Biodiversity of benthic diatom flora in the coastal zone of Puck Bay (southern Baltic Sea): a case study of the Hel Peninsula

The epilithic assemblage of the Hel Peninsula was dominated by diatoms preferring higher concentrations of nutrients and organic matter (Fig. 2). The frequency of the eutraphentic group usually exceeded 60%, with Nitzschia frustulum (Kützing) Grunow being the most important component (Fig. 3). This β-mesosaprobiont occurred abundantly at one of the sites in Chałupy (HP13/1014, ca. 80%), while was very rare in the town of Hel (HP3/1014, ca. 2%). At the latter site, Diatoma moniliformis (Kützing) D.M. Williams was regularly observed. Both species are known as oligohalobous halophilous, with the former being a β-mesosaprobiont and the latter being a β-meso-oligosaprobiont. They were accompanied by the marine species Opephora krumbeinii Witkowski, Witak & Stachura and the brackish-water species Gedaniella mutabilis (Grunow) Li & Witkowski. Less frequent were α-mesosaprobionts: Gedaniella flavovirens (Takano) Li, Witkowski & Ashworth, Halamphora coffeaeformis (C. Agardh) Levkov, Rhoicosphaenia abbreviata (C. Agardh) Lange-Bertalot, and Planothidium delicatulum (Kützing) Round & Bukhtiyarova representing β-mesosaprobionts. The eu-mesotraphentic group occurred very rarely and was usually represented by Cocconeis placentula var. placentula Ehrenberg occurring at only one site in Jastarnia (HP9/1014). In the eastern part of the Hel Peninsula, at the Hel sites (HP2–3/1014) and in Jurata (HP5/1014), mesotraphentic diatoms were observed frequently. At these sites, Navicula perminuta Grunow in Van Heurck, considered to be mesohalobous, tolerant of higher concentrations of organic matter and pollution, occurred regularly. Denticula creticola – a species preferring higher salinity – was observed with high frequency in the town of Hel (HP2/1014).

Figure 2

Figure 3

The results of the quantitative analysis indicate considerable differences in the concentration of diatom valves in sandy sediments (Fig. 2). The lowest concentration was observed at the Hel Tip. At sites HP1–4/1014, the values ranged from 1.2 to 4.2 × 10⁶ valves g⁻¹. In the vicinity of Jurata (HP6/1014), the concentration of valves increased to 28 × 10⁶ valves g⁻¹. The value decreased from 18 to 10 × 10⁶ valves g⁻¹ at the Jastarnia sites (HP7–8/1014). Along the coast of the Puck Lagoon, diatoms occurred mostly with higher abundance in sediments. The highest concentrations were observed at two sites in Kuznica, i.e. HP10/1014 (42 × 10⁶ valves g⁻¹) and HP13/1014 (60 x 10⁶ valves/g).

In the western part of the study area, the concentrations were lower and reached 36 × 10⁶ valves g⁻¹ at HP16/1014 and 19 × 10⁶ valves g⁻¹ at HP15/1014.

The results of the taxonomic analysis show that the epipsammon of the Hel Peninsula was characterized by abundant occurrence of eutraphentic taxa and i-mesosaprobionts (Fig. 2). In sandy sediments, between Chałupy and Jurata (HP5–16/1014), Nitzschia frustulum, Opephora krumbeinii, Gedaniella mutabilis and Planothidium delicatulum dominated (Fig. 3). Other diatoms, i.e. Catenula adhaerens Mereschkovsky, Gedaniella flavovirens, Rhoicosphaenia abbreviata, Tabularia fasciculata (C. Agardh) D.M. Williams & Round were observed sporadically. However, the assemblage at the Hel Tip (HP1–4/1014) was represented by α-mesosaprobionts – Halamphora coffeaeformis and Navicula gregaria Donkin, and the β-mesosaprobiont Planothidium engelbrechtii (Cholnoky) Round & Bukhtiyarova. The latter species was also found at one site near Chałupy (HP13/1014). In the region of the Hel Tip (HP1–4/1014), the mesohalobous species Navicula germanopolonica Witkowski & Lange-Bertalot, tolerant of higher concentrations of biogenic salts, was observed regularly. In the inner coast of the Hel Peninsula, two species with indifferent trophic and saprobic preferences, i.e. Fallacia clepsidroides and Navicula paul-schulzii were found. The former species was recorded with a higher frequency in Jastarnia (HP8/1014), whereas the latter – near the town of Hel (HP1-2/1014).

In general, epiphytic diatoms recorded in the study area belong mainly to eutraphentic forms and i-mesosaprobionts (Fig. 4). The main components of the diatom flora occurring on seagrasses of the eastern part of the Hel Peninsula (HP1–4/1014) were Rhoicosphaenia abbreviata, Cocconeis pediculus Ehrenberg, C. placentula var. euglypta (Ehrenberg) Grunow, C. scutellum var. scutellum Ehrenberg and Diatoma moniliformis (Fig. 5). These species show different salinity preferences, however, they developed well in moderately/slightly polluted waters with decaying organic matter. At site HP1/1014, the -i-mesosaprobiont Tabularia fasciculata occurred with abundance of ca. 40%. All these taxa occur on macroalgae, but mostly with a relatively low frequency, except for Rhoicosphaenia abbreviata, the content of which exceeded ca. 30%. Along the coast between Jurata and Kuznica (HP5–9/1014) as well as in the vicinity of Chałupy (HP15–16/1014), Nitzschia frustulum, Opephora krumbeinii, Gedaniella mutabilis and Planothidium delicatulum were observed on seagrasses with a relatively high frequency. In addition, in the Chałupy area, the diatom flora of seagrasses was represented by Diatoma moniliformis, Gedaniella flavovirens and Tabularia fasciculata. They were also observed on macroalgae in this part of the Hel Peninsula. In the diatom community recorded on seagrasses in the regions of Kuznica and Chałupy (HP10–16/1014), C. placentula var. euglypta and Rhoicosphaenia abbreviata were abundant. The content of Diatoma moniliformis was high only at HP10/1014, where it exceeded ca. 35%. The eu-mesotraphentic species Gomphonema olivaceum (Hornemann) Ehrenberg and Staurosira venter (Ehrenberg) Cleve & Möller were recorded at almost all sites. The polysaprobiont Navicula perminuta was abundant on macroalgae collected at the town of Hel (HP2,4/1014). Moreover, the brackish-water species Gomphonemopsis obscura was observed on seagrasses, particularly abundantly at Hel (HP2,4/1014), Jastarnia (HP7/1014) and Kuznica (HP12/1014), with a frequency exceeding 40%. Between Jurata and Chałupy (HP7–16/1014), the marine form of Mastogloia pumila and the halophilous form of M. smithii were also observed. C. placentula var. placentula and Ctenophora pulchella (Ralfs ex Kützing) D.M. Williams & Round were rare in the epiphyton of the Chałupy region.

Figure 4

Figure 5

The comparative analysis of diatom dominants and subdominants observed in the epilithon, epipsammon, and epiphyton enabled us to distinguish three major groups. Despite significant differences in environmental parameters between the Puck Lagoon and the Outer Puck Bay, such as hydrodynamic conditions, bathymetry and salinity, there is a group of diatom taxa observed on all types of substrates, i.e. stones, sands, seagrasses and macroalgae (Table 4). However, there are also several species observed on only one type of substrate and those found on two types of substrates.

One substrate. The first group includes diatom taxa recorded on only one type of substrate. This group included two species in the epilithon, Denticula creticola at site HP2/1014 at the town of Hel and Diatoma tenuis at HP14/1014 in Chałupy. The former species is known as an epilithic form (Snoeijs & Vilbaste 1994) and the latter one is described as an epiphytic form (Snoeijs 1993). Diatoms typical for the epipsammon were represented by Catenula adhaerens, Navicula germanopolonica and Planothidium engelbrechtii. Their habitat preferences correspond to the observations in other parts of the Baltic Sea (Snoeijs 1993; Snoeijs & Potapova 1995). The former species was more frequently observed in Kuznica and Chałupy, while the remaining species mentioned above were observed in the vicinity of Hel. In the epiphyton developing on seagrasses, the most typical were Cocconeis pediculus, C. scutellum var. scutellum, Gomphonema olivaceum, Gomphonemopsis obscura, Mastogloia pumila and M. smithii. All the above-mentioned taxa are known as epiphytic diatoms (Snoeijs 1993; Snoeijs & Potapova 1995). However, G. olivaceum, M. pumila and M. smithii were also recorded in the epilithon of the Baltic Sea (Snoeijs 1993). Cocconeis spp. were recorded in large numbers at the town of Hel. G. olivaceum occurred frequently in Kuznica, while Mastogloia spp. on the coast between Jastarnia and Chałupy. Noteworthy is the occurrence of the brackish-water species G. obscura on seagrasses at the same sites i.e. in vicinity of Hel, Jastarnia, Kuznica and Chałupy. The clear decrease in its frequency from SE to NW can be related to the distance from the open sea waters.

Two substrates. The second group consisted of diatoms observed on two types of substrates, i.e. sands and stones. It is represented by Fallacia clepsidroides and Navicula paul-schulzii. They were more frequently recorded on sands than on stones. However, the former species was observed only in the epipelon of the Baltic Sea (Witkowski 1994; Snoeijs & Potapova 1995). Ctenophora pulchella known from the Baltic epiphyton (Snoeijs 1993) and Staurosira venter considered an epipsammic species (Snoeijs & Balashova 1998) were observed on seagrasses and macroalgae at many sites of the Hel Peninsula, with a higher frequency near Chałupy.

All types of substrates. The third group includes species occurring on both types of sediments (stones and sands) and macroflora (seagrasses and macroalgae). It is represented by Gedaniella flavovirens, Halamphora coffeaeformis and Nitzschia frustulum. The former species was observed in the epilithon and epiphyton of the Baltic Sea (Snoeijs & Potapova 1995), whereas the latter only in the epilithon (Snoeijs 1993). They were observed along the whole coastline of the Hel Peninsula. Nitzschia frustulum played the key role on all types of habitats, especially in the epilithon and epipsammon. This group also includes Rhoicosphaenia abbreviata and Tabularia fasciculata, but in general their frequency in the epiphyton is much higher than in the epilithon and epipsammon. Noteworthy is their mass occurrence on seagrasses collected at the Hel Tip. The epiphytic diatoms Cocconeis placentula var. euglypta and Diatoma moniliformis (Snoeijs 1993), found on macroflora at almost all sites, were also collected on both types of sediments. The latter species is one of the main species on the macroflora in the Hel region. It is interesting that Cocconeis placentula var. placentula, referred to as an epiphytic species, was regularly observed in the epipsammon, but was very rarely found on seagrasses and macroalgae. However, this species occurred abundantly in the epilithon at site HP9/1014 located in Jastarnia. Between Jurata and Chałupy, this group comprised Opephora krumbeinii, Gedaniella mutabilis and Planothidium delicatulum. Although these species are considered to be epipsammic and epilithic forms (Snoeijs 1993; Witkowski 1994; Witak 2002; 2013), they are also known from other habitats along the whole coastline of the Hel Peninsula, except for the area in the vicinity of Hel. They were particularly common in the regions of Jurata and Chałupy. Another species associated with the sandy bottom, Gedaniella guenter-grassii, was also observed in the epilithon and epiphyton. Navicula perminuta was observed on all habitats at almost all sites. It was regularly observed in the epilithon, but it reached the maximum frequency on macroalgae in the region of Hel.

Hel Tip region. The direct impact of saline waters leads to the abundant occurrence of diatoms preferring higher salinity with occasional occurrence of freshwater forms in the vicinity of the town of Hel. Despite the major impact of open sea waters, the benthic diatom flora found on all types of substrates clearly indicates that this is the most polluted region in the study area. This is evidenced by the high frequency of the polysaprobiont species Navicula perminuta (Witak 2010; Majewska et al. 2012) in diatom communities developed on macroalgae and stones. Moreover, -mesosaprobionts were observed in large numbers. Even if Gomphonemopsis obscura and Rhoicosphaenia abbreviata belonging to this group observed in epiphyton were transported with seagrasses by hydrodynamic factors, other species, i.e. Halamphora coffeaeformis and Navicula gregaria known as epipsammon, are an autochthonous component of the diatom flora. The higher content of pollutants in this part of Puck Bay is also evidenced by the maximum frequency of species that are -i-mesosaprobionts – Cocconeis pediculus and Tabularia fasciculata. In addition, the eutraphentic species Planothidium engelbrechtii was observed in the sand of this region. These taxa were recorded in the superficial sediments of the Outer Puck Bay (Witak 2000; 2010; Witak et al. 2006; Witak & Dunder 2007). The dominance of diatoms that tolerate high concentrations of nutrients and pollutants indicates intensive eutrophication and saprobication of water in the vicinity of Hel. This phenomenon is caused by a number of anthropogenic factors, including the development of shipping, the functioning of a fishing port as well as a yacht and military port and advanced infrastructure. Due to increased tourist traffic in the 21st century, the inflow of municipal sewage to Puck Bay has also significantly increased (Andrulewicz & Witek 2002; Kruk-Dowgiałło & Szaniawska 2008; Warzocha et al. 2018). These factors caused the accumulation of biogenic methane in subsurface sediments and, consequently, chemical changes in the water column (Reindl & Bolałek 2012), dissolution of siliceous diatom valves and their resuspension. These phenomena could cause a very low concentration of diatoms in sandy sediments. Another reason could be the high hydrodynamic conditions, which cause mechanical damage to diatom frustules (Nowacki et al. 2009; Dybowski et al. 2019).

Jurata–Jastarnia region. In the coastal section between Jurata and Jastarnia, the abundance of diatoms with very high edaphic requirements indicates the presence of eutrophic waters. In Jurata, the frequency of eutraphentic forms reached 90% in the epilithon and epipsammon, which can be related to tourism and developed hotel infrastructure. However, the distribution of saprobic groups clearly indicates that the environment status in the Jurata area is more favorable than in Jastarnia. A good indicator of lower pollution in the coastal zone in Jurata is the higher content of β-mesosaprobionts associated with increased frequency of euhalobous (Opephora krumbeinii), mesohalobous (Gedaniella mutabilis, Planothidium delicatulum) and oligohalobous halophilous (Nitzschia frustulum) taxa. All mentioned taxa were observed in the Outer Puck Bay (Witak 2000; 2010; Witak et al. 2006; Witak & Dunder 2007; Li et al. 2018, Pedzinski & Witak 2019). The lower content of contaminants is likely due to the closure of a large part of the coastline between Hel and Jurata. It is also worth noting that the concentration of epipsammic diatoms is higher compared to the Hel community, which is associated with weaker hydrodynamic conditions. Slightly worse water status in Jastarnia, evidenced by the decrease in the content of β-mesosaprobionts combined with the increase of α-mesosaprobionts in the epilithon and epipsammon, may be connected with the presence of the fishing port.

Kuznica region. Compared to the coastal zone of the Outer Puck Bay, a decrease in the content of marine and brackish-water species in epilithon and epipsammon was observed in the Kuznica area. They were replaced by oligohalobous halophilous (Nitzschia frustulum) and indifferent (Cocconeis placentula + varr.) taxa. This is related to a distance from the open sea waters and the presence of the sandy barrier of the Seagull Reef, which hinders the inflow of more saline waters to the Puck Lagoon (Krzyminski et al. 2004; Kruk-Dowgiałło & Szaniawska 2008; Robakiewicz 2014). The higher content of diatoms that are α-mesosaprobionts (Gedaniella flavovirens, Halamphora coffeaeformis, Navicula gregaria) accompanied by polysaprobionts (Navicula perminuta) may be caused by the functioning of the fishing port. In addition, the waterway on the Kuznica Passage was modernized and a new harbor infrastructure was built in 2012 (Stelmaszyk-Swierczynska & Małkiewicz 2014).

Chałupy region. In the NW part of the Puck Lagoon, the freshwater species Staurosira venter known from the seagrass and macroalgae may be an autochthonous element in the diatom community. However, despite the location of this area at a fairly large distance from the open sea waters, a relatively high content of marine diatoms (Catenula adhaerens, Halamphora coffeaeformis, Opephora krumbeinii) and brackish-water forms (Gedaniella guenter-grassii, G. mutabilis, Planothidium delicatulum) was recorded. These diatom taxa were observed in different parts of the Puck Lagoon (Witak 2002). Some of these taxa [i.e. H. coffeaeformis named as Amphora coffeaeformis (C.Agardh) Kützing, G. mutabilis named as Opephora olsenii M.Møller, P. delicatulum named as Achnanthes delicatula (Kützing) Grunow] were observed in superficial sediments in the coastal zone of the Puck Lagoon (Witkowski 1990). It is very likely that these species constitute an allochthonous component of the diatom flora. Their occurrence can be explained by successive artificial expansion of the beach by the camping owners due to intensive development of tourism and water sports (windsurfing, kitesurfing) in the Chałupy region. For this reason, also the higher content of eutraphentic -mesosaprobionts is observed. Apart from local municipal sewage, the sewage treatment plant in Swarzewo may be another source of pollution in this area (Rönnbreg & Bonsdorff 2004; Dziengo-Czaja et al. 2008; Obarska-Pempkowiak et al. 2015). At some sites with reduced anthropopressure, i.e. HP13/1014 and HP16/1014, the concentration of diatom valves in epipsammon is much higher than in the camping area.