Evidence of cultural eutrophication of the Gulf of Gdańsk based on diatom analysis

W9-A Chaetoceros spp. RS – Thalassiosira levanderi DAZ (12–9 cm). The diatom community was rather rare but represented by numerous taxa (81 species and 31 genera). Most frustules were poorly preserved. The poor state of preservation and the high degree of fragmentation of frustules was probably caused by mechanical and chemical destruction. The lower part of the zone was dominated by benthic species (ca. 60%), including mainly eutraphentic (Amphora copulata, Catenula adhaerens, Opephora krumbeinii, Thalassiosira levanderi) and eu-mesotraphentic diatoms (Amphora pediculus, Cocconeis placentula var. placentula, Staurosira binodis, S. construens, S. subsalina, S. venter; Fig. 2). All these taxa belong to the β-mesosaprobous group. However, they are known from waters of either low or high salinity. The increase in the number of planktic taxa was observed in the upper part of the zone (up to ca. 60%), which is associated with a higher content of Chaetoceros spp. RS. They were accompanied by eutraphentic, α-mesosaprobous species Cyclotella atomus, C. choctawhatcheeana and Cyclostephanos dubius The overall frequency of eutraphentic species increased toward the upper part of the zone from 30 to 50%, whereas the frequency of the eu-mesotraphentic group decreased from 25 to 10%. The concentration of valves was generally low and in the range of 59–92 × 10⁶ valves g⁻¹.

W9-B Cyclotella atomus – Cyclotella meneghiniana – Thalassiosira levanderi DAZ (9–4 cm). The diatom flora was quantitatively rich (88 species and 40 genera) and much better preserved than in the lower section containing biogenic detritus. A clear increase in the frequency of diatoms preferring a high content of nutrients and organic matter was observed in the upward direction of the zone. In the planktic group, brackish-water Chaetoceros spp. RS dominated in the bottom part. They were replaced by oligohalobous, eutraphentic, α-mesosaprobous species, i.e. Aulacoseira granulata, Cyclostephanos dubius and Cyclotella atomus. The stable abundance (ca. 10%) of marine Thalassiosira levanderi was observed throughout the zone. Amongst benthic diatoms, Cyclotella meneghiniana representing the eutraphentic group and Staurosira spp. belonging to eu-mesotraphentic, β-mesosaprobous taxa occurred in large numbers in the upper part of the zone. The concentration of valves was higher than in the previous section (121–257 × 10⁶ valves g⁻¹).

W9-C Pauliella taeniata – Thalassiosira levanderi DAZ (4–0 cm). The diatom taphocoenosis was abundant, quite well preserved and represented by 88 species belonging to 36 genera. The most striking feature of the diatom assemblage is a clear increase in the frequency of oligotraphentic and oligosaprobous taxa toward the upper part of the zone. The phenomenon is associated with the replacement of diatoms thriving in eutrophic conditions, i.e. Actinocyclus normanii, Aulacoseira granulata, Cyclostephanos dubius, Cyclotella atomus, C. choctawhatcheeana, Stephanodiscus hantzschii, by taxa well adapted to low concentrations of nutrients, i.e. Pauliella taeniata, which reached a maximum frequency of 40%. The important component of the plankton was also Thalassiosira levanderi (10–20%) belonging to the eutraphentic group. The diatom concentration ranged from 80 in the bottom part to 492 × 10⁶ valves g⁻¹ in the upper part of the zone.

ZP1-A Chaetoceros spp. RS – Thalassiosira levanderi DAZ (27–22 cm). The diatom assemblage of this zone was quantitatively rich (75 species and 32 genera) and generally well preserved. However, some valves were mechanically or/and chemically destroyed. Brackish-water Chaetoceros spp. RS and marine Thalassiosira levanderi were the most important components of the planktic group (65–70%; Fig. 3). They were accompanied by eutraphentic, α-mesosaprobous taxa, i.e. Cyclotella atomus, C. choctawhatcheeana and by oligotraphentic and oligosaprobous species Pauliella taeniata. Benthic diatoms were represented mostly by Amphora copulata, A. pediculus, Fragilaria sopotensis, Opephora krumbeinii, Pseudostaurosira brevistriata and Staurosira venter, which prefer waters rich in nutrients and organic matter. The diatom concentration was relatively high and amounted to 211–313 × 10⁶ valves g⁻¹.

ZP1-B Thalassiosira levanderi – Cyclotella choctawhatcheeana DAZ (22–8 cm). The diatom flora was represented by a species-rich (79 species and 33 genera) and well-preserved assemblage. Chaetoceros spp. RS and Pauliella taeniata were replaced by Cyclotella atomus, C. choctawhatcheeana and Thalassiosira levanderi in the upper part of the zone. In total, the frequency of plankton increased to ca. 80%. Benthic diatoms, mostly eutraphentic and eu-mesotraphentic α- to β-mesosaprobous, were represented by Amphora spp., Cocconeis spp., Fragilaria spp., Opephora spp. and Staurosira spp. Their frequency was higher (3–5%) at the bottom and at the top of the zone. The diatom concentration ranged from 387 to 562 × 10⁶ valves g⁻¹ in the upper part of the zone.

ZP1-C Thalassiosira levanderi – Cyclotella choctawhatcheeana – Opephora krumbeinii DAZ (8–3 cm). The diatom flora was generally well preserved and represented by 81 species belonging to 35 genera. The decrease in the content of the planktic group is correlated with the lower frequency of brackish-water Chaetoceros spp. RS and Cyclotella choctawhatcheeana, as well as marine Thalassiosira levanderi At the same time, there was a significant increase from 5 to 20% in the abundance of the oligotraphentic and oligosaprobous species, Pauliella taeniata. In the benthic group, eutraphentic and eu-mesotraphentic species were observed commonly. Amongst them, Opephora guenter-grassii, O. krumbeinii and Staurosira venter were most frequently observed. The diatom concentration decreased from 458 to 307 × 10⁶ valves g⁻¹.

ZP1-D Pauliella taeniata – Thalassiosira levanderi DAZ (3–0 cm). The diatom community was abundant, well preserved and diverse, comprising 79 species and 28 genera. The planktic group reached its maximum frequency (30%). The most characteristic feature is the gradual increase in the frequency of marine species Pauliella taeniata and Thalassiosira levanderi up to 25%. The benthic group was represented by the same taxa as in the previous zone, but their content was usually lower. The diatom concentration increased from 263 in the lower part to 403 × 10⁶ valves g⁻¹ in the surface layer.

UP10-A Thalassiosira levanderi – Cyclotella choctawhatcheeana DAZ (33–27 cm). The diatom taphocoenosis was abundant, very well preserved, but the floristic diversity was rather low – with 53 species and 25 genera. Planktic forms occurred in large numbers (up to 90%) in the zone (Fig. 4). The main components were marine Thalassiosira levanderi, brackish-water Chaetoceros spp. RS and Cyclotella choctawhatcheeana. Benthic species were observed sporadically. Only Amphora pediculus, Catenula adhaerens, Cyclotella meneghiniana and Fragilaria martyi reached a frequency of 2%. Most diatoms found in the zone belong to the eutraphentic, α- to β-mesosaprobous group. The concentration of valves was very high, i.e. 3323–2564 × 10⁶ valves g⁻¹.

UP10-B Thalassiosira levanderi – Cyclotella choctawhatcheeana – Chaetoceros spp. RS DAZ (27–17 cm). The diatom flora was represented by a well-preserved, species-poor assemblage, in which 25 genera and 45 species were identified. The most striking features of the assemblage are visible changes in the plankton structure. The content of Thalassiosira levanderi clearly decreased in the upward direction of the zone, whereas other planktic taxa, i.e. Cyclotella caspia, C. choctawhatcheeana and Chaetoceros spp. RS, were found more often. Benthic diatoms were still very rare, with the exception of Cyclotella meneghiniana, the content of which exceeded 5%. Noteworthy is the occurrence of the cold-water marine species Fragilariopsis cylindrus in this zone. The diatom concentration decreased from 2053 at the bottom to 1651 × 10⁶ valves g⁻¹ at the top of the section.

UP10-C Thalassiosira levanderi – Cyclotella choctawhatcheeana – Staurosira subsalina DAZ (17–5 cm). The diatom assemblage comprised 86 species representing 34 poorly preserved genera. Planktic species were abundant (up to 90%) and dominated by the marine species Thalassiosira levanderi, the frequency of which increased up to 75% in the upper part of the zone. Chaetoceros spp. RS, Cyclotella caspia and C. choctawhatcheeana showed the opposite tendency. Other planktic species, Actinocyclus normanii var. normanii, Coscinodiscus granii, Pauliella taeniata and Stephanodiscus hantzschii, were observed sporadically. Benthic diatoms occurred more often than in the previous zone. They were represented mainly by Amphora copulata, A. pediculus, Catenula adhaerens, Cocconeis neothumensis, Cyclotella meneghiniana, Staurosira subsalina, which developed well in eutrophic and mesosaprobic conditions. Marine cold-water Fragilariopsis cylindrus was observed in the lower part of the zone. The diatom concentration ranged from 514 to 920 × 10⁶ valves g⁻¹ in the upper part of the zone.

UP10-D Pauliella taeniata – Thalassiosira levanderi DAZ (5–0 cm). The diatom flora of this zone was rather rare and represented by a diverse assemblage comprising 78 species and 30 genera. Diatoms were well preserved, but many frustules were mechanically and/or chemically destroyed. Planktic diatoms dominated and their total frequency reached 80–92%. The main components of this group were marine taxa: Thalassiosira levanderi, Pauliella taeniata and Coscinodiscus granii They were accompanied by taxa that prefer lower salinity, i.e. Actinocyclus normanii var. normanii, Actinocyclus normanii f. subsalsa, Chaetoceros wighamii, Cyclotella choctawhatcheeana, the frequency of which did not exceed 5%. In the benthic group, Amphora spp., Cyclotella meneghiniana, Diploneis didyma, Opephora spp. and Staurosira spp. were most frequently observed. The concentration of valves ranged from 209 to 2328 × 10⁶ valves g⁻¹.

GG-A Chaetoceros spp. RS – Actinocyclus octonarius – Pseudosolenia calcar-avis DAZ (32–15 cm). Diatoms were very rare in the taphocoenosis, but represented by 80 species belonging to 34 genera. They were poorly preserved and many broken valves were observed. The diatom flora was dominated by planktic species, whose total frequency reached 85% (Fig. 5). The main component was Chaetoceros spp. RS (30–40%), accompanied by marine species, including oligotraphentic Actinocyclus octonarius and Pauliella taeniata as well as Coscinodiscus spp., whose trophic preferences are irrelevant. Pseudosolenia calcar-avis was often observed in the lowest part of the zone. Benthic taxa were rare and included freshwater Amphora pediculus, Pseudostaurosira brevistriata, brackish-water Opephora guenter-grassii and marine Catenula adhaerens. Most of them prefer eutrophic β-mesosaprobic conditions. The concentration of valves is very low and ranged from 31 to 69 × 10⁶ valves g⁻¹.

GG-B Chaetoceros spp. RS – Thalassiosira levanderi DAZ (15–3 cm). The diatom taphocoenosis was in a poor state of preservation, but represented by as many as 72 species belonging 30 genera. Half of all diatoms were represented by resting spores of Chaetoceros. The frequency of marine species Thalassiosira levanderi gradually increased toward the upper part of the zone. In addition, Actinocyclus octonarius, Coscinodiscus granii, Cyclotella choctawhatcheeana were abundant in the planktic group. The benthic group was observed very rarely, particularly in the middle and upper part of the zone. The diatom concentration increased from 96 at the bottom to 1136 × 10⁶ valves g⁻¹ at the top of the zone.

GG-C Pauliella taeniata – Chaetoceros spp. RS – Coscinodiscus granii DAZ (3–0 cm). The diatom community was well preserved, abundant and represented by diverse flora (72 species and 28 genera). Planktic taxa accounted for over 90% of the assemblage. Marine species – Coscinodiscus granii, Pauliella taeniata and Thalassiosira levanderi – were the main component. Freshwater species, Aulacoseira spp. and Stephanodiscus hantzschii, were rarely encountered. Oligohalobous halophilous taxa were represented by Actinocyclus normanii f. subsalsa and Cyclotella meneghiniana. Oligotraphentic and oligosaprobous diatoms (Pauliella taeniata, Actinocyclus octonarius) reached the highest frequency in this zone. The diatom concentration was very high and ranged from 1006 to 2268 × 10⁶ valves g⁻¹ in the surface layer.

The first phase is recorded in the deepest sediments of all cores GG-I DAZ, UP10-I DAZ, ZP1-I DAZ and W9-I DAZ. The diatom flora preserved in these sediments was represented by planktic species. In cores W9 and ZP1, benthic forms were common in the bottom part of this phase. The diatom assemblages described in all zones were dominated by eutraphentic and eu-mesotraphentic species. Furthermore, diatoms with high edaphic requirements, such as Actinocyclus normanii var. normanii (α- mesosaprobous), Amphora copulata (α-β-mesosaprobous), Catenula adhaerens (β-mesosaprobous), Cyclotella choctawhatcheeana (α-mesosaprobous), Diploneis didyma (irrelevant), Opephora krumbeinii (β-mesosaprobous), were observed quite often. An important diatom was Cyclotella choctawhatcheeana (α- mesosaprobous). The

eu-mesotraphentic group was dominated by Amphora pediculus (β-mesosaprobous), Pseudostaurosira brevistriata (oligosaprobous) and Thalassiosira baltica (β-mesosaprobous), accompanied by Fragilaria martyi (β-mesosaprobous), Staurosira construens (β-mesosaprobous), S. subsalina (β-mesosaprobous), and S. venter (β-mesosaprobous). Resting spores of Chaetoceros were very common. According to Hajdu and Larsson (1990), Chaetoceros spp. RS dominate in the spring bloom after a cold winter in the Baltic Sea. However, ecological (e.g. trophic and saprobic) preferences of the taxon, are still unknown.

The main features of the diatom taphocoenoses preserved in the lower parts of the cores indicate the human impact. The high frequency of diatoms with high edaphic requirements observed in phase A can be related to urbanization and increased use of fertilizers in agriculture (Kowalkowski et al. 2012; Pastuszak et al. 2012) and industrialization of the catchment area. On the other hand, the eutraphentic species occurred in the Gdańsk Deep (core GG) with the lowest abundance. The diatom flora in this region was represented mainly by oligotraphentic taxa, which prefer clean waters and a low content of nutrients in pelagic zone, i.e. Actinocyclus octonarius and Pauliella taeniata in the water column and Opephora guenter-grassii in the benthic group. This shift indicates that changes in the diatom assemblages are directly associated with the distance from the Vistula estuary, which is the main source of biogenic substances in the studied area. The relation between the trophic and saprobic groups indicates that waters in the first phase were rich in biogenic compounds and organic matter. The concentration of diatom valves was generally the lowest and ranged from 31 to 69 × 10⁶ valves g⁻¹ with one maximum of 3323 × 10⁶ valves g⁻¹ in UP10-A. The low concentration in the deepest part of the cores could result from the dissolution of valves in the water column and mechanical destruction.

The second phase of environmental changes in the study area is described on the basis of the diatom flora of GG-II DAZ, UP10-II DAZ, UP10-III DAZ, ZP1-II DAZ, ZP1-III DAZ and W9-II DAZ. In phase B, significant changes in ecological groups, i.e. the trophic and saprobic ones, were observed. A very strong human impact on the species composition is documented in the planktic group with mainly small-sized forms. This fact can be associated with the decreasing depth of the euphotic zone, which is a result of the eutrophication process (Andrén et al. 1999; Witak 2010). The significant increase in the abundance of eutraphentic, eu-mesotraphentic and α- mesosaprobous taxa was observed in all diatom assemblages.

One of the most important species, especially in DAZ W9-II, was Cyclotella meneghiniana, considered to be one of the most typical indicators of anthropogenic assemblages (Witkowski 1994; Witkowski & Pempkowiak 1995; Lesniewska & Witak 2008; Witak 2010; Witak & Pedzinski 2018). The species prefers eutrophic and hypertrophic conditions and tolerates polluted waters (van Dam et al. 1994; Stachura-Suchoples 2001). It was accompanied by Cyclotella atomus and C. choctawhatcheeana. Both species are eutraphentic/α-mesosaprobous and are abundant in waters with higher concentrations of phosphorous and inorganic nitrogen, moderately polluted, containing decaying organic matter (Stachura-Suchoples 2001; Bąk 2004). The small-celled cosmopolitan species, C. choctawhatcheeana, was confirmed to be one of the most tolerant diatoms in the study area (Witkowski 1994; Stachura-Suchoples 1999; 2001). Cyclotella atomus was abundant in core W9 collected to the north of the Vistula mouth. This taxon often occurs in bays and near estuaries, where the content of biogenic substances and organic matter is high in the water column.

The highest abundance of Thalassiosira levanderi, a very common marine planktic species in the surface sediments, is a very important observation. This taxon was considered tolerant of increased concentrations of phosphorus and nitrogen and is the main component of the anthropogenic assemblage in the Gulf of Gdańsk (Witkowski 1994; Witkowski & Pempkowiak 1995; Witak 2000). Opephora krumbeinii, a small-sized species, was frequently observed in the benthic group, particularly in the upper part of ZP1-C DAZ. Opephora krumbeinii is known as a eutraphentic, β-mesosaprobic form. Eu-mesotraphentic species were also found in the oligohalobous group, i.e. Pseudostaurosira brevistriata (oligosaprobous), Staurosira subsalina (β-mesosaprobous), S. venter (β-mesosaprobous). The highest frequency of these species was recorded in the vicinity of the Vistula mouth (core W9) and in the Outer Puck Bay (core ZP1). In this phase, resting spores of Chaetoceros spp. were most abundant. According to Grimm & Gill (1994), the occurrence of Chaetoceros spp. RS is associated with nutrient depletion toward the termination of a phytoplankton bloom. The abundance of eutraphentic/eu-mesotraphentic and pollution-tolerant taxa indicates a strong human impact and the most intensive cultural eutrophication in this phase. This is related to increased amounts of biogenic substances supplied by rivers, mainly the Vistula, since the 1980s. Excessive loads of biogenic salts come from the use of fertilizers in agriculture, urbanization, soil erosion, domestic, animal livestock and industrial sewage disposal leading to the degradation of water quality. As a consequence, nutrient enrichment leads to increased primary production and reduced light penetration, oxygen depletion, changes in biochemical cycles and biological structures, including the loss of biodiversity. The diatom valve concentration was generally higher compared to the previous interval with an amount of 96–2053 × 10⁶ valves g⁻¹. This indicates that the diatoms were generally well preserved. The high concentration of diatoms may also be related to the high content of available biogenic substances. However, valves were sometimes mechanically and/or chemically destroyed, making it difficult to count them.

The last phase C is recorded in GG-III DAZ, UP10-IV DAZ, ZP1-IV DAZ and W9-III DAZ. The most characteristic feature of these taphocoenoses is the abundance of marine forms, i.e. Thalasiossira levanderi, Opephora krumbeinii, which belong to the eutraphentic group. On the other hand, the highest frequency of Pauliella taeniata, Actinocyclus octonarius and Opephora guenter-grassii was observed. These species occurred in waters with low concentrations of phosphorous and inorganic nitrogen and developed in slightly polluted waters (Stachura-Suchoples 1999;

The frequency of the cosmopolitan neritic planktic species, i.e. Actinocyclus octonarius (oligotraphentic, β-mesosaprobous), was high in GG-III DAZ. Coscinodiscus granii was often found and proved to be the dominant diatom blooming in early autumn, while Pauliella taeniata, Thalassiosira levanderi and Chaetoceros spp. RS dominate in the spring bloom after cold winters with extensive ice cover (Hasle & Syversten et al. 1997; Andrén et al. 2000). The most important component of this phase is Pauliella taeniata, which dominates in the spring bloom (Hajdu & Larson 1990).

Irrespective of the location, Pauliella taeniata was most frequently observed in the uppermost part of all cores. Perhaps the highest frequency of this species observed only in the upper parts of the cores results from the very low preservation potential (Denys 1991). Pauliella taeniata has a delicate, thin frustule and rarely occurs in the fossil state (Witak 2010). Such a high content of this taxon in the superficial sediments may be related to ice cover. It is an arctic species associated with the sea ice (Haecky et al. 1998) and an indicator of ice-cover conditions in the Baltic Sea. According to Höglander et al. (2004), this species occurs with a lower frequency in the spring bloom when the ice cover is limited. During harsh winters, characterized by the presence of thick ice cover, a high frequency of Pauliella taeniata was observed. These changes may have been induced by the shift in the NAO (North Atlantic Oscillation) index from a negative to positive phase (Tuovinen et al. 2010).

Another explanation of the abundance of Pauliella taeniata in the uppermost part of all cores may be related to the trophic conditions in the Gulf of Gdańsk. Undoubtedly, the changes in the trophic status in the Gulf of Gdańsk were caused by the use of fertilizers in agriculture and progressing urbanization of the catchment. Differences in the trophic status may result from the reduced use of fertilizers in the 1990s.

As a result, the gradual improvement in the water quality in the Gulf of Gdańsk has been observed. Therefore, the higher frequency of Pauliella taeniata could be related to these environmental changes. This hypothesis is confirmed by research conducted by Mazur-Marzec et al. (2012). The results show that symptoms of eutrophication are regressing. Recent studies have shown lower frequency and lower intensity of cyanobacterial blooms. This shift may be an indication of ongoing changes induced by the reduced nutrient loading and/or climate change. The diatom valve concentration was similar to the previous phase with a range of 80–2268 × 10⁶ valves g⁻¹. This demonstrates good conditions during the fossilization process, which is due to the relatively short exposure of the surface and shallow subsurface (0–4 cm) sediments to the chemical dissolution and mechanical destruction.