constricta, Opephora guenter-grassii, Planothidium delicatulum were prevalent. 1-II Catenula adhaerens Thalassiosira levanderii Planothidium delicatulum DAZ ( cm). The main components of this assemblage were benthic forms represented by various halobian groups. Amongst the euhalobous group, Catenula adhaerens were observed most often, accompanied by many taxa belonging to the genera Cocconeis, Diploneis, Grammatophora and Opephora. The main taxa of the mesohalobous groups were Planothidium delicatulum and Opephora guenter-grassii, with less frequent observations of Fragilaria atomus, F. geocollegarum and Navicula perminuta. Representatives of oligohalobous halophilous forms, Achnanthes lemmermannii, and indifferent taxa, Cocconeis placentula, Diploneis boldtiana, Pseudostaurosira brevistriata, Staurosira construens, and Staurosirella pinnata, were encountered. Planktonic forms did not exceed 30% of diatom abundances, with Achnanthes taeniata, Aulacoseira spp., Thalassiosira levanderi and Hyalodiscus scoticus commonly observed.

8 Fig. 2. Diatom record of the core 1/2001.

9 Fig. 3. Diatom record of the core 5/2001.

10 12 M. Witak, J. Dunder 5-I Fragilaria martyi Stauroneis construens + varr. Catenula adhaerens Opephora spp. DAZ ( cm). The diatom community of this section was dominated by benthic forms, abundances ranging from 95% of the total at the bottom to 75% in the upper part of the zone. In the lowest part freshwater flora, mostly represented by Fragilaria martyi and Staurosira construens were common. Moving up the zone these taxa were replaced by both marine (Catenula adhaerens and Opephora schwarzii,) and brackish-water (Opephora guenter-grassii, O. mutabilis and Planothidium delicatulum) diatoms. Marine planktonic taxa including Thalassionema nitzschioides, Rhizosolenia hebatata f. hebatata and Pseudosolenia calcar-avis were observed sporadically. 5-II Thalassionema nitzschioides Pseudosolenia calcar-avis Rhizosolenia hebatata f. hebatata DAZ ( cm). A distinct increase in the abundance of marine plankton was noted in the diatom assemblage of this zone. The taxa Thalassionema nitzschioides, Rhizosolenia hebatata f. hebatata and Pseudosolenia calcar-avis were very abundant. They were accompanied by marine benthic forms including Diploneis bombus, D. smithii and Opephora schwarzii. Brackish water taxa were primarily represented by Fallacia spp. and Fragilaria spp., and the main components of the freshwater flora were Amphora pediculus, Diploneis boldtiana, Pseudostaurosira brevistriata and Staurosira construens. 5-III Opephora guenter-grassii Opephora mutabilis DAZ ( cm). The diatom assemblage in this zone was dominated by benthic forms. In the euhalobous group Catenula adhaerens occurred often. Amongst the mesohalobous forms Opephora guenter-grassii, O. mutabilis and Planothidium delicatulum dominated, accompanied by Fallacia florinae, Fragilaria spp. and Navicula perminuta. Oligohalobous halophilous occurred rather rarely, being represented most often by Achnanthes lemmermannii. Indifferent taxa were abundant, with Cocconeis placentula, Pseudostaurosira brevistriata and Staurosirella pinnata most common. Planktonic forms occurred sporadically, with Achnanthes taeniata, Aulacoseira granulata and Thalassiosira levanderi being the most abundant. 5-IV Catenula adhaerens Planothidium delicatulum Cocconeis placentula + varr. DAZ (177 0 cm). The most streaking feature of the diatom flora preserved in this zone is the high frequency of benthic forms, which usually exceeded 90% of total diatom abundances. The main components of the assemblage were brackish water taxa, notably Navicula perminuta, Opephora guenter-grassii, Planothidium delicatulum and Thalassiosira levanderi. The euhalobous group was most represented by Catenula adhaerens, with members Copyright by Institute of Oceanography, University of Gdańsk, Poland

11 Holocene diatom biostratigraphy of the Southern Baltic Sea 13 of the genera Cocconeis and Diploneis also encountered. Freshwater taxa were less frequent, although Cocconeis placentula was seen at its most abundant of the study in this sample, and Pseudostaurosira brevistriata, Staurosira construens and Staurosirella pinnata were also seen. Amongst oligohalobous halophilous forms Krayevia clevei var. bottnica and Achnanthes lemmermannii exceeded 3% of total abundance. DISCUSSION The diatom flora preserved in both cores show many differences including number of identified taxa, the ratio of benthic/planktonic species and the relative abundances of halobian groups. Freshwater planktonic diatoms were noted much more often in core 1/2001, particularly in its lower part, than in core 5/2001, which was dominated by benthic diatoms, they preferring marine and brackish-waters. In spite of these differences, which result from the core location, diatom assemblage zones distinguished in the sediments can be correlated. Based on a paleoecological interpretation of these correlations, three distinct stages can be described in the development of the area (Fig. 4). Fig. 4. The correlation of the diatom assemblages zones of the studied cores. The lithological description based on Krzymińska, Michałowska (unpublished).

12 14 M. Witak, J. Dunder Mastogloia stage The diatom taphocoenose of 5-I DAZ preserved in the bottom part of the sandy sediments of core 5/2001 was the oldest of the material studied. Freshwater benthic diatoms (Fragilaria martyi, Staurosira construens and Staurosirella pinnata) were displaced by marine (Catenula adhaerens, Opephora schwarzii) and brackish-water forms (Opephora guenter-grassii, O. mutabilis, Planothidium delicatulum). A rapid drop in the content of oligohalobous-indifferent strains and an increase of the frequency of euhalobous and mesohalobous strains was also observed in 6-II DAZ preserved Late Boreal and Early Atlantic sediments (Witak et al. 2006). Such changes in the diatom record indicate the increase of the water salinity, which is connected to incursions of saline waters into the Baltic basin during the Mastogloia Sea stage. According to Uścinowicz (2003) marine transgression with associated sea-level rises of 14 mm/a in the period ka BP, and 12 mm/a in ka BP caused the shoreline to migrate up to 5 km in the Gulf of Gdańsk, and as a consequence areas 15 m below sea level were flooded. In the outer part of Puck Bay the freshwater eutrophic basin (Ancylus Lake) transformed into a brackish-water one as a result of the sea water influx. Littorina Stage The next stage of development of the study area is indicated in 5-II DAZ, preserved in the sandy muds and silts, and in 1-I DAZ, in the layer of fine sands occurring in the lowest part of the core and in the overlaying silty sediments. In both cores whole and fragments of marine mollusc shells were seen. The features of taphocoenoses, mainly the presence of marine/oceanic planktonic diatoms represented by Thalassionema nitzschioides, Rhizosolenia hebatata f. hebatata and Pseudosolenia calcar-avis, indicates that both assemblages correspond to the Littorina Sea stage. There is a distinct difference in the frequency of these indicators observed in the cores. The content of T. nitzschioides reached 40% in 5-II DAZ, whereas it did not exceed 5% in 1-I DAZ. Its abundance was also observed in Littorina sediments found in core 6/2001, while a low abundance was noted in analogous deposits in cores 3/2001 and 4/2001 (Witak et al. 2006). P. calcar-avis was more often recorded in 5-II DAZ (up to 20%) than in 1-I (up to 3%). This species occurred frequently in the Littorina section of the core 3/2001, and rather rarely in 4-III and 6-III DAZ (Witak et al. 2006). The content of R. hebatata f. hebatata did not exceed 3% in 5-II and 1-I DAZ, or in 3-II or 6-III. Moreover, a subtropical neritic form, Thalassiosira oestrupii, was noted in 5-II. This species was also sporadically observed in Littorina sediments of cores 3/2001, 4/2001 and 6/2001. The marine benthic diatoms, Catenula adhaerens, Cocconeis hoffmannii, Diploneis bombus, Copyright by Institute of Oceanography, University of Gdańsk, Poland

13 Holocene diatom biostratigraphy of the Southern Baltic Sea 15 D. smithii, Grammatophora spp., and Opephora schwartzii occurred more frequently in 5-II than in 1-I DAZ. Of them only C. adhaerens was a permanent component of the Littorina flora preserved in the sediments of the three cores discussed previously (Witak et al. 2006). D. bombus was recorded rather rarely in 3-II and 6-III DAZ. The abundance of mesohalobous forms was higher in 5-II (up to 30%) than in 1-I (up to 10%). In 5-II littoral species Opephora guenter-grassii, O. mutabilis and Planothidium delicatulum dominated, while of these only P. delicatulum reached 3% abundance in 1-I. These diatoms were all regularly noted in 3-II and 6-III, but rarely in 4-III (Witak et al. 2006). The opposite tendency was seen in the percentage abundance of freshwater diatoms. Their abundances ranged from 60 to 85% in 1-I, whereas in 5-II they were only 20 to 40%. This incongruity is a consequence of the frequency of observation of the riverine planktonic form Aulacoseira spp. Their abundance in 1-I was similar to that observed in 4-III (Witak et al. 2006). Conversely, oligohalobous-indifferent strains in 5-II were dominated by benthic forms with high edaphic preferences: Amphora pediculus, Diploneis boldtiana, Pseudostaurosira brevistriata and Staurosira construens. These taxa were regularly recorded in 3-II DAZ, and occasionally in the Littorina sediments of cores 4/2001 and 6/2001. These distinct differences in the distribution of various ecological groups established the variability of paleoenvironmental conditions in the study area during the Littorina stage. The diatom flora of 5-II DAZ clearly demonstrates a strong influence of sea water in the deep part of Puck Bay. As geological studies have shown the sandy barrier of the Hel Peninsula was much shorter during that time than at the present, and reached beyond Jastarnia and Jurata in the middle Holocene (Tomczak 1995). Uścinowicz (2003) suggested that this barrier, and others on the southern Baltic coast, was narrow and low in the Atlantic chronozone. Therefore the spit was often broken by storms which occurred with high frequency and force during the climatic optimum of the Holocene. The regional factor, namely the Littorina transgression, overlapped with the local factor, the occurrence of heavy storms, determined the abundance of marine planktonic diatoms. An analogous diatom flora was observed in the sediment of core 6/2001, which was pollen dated to the Early Subboreal (Witak et al. 2006). Results from the core 4/2001 indicate that the paleohydrology of the shallow part of the study area was shaped by Vistula River waters, implied by the dominance of diatoms belonging to genus Aulacoseira. In the Littorina stage the Vistula River flowed into the Gulf of Gdańsk (Mojski 1988). An increase of temperature and humanity observed in the Middle Holocene caused the intensification of the riverine inflow (Starkel 2001) and in consequence a freshening of the neighbouring area. Moreover, the Vistula River carried a great

14 16 M. Witak, J. Dunder nutrient load, which can cause trophic status to change. The increase of the trophy in the Littorina stage is indicated by the abundance of eutraphentic forms noted in 3-II, 5-II and 6-III DAZ. The same phenomenon was also observed by Stachura-Suchoples (2006). Post-Littorina stage Post-Littorina sediments have been found in the upper parts of both cores. They are represented by silts and silty muds in core 1/2001, whereas in core 5/2001 silty sediments with mollusc shells in their middle section prevailed. Reconstruction of environmental changes that happened in the study area can be inferred from the diatom communities of 1-II, 5-III and 5-IV DAZ. The most characteristic feature of these taphocoenoses is an almost entire disappearance of marine planktonic forms. These were replaced by benthic flora, typical for the littoral zone, with components of the various halobian groups. Amongst the euhalobous diatoms C. adhaerens was observed most frequently. Its abundance was relatively stable and reached 20% in 1-II DAZ and 15% in 5-III DAZ. A similar pattern was seen in the diatom flora preserved in Post-Littorina sediments of cores 3/2001 and 4/2001 (Witak et al. 2006). Moreover, the group of brackish water taxa was a significant component of the fossil assemblages, its frequency increasing to 20-30% in 1-II DAZ and 5-III DAZ. The primary representatives of mesohalobous strains in 5-III DAZ were Navicula perminuta, Opephora guenter-grassii, O. mutabilis and Planothidium delicatulum. These species all occurred abundantly in 3-III DAZ, although in 4-IV and 6-IV DAZ N. perminuta was absent (Witak et al. 2006). In 1-II DAZ P. delicatulum was the dominant taxon and O. guenter-grassii was rarely seen. Furthermore, in that assemblage a drastic drop in the frequency of oligohalobous-indifferent strains was observed. This pattern is reflected in a rapid decrease of the riverine plankton content represented by Aulacoseira spp, the same phenomenon having been recorded in 4-IV DAZ. The main components of the freshwater benthos were Cocconeis placentula, Pseudostaurosira brevistriata, Staurosira construens and Staurosirella pinnata, which occurred abundantly in 5-III DAZ, although rarely in 4-IV and 6-IV DAZ. In 3-III DAZ the two first species were noted relatively often, whilst the others were seen only sporadically (Witak et al. 2006). The silty sediments of the upper part of core 5/2001, 5-IV DAZ, contained a low abundance of planktonic taxa. They were primarily represented by Achnanthes taeniata and Thalassiosira levanderi, reaching up to 2 and 8% of total abundance respectively, both being species that prefer environments rich in nutrients and organic matter. Both species were also observed in the upper part of core 4/2001, where they occurred more frequently. A high number of benthic Copyright by Institute of Oceanography, University of Gdańsk, Poland

15 Holocene diatom biostratigraphy of the Southern Baltic Sea 17 forms were also observed in 5-IV DAZ, abundant among them Catenula adhaerens, Navicula perminuta, Opephora guenter-grassii, Planothidium delicatulum and Cocconeis placentula. Of these, O. guenter-grassii, P. delicatulum and C. placentula had also been observed in the sub-bottom sediments of cores 3/2001, 4/2001 and 6/2001, although as smaller proportions of the total abundance there (Witak et al. 2006). The diatom flora of the Post-Littorina stage is indicative of the environmental changes in the study area. In the outer part of Puck Bay, in the vicinity of core 5/2001, a decrease of water salinity was recorded. Changing halobian conditions can be ascribed to both large-scale and local processes, although this phenomenon is clearly connected with the Baltic trend of a gradual drop of salinity caused by the shallowing of the Danish Straits over the last 4000 a (e.g., Berglund and Björck 1994, Hyvärinen 2000). In addition, the inflow of less saline waters was restricted by the intensive extension of the Hel Peninsula in the Post-transgression phase (e.g., Rosa 1994, Zawadzka-Kahlau 1999). As a result of this process the barrier was elongated and broadened in its southeastern part and became a more stable structure separating the bay from the open ocean (Mojski 2000, Uścinowicz 2003). The floristic changes recorded in core 1/2001 show the opposite trend in the development of this part of area, the diatom flora clearly indicating an increase in salinity in comparison to the earlier phase. Previous geological and archeological studies have shown that at the beginning of the Subatlantic chronozone the many branches of the Vistula River discharged into the Vistula Lagoon (Cyberski 1982, Starkel 2001). Thus, at that time the influence of the riverine water on the environmental conditions of the study area was restricted. Unfortunately, in the sub-bottom sediments of core 1/2001 the diatom flora is poorly preserved. Because of the lack of the socalled anthropogenic assemblage with small planktonic pollution tolerant taxa, which was observed in this area by Stachura-Suchoples (2006) and Witak et al. (2006), it is impossible to discuss the role of the Vistula River on the environmental status of this area in the last 200 years. CONCLUSIONS Diatomological studies of two cores taken between the Hel Peninsula and the Tri-City area of the Puck Bay show abundant and diverse flora that indicate temporal and spatial variability of paleoenvironmental conditions in that area. In the bottom part of core 5/2001 the oldest sediments represent the Mastogloia Sea stage. The replacing of freshwater diatoms by euhalobous and mesohalobous taxa clearly indicates an increase in water salinity in the SW part of the Gulf of Gdańsk, which is typical for this stage of the Baltic Sea evolution. The next stage, the Littorina Sea, is indicated by the presence of oceanic

16 18 M. Witak, J. Dunder planktonic taxa in both cores, with the floristic spectra implying the spatial difference of the environmental status imparted by the different sites of core collection. In the outer part of Puck Bay, in the vicinity of core 5/2001, there is a strong inflow of saline open-sea water, whilst the hydrology of the region located near core 1/2001 is heavily influenced by Vistula River inputs. The intensive inflow of riverine waters causes the freshening of this of the basin, but with it the nutrient load increases the trophic status. In the Post-Littorina Sea stage the distinct environmental changes are registered in diatom taphocoenoses of both cores. In the region of core 5/2001 a decrease of salinity was indicated, whilst the opposite tendency was implied to occur around core 1/2001. The high abundance of diatoms preferring marine and brackish water conditions seen in the upper part of core 1/2001 indicates the relatively recent increase of salinity in this part of the study area, probably linked with limitation of the inflow of Vistula River water. REFERENCES Battarbee, R.W. (1986). Diatom analysis. In B.E Berglund (Ed.), Handbook of Holocene palaeoecology and palaeohydrology (pp ). London: John Wiley & Sons Ltd. Berglund, B.E. & Björck S. (1994). Late Weichselian and Holocene shore displacement in Blekinge, SE Sweden. Acta Universitatis Nicolai Copernici, Geografia, 27 (92), Borówka, R.K., Osadczuk A., Witkowski A., Wawrzyniak-Wydrowska B. & Duda T. (2005). Late Glacial and Holocene depositional history in the eastern part of the Szczecin Lagoon (Great Lagoon) basin NW Poland. Quat. Inter. 130, DOI: /j.quaint Cyberski, J. (1982). Charakterystyka hydrologiczna. In B. Augustowski (Ed.), Dolina Dolnej Wisły (pp ). Gdańsk: Ossolineum. Cyberski, J.(1995). Hydrography of Żuławy Wiślane (Vistula Delta) and its changes over the historical period. In K. Rotnicki (Ed.) Polish Coast: Past, Present and Future (pp )., J. Coastal Res. Spec. Issue 22. Cyberski, J. Grześ M., Gutry Korycka M., Nachlik E. & Kundzewicz Z.W. (2006). History of floods on the River Vistula. Hydrol. Sci. J. 51 (5), Hyvärinen, H. (2000). The history of the Baltic Sea In P. Sandegren (Ed.), Environmental changes in Fennoscandia during the Late Quaternary (pp ). Lund University, Department of Quaternary Geology, LUNNQUA Report 37. Kramarska, R., Uścinowicz S. & Zachowicz J. (1995), Origin and evolution of the Puck Lagoon In K. Rotnicki (Ed.) Polish Coast: Past, Present and Future (pp ). J. Coastal Res. Spec. Issue 22. Kolbe, R.W. (1927). Zur Ökologie, Morphologie, und Systematik der Brackwasser-Diatomeen. Jena: Pflanzenforschung. Majewski, A. (1969). Rozwój hydrograficzny delty Wisły w okresie historycznym. Przegląd Geofizyczny, 14 (1), Mojski, J.E. (1988). Development of the Vistula River delta and evolution of the Baltic Sea, an attempt at chronological correlation. In B. Winterhalter (Ed.), The Baltic Sea (pp ). Geol. Surv. Finland, Spec. Papers, 6. Copyright by Institute of Oceanography, University of Gdańsk, Poland

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18 20 M. Witak, J. Dunder Uścinowicz, S. (2006). A relative sea-level curve for the Polish Southern Baltic Sea. Quat. Intern , DOI: /j.quaint Uścinowicz, S. & Miotk-Szpiganowicz, G. (2003). Holocene shoreline migrations in the Puck Lagoon (Southern Baltic Sea) based on the Rzucewo Headland case study. Landform Analysis 4, Witak, M., Jankowska D. & Piekarek-Jankowska H. (2006). Holocene diatom biostratigraphy of the SW Gulf of Gdańsk, Southern Baltic Sea (part I), Oceanological and Hydrobiological Studies 35(4), Zawadzka-Kahlau, E. (1999). Tendencje rozwojowe polskich brzegów Bałtyku Południowego, Gdańsk: GTN. Copyright by Institute of Oceanography, University of Gdańsk, Poland