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Article

The Relationship between Total Mercury, Its Fractions and Species Diversity of Diatom Taphocoenoses Deposited in Surface Sediments (Southern Baltic Sea)

Institute of Oceanography, University of Gdańsk, Piłsudskiego 46, 81-378 Gdynia, Poland
*
Author to whom correspondence should be addressed.
Water 2023, 15(22), 3907; https://doi.org/10.3390/w15223907
Submission received: 12 October 2023 / Revised: 3 November 2023 / Accepted: 7 November 2023 / Published: 9 November 2023

Abstract

:
Mercury is a toxic metal that in excessive amounts negatively affects the human nervous system. It can biomagnify to enrich successive levels of the trophic web with higher and higher concentrations of Hg. Diatoms are cosmopolitan microalgae that represent the base of the trophic web. The objective of this study was to determine the importance of diatoms in the accumulation of Hg in surface sediments in the example of Puck Lagoon (southern Baltic Sea). The study verified the relationship of mercury forms with diatoms’ characteristic salinity groups and individual diatom species. The identified diatom flora permitted the distinguishment of three regions with different salinity preferences. The obtained results suggest that, together with organic matter and fine sediment fraction, diatoms are an important element of the marine environment in accumulating Hg. Area I, dominated by euhalobous and mesohalobous diatom species, was characterized by a high statistically significant correlation of mesohalobous with HgSO4, Hg in complexes with organic matter and HgS. In Area II, associated with the highest Hg concentrations (ca. 100 ng g−1), euhalobous showed a statistically significant linear correlation with HgSO4. In contrast, in Area III, the freshwater diatom community was associated with stable Hg forms. The identified species showed a connection with labile forms of Hg that can undergo biomagnification.

1. Introduction

Mercury is one of the most toxic metals negatively affecting the human nervous system. It may even contribute to Parkinson’s disease and Alzheimer’s disease [1]. Human activity has been recently showing the greatest impact on the increase in the concentration of mercury in the environment. In Europe and Asia, anthropogenic sources of Hg are dominated by fossil fuel [2]. Hg, along with sewage, rivers and terrestrial runoff, enters the coastal zone of the seas. There, it is incorporated into the trophic web where it undergoes biomagnification in subsequent levels, reaching concentrations up to a million times higher in marine mammals compared to the surrounding water [3,4,5]. The main source of Hg into the human body is the consumption of fish and seafood. An important vector incorporating Hg into the trophic web is phytoplankton. Due to their high abundance and frequency of occurrence, diatoms play a major role. Mercury is easily accumulated by diatoms, which create the entry point for the food web [6,7,8]. Diatoms are one of the most numerous groups of microorganisms dominant in terms of biomass abundance [9]. It is estimated that diatoms produce about 25% of the total carbon bound on Earth [10]. Data presented by Legrand et al. [11] and Staniszewska et al. [12] show that in the Baltic Sea, planktonic diatoms can account for up to >90% of the biomass during the bloom period (spring, autumn). Previous studies show that diatoms accumulate metals intensively, including Hg [13,14]. Therefore, diatom flora may be an important determinant of the mercury concentration in estuarine sediments and at the same time, a major vector allowing mercury into the food web.
Research on mercury (particularly forms of mercury) accumulated by diatoms in the sediments is relatively new. Previous studies have observed the relationship between changes in total mercury (Hgtot) concentration in sediments and the number of diatom valves [15]. In addition, the possible role of diatoms in Hg concentration in sediments has so far been postulated based on biogenic Si content [16]. Research conducted in Antarctica showed that mercury can be stored in diatom flora, becoming the main sink for Hg deposited in sediments [16]. Moreover, previous studies on individual diatom species have confirmed the persistence of diatoms under high concentrations of heavy metals (including mercury) under laboratory conditions and their ability to convert mercury to forms harmless to diatoms [17,18,19,20]. Hg accumulation by diatoms can be controlled by other environmental factors [13,21]. Therefore, it is important to investigate the accumulation of Hg by diatoms in diverse marine environments.
A valuable test site for identifying environmental processes is Puck Lagoon, which is a basin with different hydrological conditions [22]. It is an area exposed to high concentrations of organic substances (e.g., pesticides) and metals, including mercury [23]. Characteristic of Puck Bay is the occurrence of the beach wracks, which provide storage for, among other things, Hg [24]. This creates a secondary source of Hg to the coastal water. Since 1990, a gradual decrease in the emission and deposition of Hg has been observed [25]. Consequently, a decrease in mercury concentrations in Baltic Sea surface sediments has been recorded over the years [26]. Nevertheless, the Puck Lagoon area is still exposed to Hg supply through river inflow, atmospheric deposition and cliff erosion [27,28]. Considering the fact that diatoms constitute one of the most abundant groups of microorganisms in Puck Lagoon and the fact that microorganisms efficiently accumulate Hg, it is possible that they may accumulate a considerable mass of mercury, potentially undergoing biomagnification. Conducted research indicates that Hg concentrations of epilithic diatoms reached lower values compared to the epiphytic and planktic assemblage but reached much higher values than mercury contained in sediments and macroalgae [6].
The concentration of mercury (Hg) as well as other metals in sediments is related to the presence of organic matter (OM) and the fine sediment fraction (FSF) [29]. The shallow water part of Puck Bay on the Polish coast of the Baltic Sea was characterized by benthic diatom biomass ranging from 50% (spring) to 59% (autumn) [30]. Therefore, the objective of this study was to determine the role of diatoms next to OM and FSF in Hg accumulation in Baltic sediments at the base of a lagoon in the southern Baltic Sea (Puck Lagoon), as is the case in the Arctic region. The scientific novelty will be the estimation of the impact of discrete salinity groups (euhalobous, mesohalobous, oligohalobous halophilous and oligohalobous indifferent) and individual diatom species on the accumulation of Hg and its forms in the sediments. The analyses were extended to include studies on five fractions of different forms of mercury. The studies were carried out on the sediments of Puck Lagoon, however, the processes discussed could take place in other analogous embayments of the world. Due to the risk of contamination, mercury was analysed from bulk sediments, and not in the diatom. Diatoms are microalgae ranging in the size of 1.5–5000 μm [31]. In contrast, in the present study, diatoms ranged in size from 4 to 150 μm. Therefore, it is extremely difficult to isolate them from among FSF and OM under the conditions and quantities required for trace analyses. Statistical analyses were used to verify the hypothesis of the study.

2. Materials and Methods

2.1. Study Area

Puck Lagoon is the north-western part of Puck Bay, belonging to the Gulf of Gdańsk. It covers an area of 104.8 km2 (approximately 40% of Puck Bay) [32]. It is confined to the Hel Peninsula (from Kuźnica to Władysławowo) and by land from Władysławowo to Rewa (Figure 1). Puck Lagoon is a shallow water body with an average depth of 3 m. Greater depths are found in the Kuźnica Deep (max. depth 9.7 m), the Rzucewo Deep (max. depth 5.7 m) and the Chałupy Deep (max. depth 4 m) [32]. The hydrological conditions of Puck Lagoon are influenced by rivers (Reda, Płutnica and Gizdepka) flowing into it [33]. The most important one is the Reda River, introducing approximately 0.187 km3 of water into Puck Lagoon per year [34] and accounting for approximately 66% of river inflow to the lagoon [35]. Furthermore, the Seagull Sandbar between Puck Lagoon and the outer Puck Bay limits the water exchange occurring in the northern part through the Kuźnica Passage and in the southern part through Głębinka Passage [36].
Hydrodynamic conditions in the inner Puck Bay (Puck Lagoon) are shaped by water inflow from the outer Puck Bay and wave action [37]. At the bottom of Puck Lagoon, weak bottom currents (10–20 cm s−1) are observed [38]. The main factors generating currents in Puck Bay are the wind and sea level changes associated with the wind [39]. The direction of currents in the inner Puck Bay is parallel to the shoreline, shoals and basins present in the area [39]. Two clockwise circulation systems can be distinguished in Puck Lagoon. The first one in the area of Puck Basin (from Głębinka Passage through Puck to Swarzewo City), while the second one in the area of Kuźnica Basin [39].
The water temperature in Puck Lagoon from June to October in the shallow water zone is 17.1–17.5 °C and drops to 15.1–15.5 °C in the area of Jama Kuźnicka and near Głębinka [32]. However, from December to April the temperature ranges from ca. 3.12 °C in the area of the coastal zone of Władysławowo and Hel Peninsula to 2.88 °C in the area of Głębinka and Kuźnickie Passage [40]. The salinity in Puck Lagoon (annual mean 7.31) is influenced by limited water exchange with the outer Puck Bay (annual mean 7.65) and the inflow of fresh river water [32].
The distribution of sediments in Puck Bay is related to the depth and changes in bottom relief [41]. For further detail on the sediment grain size in the inner Puck Bay, see the Supplementary Material Figure S1. In the whole area of Puck Lagoon, sediment mixing occurs. According to the study by Graca et al. [42], intensive mixing in Puck Bay is likely to occur down to a depth of approximately 80 m. The majority of the bottom of Puck Lagoon is covered by fine-grained sands [41]. However, the coastal zone of Puck Lagoon is associated with the occurrence of coarse-grained sands admixed with gravels. Medium-grained sands are observed up to the 1 m isobath [41]. Deeper areas (Rzucewo Deep, Chałupy Deep and Kuźnica Deep) and near the mouth of the Reda River are covered by silty sand [41]. In addition, sandy silt is present in the deepest part of the Kuźnica Deep [41]. Sources of the material deposited in Puck Bay include fluvial inflow, storm overflows or eolian processes [41].
The rate of deposition in Puck Lagoon is variable. In Kuznica and Rzucewo Deep, changes in the rate of deposition are observed seasonally (autumn–winter about 2.5 mm year−1; spring–autumn about 5 mm year−1) [43]. In addition, there are spatial variations in the rate of accumulation. Near the mouth of the Reda River, the accumulation rate can be as high as 4.5 mm year−1 [44]. A similar accumulation bluntness is observed in Władysławowo pit. In Głębinka Passage, on the other hand, it is approximately 10 mm year−1 [43].
Puck Lagoon is a relatively shallow body of water (mean 3 m of depth) with a strongly restricted exchange with the open waters from the Gulf of Gdansk. This makes the dynamics of the processes like resuspension, remobilisation and transport limited. Allochthonous as well as autochthonous diatom species can occur throughout the study area. They may originate from rivers or the outer part of the Puck Bay, and epiphyton, in addition to plankton, may be found in a shallower area and benthos in a deeper area. The water exchange between Puck Lagoon and the outer Puck Bay is poor. Furthermore, material was not collected from the estuary itself. Therefore, the authors felt that they could exclude the influence of these factors on the diatom flora present and its origin.

2.2. Sample Collection

Samples for analysis were collected in August and September 2019 using a pontoon on Puck Lagoon (inner Puck Bay), the shallowest part of the Gulf of Gdańsk, southern Baltic Sea. During sediment collection, good atmospheric conditions prevailed. The salinity of the water was 7.1–7.2 and the water temperature ranged from 14 °C to 21.3 °C. Further details on the atmospheric conditions prevailing at the time of sampling and sampling depth are presented in Supplementary Material Table S1.
In the AREA I and AREA III shoreline area, sampling was dependent on access to the sea. Therefore, the distribution of points in this area is irregular. However, areas influenced by human activity, undisturbed areas and areas close to river mouths were selected. The distance of the measurement points in the shoreline zone is 0.3–4 km. In deeper areas of AREA III, which were easily accessible for sampling by pontoon, the distance between locations ranged from 0.5–2 km. Such a dense grid of measurements in AREA III was planned due to a large number of potential factors influencing this area (river water inflow, human activities and the influence of water inflow from the outer Puck Bay through Głębinka Passage). In AREA II, a less frequent measurement grid was planned, as this area, in contrast to AREA III, is influenced by a smaller number of factors (mainly, the influence of the outer Puck Bay through the Kuźnica Passage). In AREA II, the distance between points was 1–3 km. A total of 129 sandy and muddy surface sediment samples were collected from the bottom of the entire lagoon (depth ≤ 9 m) by means of a Van Veen Grab sampler, with a surface area of 250 cm2 and a depth no more than 10 cm (Figure 1).

2.3. Diatom Analysis

The diatom analyses were prepared according to the standard Battarbee [45] procedure. To remove calcium carbonate and organic matter, all samples for diatom analysis (approximately 1 g of dry sediment) were treated with 10% HCl and 30% H2O2, respectively. Quantitative and qualitative diatom analyses were carried out on all samples. Permanent diatom slides were made using Naprax, UK (refractive index nD—1.73). The concentration of diatom valves per unit weight of dry sediment was estimated according to the Bodén [46] method. The analysis was performed under a light microscope Nikon Eclipse Ci, Japan, under a 100× oil immersion objective. The counting method of Schrader and Gersonde [47] was used, and in each sample, approximately 300–500 valves were counted in order to estimate the percentage abundance of particular taxa. Taxonomic identification was carried out based on Hustedt [48], Krammer and Lange-Bertalot [49,50,51,52], Pankow [53], Lange-Bertalot [54], Bąk et al. [31], Snoeijs [55], Snoeijs and Balashova [56], Snoeijs and Kasperovicˇiene [57], Snoeijs and Potapova [58], Snoeijs and Vilbaste [59], and Witkowski et al. [60]. The ecological preferences, including habitat [9], salinity [61], pH [48] and trophic [62] and saprobic status [63], were determined based on OMNIDIA 6.08 software. For further detail on the division of diatoms according to the analysed ecological preferences, see Supplementary Material Table S2.

2.4. Hg Analysis

The collected study material was preserved at −20 °C until mercury analysis. Before analysing mercury contained in the sediment, the samples were freeze-dried and homogenized using a ball mill. The analysis of total mercury (Hgtot) and mercury groups was carried out by the thermodesorption method using a DMA-80 analyser by Milestone, Italy. The method presented earlier by Reis et al. [64] has been improved and can be used to analyse sediment with high OM and even biological matter content [65]. The analysis was performed on material representing 0.1–0.4 g of dry sediment. The research material was placed in nickel boats, previously roasted for 1 min at 800 °C. The resulting research material was subject to total mercury analysis and 5-step mercury analysis in accordance with the procedure presented by Bełdowska et al. [66]. Total mercury analysis involved measuring the concentration of the sample burned at 750 °C. Mercury fraction analysis involved measuring the concentration of mercury groups released when the material was burned at specific temperatures. The first analysed fraction of Hg related to halides (HgF1) and can be adsorbed on sediment and diatom shells and is released at 175 °C. The second analysed fraction absorbed inside the cells (embedded in diatom frustules) (HgF2) was Hg forming complexes with organic matter released at 225 °C. The last labile form adsorbed on diatom frustules and released at 475 °C was associated with mercuric oxide (HgO) and mercuric sulphate (HgSO4) (HgF4). In contrast, the analysed stable forms included mercury sulphide (HgF3) released at 325 °C and mercury associated with the matrix (HgF5) released at 750 °C. The determination of Hgtot was verified by the analysis of certified reference materials: soil NCS DC 87103 (HgTOT: 17 ng g−1) and marine sediment GBW 07314 (HgTOT 43 ng g−1) at mean recovery > 95% and standard deviation (SD) > 85%. The limit of detection (LOD) was calculated based on the SD of the Hg concentration in the blank samples, measured in 10 repetitions (LOD = 3 × SD). In the case of analysis of HgTOT, LOD was 1 pg Hg. More details are described by Wilman et al. [67].
There are no reference materials for mercury speciation (except for MeHg). Therefore, the method was tested on certified reference materials (tea leaves—INCT-TL-1; soil—NCS DC 87103), as well as on 3 synthetic materials (HgCl2, HgS and HgO) (Sigma Aldrich Co., St. Louis, MO, USA, purity ≥ 97%) and two natural materials (seal muscle and seal faeces). More details are disrobed by Wilman et al. [67]. The accuracy of the method was verified by comparing the sum of the Hg concentrations measured by the 5-step fractionation method with the result of the HgTOT analysis. The accuracy of the Hg forms analysis averaged 94%, with a standard deviation not exceeding 5%. The accuracy and precision of the method were verified by the analysis of certified reference materials (soil NCS DC 87103—HgTOT 17 ng g−1 and marine sediment GBW 07314—HgTOT 43 ng g−1). The analysis of reference materials was carried out in three replicates, for which the recovery was found to be at levels of 96 and 98%, respectively [68].

2.5. Loss on Ignition, Wetness and Granulometry Analysis

The percentage of organic matter (OM) in the analysed sediment was measured by means of loss on ignition (LOI). For this purpose, the sediment was burned at 550 °C for 6 h. Moreover, sediment wetness was determined by the weight ratio of wet sediment to dried sediment at 105 °C [69]. The sediment was then subject to granulometric analysis by means of the sieve method [70]. The sediment was separated on a column of sieves with a mesh diameter ranging from −3 φ (8 mm) to 4 φ (0.063 mm). Sediment with grain diameter > 4 φ (<0.063 mm) was classified as the fine sediment fraction (FSF) based on the classification by Gradziński et al. [71].

2.6. Data Processing

The results of the concentrations of Hgtot (ng g−1) and its fractions (HgF1, HgF2, HgF3, HgF4 and HgF5) were presented as concentrations in dry sediment. Statistical analyses were performed to determine the association of Hg to the identified diatom species. The percentage of each diatom group was used to characterize diatom ecological groups in relation to the Hg concentration (similar to the content of organic matter and fine sediment fractions). For the characterization of diatom species in relation to Hg concentration, the percentage of individual species was taken into account.
Statistical analysis (PCA, corelation analysis) and graphic representation of the results of mercury analysis were carried out using STATISTICA 13 (StatSoft, Kraków, Poland). The results of the diatom analysis were presented using TILIA 2.0.37 [72]. The diatom diagram shows salinity preference of the species that occurred with ≥5% frequency in at least one sample. Cluster analysis was performed by means of CONISS [72] and presented as percentage diagrams. The map of the study area and maps showing spatial distribution of Hgtot concentrations, contents of diatom valves, %OM and %FSF were created using ArcGIS Pro 2.7.0 (ESRI, New York, NY, USA) with the WGS1984 geographic coordinate system.

3. Results and Discussion

3.1. Diatom Analysis

The analysed diatom flora preserved in the sandy and silty sediments of Puck Lagoon were abundant and well preserved. A total of 159 species belonging to 57 genera were identified (Figure S2). CONISS cluster analysis showed three prominent areas differing in salinity preferences of the dominant diatom species. For further detail on the characteristic of diatom taxa, see Supplementary Material Figure S2.
AREA I (PL1–15) along the Hel Peninsula, due to its shallow depth (<1 m), was characterised by the dominance of benthic species (approximately 100%) (Figure S3). For further detail on the percentage content of the diatom ecological groups, see Supplementary Material Figure S3. Euhalobous species were represented by Halamphora coffeaeformis (C. Agardh) Mereschkowsky and Nanofrustulum krumbeinii (Witkowski, Witak and Stachura) Morales (Figure 2). Mesohalobous species preferring waters less polluted with organic substances (β-mesosaprobionts) include Gedaniella flavovirens (Takano) Li, Witkowski and Ashworth, G. mutabilis (Grunow) Li and Witkowski, G. guenter-grassii (Witkowski and Lange-Bertalot) Chunlian Li, Sato and Witkowski, and Planothidium delicatulum (Kützing) Round and Bukhtiyarova. Furthermore, oligohalobous halophilous species, including Nitzschia frustulum (Kützing) Grunow and Planothidium lemmermannii (Hustedt) Morales, were observed abundantly.
All species except P. lemmermannii were observed in surface sediments along the Hel Peninsula by Witak et al. [73]. Like the other species, P. lemmermannii was commonly observed in different parts of the Gulf of Gdansk [74,75]. The concentration of diatom valves in sediments along the Hel Peninsula ranged from 9 × 105 valves g−1 (PL5, Pl8) to 71 × 105 valves g−1 (PL2) (Figure S3). The estimated concentration in the analysed material was lower in comparison to the results presented by Witak et al. [73], where the concentrations ranged from 42 × 106 valves g−1 to 60 × 106 valves g−1.
AREA II (PL16–81), in the central part of Puck Lagoon, like AREA I, showed the dominance of benthic species (84–100%). Planktic taxa were also observed, however, they were particularly abundant in the area of the Kuźnica Deep (PL29–30), with a frequency of 5–12% (Figure S3). Planktic species were mostly represented by Cyclotella chactawhatcheeana Prasad and Chaetoceros spp. RS. The diatom flora of AREA II are dominated by oligohalobous indifferent taxa, including Amphora pediculus (Kützing) Grunow, accompanied by species of genera Cocconeis and Staurosira (Figure 2). Their occurrence is related to the inflow of the waters of the Płutnica River. Relatively high frequency was recorded for brackish species Gedaniella mutabilis and Planothidium delicatulum, constituting permanent components of the diatom community of the shallow zones of Puck Bay and the Gulf of Gdansk [74,76,77]. Among marine species, Cocconeis scutellum Ehrenberg, Martyana schulzii (Brockmann) Snoeijs, Opephora horstiana Witkowski and Gedaniella guenter-grassii were observed more frequently in the northern part of AREA II. In the southern part of AREA II, however, Catenula adhaerens Mereschkowsky and Navicula germanopolonica Witkowski and Lange-Bertalot were observed in the community. The frequency of these taxa suggests that the hydrology of this area is shaped by higher salinity waters flowing from the outer Puck Bay through the Głębinka Passage to the south and through the Kuźnica Passage to the north. Furthermore, the oligohalobous halophilous Planothidium lemmermannii is also highly frequent in this area, potentially due to the inflow of the Płutnica River. The species has been observed in the shallow water zone of Puck Lagoon [76,78] and the Gulf of Gdańsk [74]. AREA II is characterised by valve concentration ranging from 5 × 105 valves g−1 (PL17) to 127 × 105 valves g−1 near the Kuźnica Deep (station PL30). A high concentration was also observed near the Swarzewo coastal area (PL63), where it increased to 81.67 × 105 valves g−1 (Figure S3). AREA II features the highest concentration of valves of the entire study area.
AREA III (PL82–129), covering the southern part of Puck Lagoon, showed an increased frequency of planktic species (≤37%), particularly in the vicinity of the Rzucewo Deep (Figure S3). Their primary representative was the anthropogenic species Cyclotella choctawhatcheeana, often observed in the sediments of the nearby Port of Gdynia [79]. AREA III was characterised by an increased frequency of mesohalobous taxa (19–64%) and a decreased percentage content of oligohalobous indifferent taxa (3–60%), compared to AREA II. Gedaniella mutabilis and Planothidium delicatulum represented the first group, while the second group was represented by Amphora pediculus. This distribution points to a greater role of brackish waters in shaping the hydrology of waters in the southern part of the lagoon. In the vicinity of the Reda River, a higher percentage of eutraphentic species tolerating high concentrations of nutrients in water was recorded, namely, Halamphora coffeaeformis and Nitzschia frustulum (Figure 2). They could be associated with the presence of recreational centres in this part of the lagoon’s coastal zone. The species Catenula adhaerens, preferring less polluted waters, was observed more frequently at the other stations. In comparison to AREA I, lower concentrations of diatom valves in the sediment were observed in AREA III. In AREA III, their concentration ranged from 4 × 105 valves g−1 to 102 × 105 valves g−1 in the Rzucewo Deep (PL115). Moreover, near the mouth of the Reda River (PL120), the concentration of diatom valves in the sediment increased to 53 × 105 valves g−1 (Figure S3).

3.2. Hg in Surface Sediments

The average concentration of total mercury (Hgtot) in surface sediments of Puck Lagoon was low, reaching 4.6 ng g−1 (median 1.1 ng g−1). The highest percentage of Hgtot was represented by mercury in complexes with organic matter HgF2 (37%), HgF3 (36%) and mercury adsorbed in the form of HgSO4 and HgO (HgF4) (21%). The lowest percentage of Hgtot was matrix-bound mercury HgF5 (5%) and mercury related with halides HgF1 (1%) (Figure 3). According to the study by Kwasigroch et al. [68], such a distribution is characteristic of this region (Gdańsk Basin).
AREA I (PL1–15) was the region with the lowest concentrations of Hgtot (≤2.3 ng g−1). Also, some of the lowest values over the entire Puck Lagoon were measured for the analysed forms of mercury. In this area, the highest concentration in sediment was measured for HgF2 (1.2 ng g−1). Such low Hg concentrations in this area were associated with low contents of diatom valves (≤71.2 × 105 valves g−1), OM (≤1.3%) and FSF (≤1.7%) in the surface sediment (Figure S4). In Puck Lagoon, the average organic matter content ranges from 1% up to 5% [80,81,82]. The low content of Hg, OM, FSF and diatom valves may result from the lack of supply sources for those in close vicinity to the area. In addition, high water dynamics in this location make it difficult for diatom flora to attach to the substrate. Further details on spatial variability of the concentrations of Hgtot, contents of diatom valves, %OM and %FSF, are presented in the Supplementary Material Figure S4.
The northern deepest part of the basin (stations PL29–31, AREA II) (Figure 1) was characterized by the highest concentrations of Hgtot, reaching almost 100 ng g−1 (Table 1) and exceeding the value accepted as good environmental status (70 ng g−1) as defined by the Directive 2008/56/EC of the European Parliament and of the Council. High Hg concentrations in this area resulted from its location; it is a deposition zone that contributes to the accumulation of pollutants, including mercury [83]. The increase in bottom depth is associated with an increase in sediment dispersion [41]. Moreover, fine sediment fraction and organic matter delivered by currents in Puck Lagoon are deposited in the Kuźnica Deep depression. Therefore, this area also had the highest diatom valve concentration (127 × 105 valves g−1), content of organic matter (22.7%) and fine fraction (21.69%) in the analysed sediment (Figure S4). The increase in the content of silt minerals in sediments associated with the deepwater area is accompanied by an increase in the concentration of OM [80]. Moreover, the high share of OM in bottom sediments of deep water areas is associated with more intense phytoplankton blooms in these areas [80,84]. This is also indicated by the results of the conducted study. Moreover, vegetation overgrowing the bottom of the shoreline zone contributed to stable conditions, allowing for the retention of organic matter which consequently may have contributed to increased Hg concentrations. On the other hand, mercury in the deeper regions of Puck Lagoon (Kuźnica and Rzucewo Deep) could probably have been transported from the outer Puck Bay with the inflow of more brackish waters through the Kuźnica and Głębinka Passage [85] (Figure 1).
In the southern part of the lagoon, near the shore zone of Swarzewo (station PL40–AREA II), in the vincity of the Płutnica (station PL67–AREA II), Gizdepka (station PL97–AREA III) and Reda rivers (stations PL111, PL118–AREA III), and in the area of the Rrzucewo Deep (station PL101–AREA III), an increase in Hgtot concentrations of up to approximately 20 ng g−1 was also observed. In these areas, the concentration of diatom valves (up to 127 × 105 valves g−1) and the percentage of organic matter (up to 22.7%) and fine fraction (up to 23.7%) in the sediment increased simultaneously. River water was a substantial source of Hg in the study area. The Reda River is therefore of particular importance, with an average water flow of 5.9 m3 s−1, contributing metal from a catchment area of 485.2 km2 [22,84]. Rivers introduce >12.5 tons of sedimentary material into Puck Lagoon, which is deposited near the river mouths [86]. This contributes to the increase in silty and clayey fractions in the surface sediment in this region [81,86]. Cliff erosion is also a prominent source of Hg in this part of Puck Lagoon [87]. The coastal zone of Puck Lagoon is fed by sediment derived from the cliffs along the reservoir [41]. This results in a lower content of fine sediment fraction (≤1.72%) and organic matter (0.4–2.6%) in this area, which translates into lower Hg concentrations (0.3–10.85 ng g−1) in the sediment of the AREA III coastal zone of Puck Lagoon. At the same time, this area is characterized also by low content of diatom valves (4.9–31.3 × 105 valves g−1) (Figure S4).

Factors Influencing Hg Concentration in the Surface Sediment

The accumulation of metals (including Hg) in sediments is primarily related to the presence of organic matter (OM) and fine sediment fraction [29]. The statistical analyses in the study area confirmed a high statistically significant correlation of Hgtot concentration with organic matter contained in the sediment (r = 0.9, p < 0.05) (Table 2) and with the fine sediment fraction (r = 0.5, p < 0.05). Studies of cores conducted in Arctic lakes [15,16] and the north-west Pacific [88] have revealed another factor influencing Hgtot accumulation in sediments: diatom valves. The diatom flora is an important source of organic matter in Puck Lagoon. Diatoms are one of the most abundant and diverse microorganisms in this area [89]. Diatom valves preserved in the sediment showed a statistically significant relationship with the total mercury deposited in the sediment (r = 0.6, p < 0.05), which is also supported by the PCA analysis (Figure 4). This shows that in the study area, mercury is more easily accumulated in the diatom flora than in the fine fraction.
Among the analysed mercury fractions, HgF2 and HgF1 showed a linear correlation (p < 0.05) with the concentration of diatom valves deposited in the sediment of Puck Lagoon (Table 2). Mercury in complexes with organic matter (HgF2) was absorbed inside the diatom valves (r = 0.6, p < 0.05). Mercury in this form can biomagnify, which is of importance for the marine ecosystem. HgF2 accounted for the largest percentage of Hgtot (4–99%, mean 34%). Linear correlations showed that mercury in halide bonds (HgF1) was associated with diatoms to a lesser extent (although the correlation was statistically significant) (r = 0.3, p < 0.05). This is probably related to the fact that these forms of Hg adsorb to the surface of the diatoms, and because they are labile, they can be easily leached back into the surrounding supra-bottom or pore water.
PCA analysis showed that two factors explained 71% of the cases (Figure 4). The first factor was the accumulation of Hg in surface sediments. It confirmed the role of OM and FSF, as well as diatom valves, in the accumulation of Hgtot, HgF1, HgF2 and HgF5 in sediments.
It shows that with the hydration of sediments and increasing content of organic matter, as well as diatom valves deposited in the sediment and the fine sediment fraction, the concentration of total mercury, primarily associated with halides and organic matter, increased. The second factor was the processes of the formation of stable HgS (HgF3) from HgSO4 (HgF4). The resulting mercury sulphide was affected by the reducing conditions and appropriate concentration of sulphate ions and the presence of sulphur bacteria [90]. This suggests that the conditions for the formation of HgS from HgSO4 were important throughout the study area.

3.3. Hg and Diatoms Salinity Regions

The retention of mercury in sediments in AREA I depended largely on the concentration of total mercury (Hgtot) and brackish diatom species (r = 0.5, p < 0.05) (Table 3). Considering the analysed Hg fractions, diatoms preferring salinities of 5–20 PSU (mesohalobous) absorbed HgF2 inside the cells (r = 0.4, p < 0.05). However, this community showed a better correlation with sulphide (HgF3) and HgF4 (r = 0.5, p < 0.05) adsorbed on the surface of diatom valves. The high affinity of HgF3 with the brackish diatom community was probably a result of the transformation of HgSO4 adsorbed on the valves to HgS, as supported by the correlation of HgF4 and HgF3 (r = 0.9, p < 0.05).
A probable source of mercury in the northern Puck Lagoon (AREA I) could be the transport of Hg from remote, more saline parts of the bay. This is evidenced by the intercorrelation of all Hg fractions and the correlation of Hg forms with the mesohalobous group, except for mercury associated with halides (HgF1). This was further confirmed by carbon isotope studies by Jędruch et al. [87], where in the N part of Puck Lagoon, δ13C values in organic matter were typical of OM of marine origin. This is due to the occurring currents along the Hel Peninsula. These currents transport matter to Puck Lagoon from deeper areas of the Gulf of Gdańsk [39]. The lack of connection of salinity groups with HgF1 may have been related to its dissolution and leaching back into the water. The fact that mercury is more readily accumulated by mesohalobes is further confirmed by an inverse correlation of total mercury (Hgtot) which was observed with oligohalobous halophilous taxa (r = −0.5, p < 0.05), with an optimum at a salinity of 5 PSU [61] along the Hel Peninsula. Among the analysed mercury forms, it showed a negative correlation with HgF3 (r = −0.5, p < 0.05) and HgF4 (r = −0.5, p < 0.05) (Table 3). This may indicate a more persistent accumulation of mercury by marine diatom species, probably related to the strong affinity of Hg for sulphur. The occurrence of this group of diatoms in AREA I indicates substantially higher salinity in this region. On the other hand, sulphate concentrations in the water increase with increasing salinity. Previous studies indicate the transport of OM and FSF from more saline regions [87]. The studies presented here also indicate an important role for marine diatoms (from deeper regions) in the storage of Hg in this area of Puck Lagoon.
In the sediments collected from AREA II, an evident relationship between Hgtot, particularly HgF4 (mainly HgSO4), and the euhalobous group was observed (r = 0.3, p < 0.05). Like in AREA I, mercury was more easily accumulated by species preferring saltier waters. The higher frequency of euhalobous taxa was mainly associated with higher Hg concentrations in the Kuźnica Deep and near the Rzucewo Deep. These regions were the deepest where the analysed material was deposited.
The analysis of sediments collected from AREA III (Figure 1) showed a relationship between total mercury as well as its individual fractions with freshwater diatom flora. In this part of Puck Lagoon, the inflow of river water has a significant influence on the occurrence of diatom flora, as well as on the level of chemical substance concentrations. Rivers are an important source of terrestrial Hg inputs to the bay [91]. They probably influenced the potential accumulation of Hgtot with oligohalobous indifferent taxa preferring low salinity (r = 0.3, p < 0.05) in this area. The dominant role of river transport and freshwater diatom communities is also supported by the negligible relationship between mercury and other diatom groups. This may have influenced a decrease in the correlation with marine diatom flora. Among the analysed mercury fractions, the freshwater community showed a significant correlation with stable forms of mercury: HgF3 (r = 0.4, p < 0.05) and HgF5 (r = 0.4, p < 0.05) (Figure 3). According to the study by Saniewska et al. [83], labile forms of mercury were primarily present in the sediments of rivers flowing into Puck Lagoon (Płutnica, Reda). The HgS form was present to a smaller extent. In comparison with Reda and Płutnica, however, higher concentrations of stable forms were measured in the sediments of the Gizdepka River. This suggests that the diatom flora occurring in AREA III are associated with freshwater and prefers terrestrial mercury. As with AREA I, an inverse correlation was also found between total mercury (Hgtot) and oligohalobous halophilous taxa (r = −0.4, p < 0.05). Moreover, this diatom group showed an inverse correlation with HgF3 (r = −0.3, p < 0.05). There was also an inverse correlation with euhalobous taxa in this region (r = −0.3, p < 0.05). The negative correlation with groups of oligohalobous halophilous and euhalobous taxa suggests that the increased salinity does not favour Hg binding to diatom flora in the southern part of Puck Lagoon. A similar relationship was observed in AREA I.

3.4. Mercury Versus Diatoms

Previous studies regarding Hg and the benthic and planktic diatom flora have been focused on the effects of mercury in breeding species under laboratory conditions [20,21]. The present study observed the association of mercury with benthic species and planktic Chaetoceros resting spores that were a minority in each region (AREA II; AREA III). An exception is AREA I, located along the Hel Peninsula, where the mercury concentration was seen not to deposit on particular diatom species, as indicated by the lack of correlation with particular diatom species.
AREA II, covering the central part of Puck Lagoon, was characterised by a correlation of mercury with planktic Chaetoceros resting spores and benthic taxa. The correlating salinity groups included the following taxa: euhalobous (Chaetoceros diadema RS (Ehrenberg) Gran, Rhabdonema adriaticum (C.A. Agardh) Kützing, R. minutum Kützing), mesohalobous (Chaetoceros spp. RS, Navicula phyllepta Kützing), oligohalobous halophilous (Pseudostaurosira perminuta (Grunow) Sabbe and Wyverman), oligohalobous indifferent (Epithemia adnata (Kützing) Brébisson, E. frickei Krammer, E. goeppertina Hilse, Staurosirella martyi (Héribaud) Lange-Bertalot, F. inflata var. istvantfyi (Pantoscek) Hustedt, Navicula cryptocephala Kützing and Surirella brebissonii Krammer and Lange-Bertalot). All of these species showed a moderate to high but statistically significant correlation (p < 0.05) with HgF2 and with HgF1 (Table 4).
In AREA II, most species reached the highest frequency in the vicinity of the Kuźnica Deep, ranging from 0.36% to 20.5%. The highest concentrations of Hgtot, HgF1 and HgF2 were also measured in this part of Puck Lagoon. A prominent taxon was planktic Chaetoceros seiracantus RS. This taxon is potentially associated with HgF5 (r = 0.43, p < 0.05). This suggests that it incorporated toxic Hg into the valves.
A significant correlation (r = 0.52, p < 0.05) existed between HgF1 and benthic taxa Epithemia goeppertina, Fragilaria inflata var. istvantfyi, Pseudostaurosira perminuta and Rhabdonema adriaticum. Moreover, an evident relationship was observed between this form of mercury and benthic Epithemia adnata (r = 0.32, p < 0.05), Chaetoceros spp. RS (r = 0.31, p < 0.05) and Staurosirella martyi (r = 0.33, p < 0.05). The benthic brackish species Navicula phyllepta, most abundantly observed near Swarzewo, is also worth attention (PL40, 24%). The species absorbed HgF1 onto its surface (r = 0.6, p < 0.05). This suggests that all species correlating with HgF1 could be live diatoms. HgF1 is a labile fraction that is most likely leached from dead diatoms into the surrounding water.
Moreover, in contrast to other species, N. phyllepta showed a significant correlation with adsorbed HgF4 (r = 0.4, p < 0.05). In the waters of Puck Lagoon, there are regions with reducing conditions, i.e., conditions favourable for the formation of mercury sulphide, (HgF3) with which N. phyllepta showed a high correlation (r = 0.74, p < 0.05). Previous studies on the effects of mercury on diatoms have shown that mercury reaching diatom frustules is transformed to less toxic forms through reacting with proteins [20,92,93]. This may indicate that N. phyllepta probably protects itself from Hg toxicity in this way.
The mentioned taxa (Chaetoceros spp. RS, Epithemia adnata, Epithemia goeppertina, Fragilaria inflata var. istvantfyi, Fragilaria maryti and Rhabdonema adriaticum) are species that are relatively larger in terms of the size of their valve and therefore can accumulate more Hg. Only Pseudostaurosira perminuta and Navicula phyllepta are smaller taxa, and therefore, they have a smaller adsorption area. Chaetoceros in the study material are associated with marine and brackish waters [94]. Resting spores are planktic taxa. Their connection with Hg, however, may result from their long persistence in the sediment with very high mercury concentrations.
AREA III, including the southern Puck Lagoon, showed a significant correlation of benthic species Staurosirella martyi with Hgtot (r = 0.44, p < 0.05), HgF1 (r = 0.57, p < 0.05), HgF2 (r = 0.32, p < 0.05), HgF3 (r = 0.36, p < 0.05) and HgF4 (r = 0.44, p < 0.05). This phenomenon was also observed in AREA II. Excessive nutrient supply contributed to the formation of diatom resting spores (in AREA III Chaetoceros seiracantus RS), efficiently accumulating Hg. Importantly, in the southern part of Puck Lagoon, they are potentially associated with the stable form of mercury HgF3 (r = 0.43, p < 0.05) which does not biomagnify.
The association of individual species in AREA II and AREA III with mercury could have been from relationships with high Hg concentrations in the area where these species occur (Kuźnica Deep; Swarzewo river mouth). This would explain the lack of connections of Hgtot and individual forms of mercury with diatom species found in AREA I.

4. Conclusions

Our results showed that diatom taphocoenoses (r = 0.6, p < 0.05), next to organic matter (r = 0.9, p < 0.05) and fine sediment fraction (r = 0.5, p < 0.05), are an important element of the marine environment in terms of absorbing and adsorbing mercury. It seems that, depending on environmental conditions and diatom species composition, they can interact with mercury in several ways. The linear correlation showed a connection of salinity groups with adsorbed (HgS; HgSO4) and absorbed (built into OM) mercury. In the northern part of Puck Lagoon (AREA I) and in areas distant from the coastal zone (AREA II), the forms of mercury showed a statistically significant correlation with species preferring higher salinity (euhalobous and mesohalobous taxa). In these diatom groups, Hg was retained as mercury sulphide (HgF3; r = 0.5, p < 0.05) and mercury sulphate (HgF4; r = 0.5, p < 0.05). In contrast, a higher affinity of mercury with freshwater diatom flora was observed near river mouths (AREA III; r = 0.3, p < 0.05).
This is the first study to demonstrate an association between mercury fractions and individual diatom species living in the marine environment. Correlating species were associated with deposition sites (Kuźnica Deep) and the river inflow of mercury (Płutnica, Reda, Gizdepka), at which the highest Hg concentrations were measured in the entire Puck Lagoon (approximately 20–100 ng g−1). Species showing high statistically significant correlation with mercury forms were a minority in the analysed diatom community of Puck Lagoon. These species, however, primarily correlated with mercury forms (HgF2; r = 0.5, r = 0.6, p < 0.05) potentially undergoing biomagnification, enriching subsequent levels of the food web with mercury.
Taking into account the different degrees of Hgtot and its fractions storage by living and dead diatoms, it seems reasonable to conduct analogous studies seasonally—before diatom bloom, during diatom bloom, immediately after diatom bloom and several months after diatom bloom—to determine the percentage of live and dead of individual species.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w15223907/s1, Information about spatial variability of individual diatom ecological groups; Figure S1: Sediment grain size in the inner Puck Bay; Figure S2: Characteristic of diatom taxa; Figure S3: Percentage content of the diatom ecological groups; Figure S4: Spatial variability of the: A—concentrations of Hgtot (ng g−1), B—%FSF, C—%OM and D—contents of diatom valves (valve × 105 g−1); Table S1: Environmental conditions during collection of study material; Table S2: Ecological preferences of diatoms in terms of habitat, salinity, pH, trophic and saprobic status. References [95,96,97] are citied in the Supplementary Materials.

Author Contributions

Conceptualization, D.H., M.W. and M.B.; investigation, D.H.; formal analysis, D.H.; resources, M.W. and M.B.; visualization, D.H.; a major contributor in writing—original draft, D.H.; writing—original draft, M.W. and M.B.; writing—review and editing, D.H., M.W. and M.B. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financed by Ministry of education and Science via Statutory Research Project of the Department of Chemical Oceanography and Marine Geology, University of Gdańsk.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank the anonymous reviewers for their comments, which improved the manuscript. We thank Dominika Saniewska and Bartłomiej Wilman, Department of Marine Chemistry and Environmental Protection, Institute of Oceanography, University of Gdańsk, for their invaluable consultation and assistance during mercury analysis. We thank Jarosław Pędziński, Department of Marine Geology, Institute of Oceanography, University of Gdańsk, and Dominika Wiśniewska, student of the University of Gdansk, for helping with collecting the material studied.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location of the analysed samples.
Figure 1. Location of the analysed samples.
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Figure 2. Frequency of the main diatom species: Eu—euhalobous, Meso—mesohalobous, Oh—oligohalobous halophilous, Oi—oligohalobous indifferent.
Figure 2. Frequency of the main diatom species: Eu—euhalobous, Meso—mesohalobous, Oh—oligohalobous halophilous, Oi—oligohalobous indifferent.
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Figure 3. Average percentage of labile and stable forms of mercury in the surface sediment of Puck Lagoon.
Figure 3. Average percentage of labile and stable forms of mercury in the surface sediment of Puck Lagoon.
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Figure 4. Principal component analysis (PCA) for concentration of diatom valves, Hgtot, fractions of mercury (HgF1, HgF2, HgF3, HgF4 and HgF5), LOI and wetness.
Figure 4. Principal component analysis (PCA) for concentration of diatom valves, Hgtot, fractions of mercury (HgF1, HgF2, HgF3, HgF4 and HgF5), LOI and wetness.
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Table 1. Statistical characterization of the Hgtot and its fractions (ng g−1), concentrations of diatoms valves (valve × 105 g−1), percentage of organic matter (OM) and fine fraction (FSF) in the surface sediments: * <LD—below the level of detection.
Table 1. Statistical characterization of the Hgtot and its fractions (ng g−1), concentrations of diatoms valves (valve × 105 g−1), percentage of organic matter (OM) and fine fraction (FSF) in the surface sediments: * <LD—below the level of detection.
HgtotHgF1HgF2HgF3HgF4HgF5Concentration of Diatom ValvesOMFSF
AllMin.0.10.00.0<LD *<LD *0.05.00.10.0
Max.99.90.593.613.25.88.2127.022.723.7
Mean4.60.02.81.00.50.131.01.92.9
Median1.10.00.30.30.20.024.01.00.7
AREA IMin.0.20.00.10.00.00.08.80.20.0
Max.2.30.01.21.30.60.171.21.31.7
Mean0.80.00.40.20.10.030.40.60.5
Median0.60.00.30.10.10.024.50.60.2
AREA IIMin.0.10.00.0<LD *<LD *0.05.40.10.0
Max.99.90.593.613.22.28.2127.022.721.7
Mean6.60.04.81.00.40.231.72.42.1
Median1.00.00.20.30.10.024.71.10.9
AREA IIIMin.0.20.00.00.00.00.03.80.20.0
Max.14.70.12.48.35.80.4101.75.223.7
Mean3.00.00.61.30.80.125.01.54.7
Median1.90.00.50.60.40.119.31.11.1
Table 2. Correlation coefficients determined for Hgtot, labile and stable fractions of mercury with OM%, and the concentration of diatoms valves and FSF% in the surface sediments. Statistically significant correlations are marked in bold.
Table 2. Correlation coefficients determined for Hgtot, labile and stable fractions of mercury with OM%, and the concentration of diatoms valves and FSF% in the surface sediments. Statistically significant correlations are marked in bold.
HgtotHgF1HgF2HgF3HgF4HgF5OM%FSF%Concentration of Diatoms ValvesWetness
Hgtot1.000.730.990.020.210.050.880.540.590.54
HgF10.731.000.640.310.280.190.720.380.330.54
HgF20.990.641.00−0.100.110.000.850.420.570.52
HgF30.020.31−0.101.000.87−0.030.120.420.080.30
HgF40.210.280.110.871.00−0.020.230.480.140.28
HgF50.050.190.00−0.03−0.021.000.370.070.070.10
OM%0.880.720.850.120.230.371.000.520.510.64
FSF%0.540.380.420.420.480.070.521.000.300.59
Concentration of diatoms valves0.590.330.570.080.140.070.510.301.000.54
Wetness0.540.540.520.300.280.100.640.590.541.00
Table 3. Correlation coefficients determined for Hgtot, labile and stable fractions of mercury with diatoms salinity groups in distinguished areas. Statistically significant correlations are marked in bold: Eu—euhalobous. Meso—mesohalobous. Oh—oligohalobous halophilous. Oi—oligohalobous indifferent.
Table 3. Correlation coefficients determined for Hgtot, labile and stable fractions of mercury with diatoms salinity groups in distinguished areas. Statistically significant correlations are marked in bold: Eu—euhalobous. Meso—mesohalobous. Oh—oligohalobous halophilous. Oi—oligohalobous indifferent.
HgtotHgF1HgF2HgF3HgF4HgF5
AREA IHgtot1.000.010.680.650.450.21
HgF10.011.00−0.300.550.730.62
HgF20.68−0.301.000.08−0.01−0.24
HgF30.650.550.081.000.910.12
HgF40.450.73−0.010.911.000.48
HgF50.210.62−0.240.120.481.00
Eu0.010.070.01−0.19−0.070.16
Meso0.520.100.460.520.540.26
Oh−0.54−0.14−0.40−0.51−0.58−0.37
Oi−0.09−0.06−0.290.190.14−0.17
AREA IIHgtot1.000.690.770.810.780.48
HgF10.691.000.700.560.520.05
HgF20.770.701.00−0.030.16−0.01
HgF30.810.56−0.031.000.69−0.04
HgF40.780.520.160.691.00−0.07
HgF50.480.05−0.01−0.04−0.071.00
Eu0.330.03−0.060.200.360.02
Meso−0.130.230.050.16−0.040.01
Oh0.00−0.09−0.10−0.06−0.04−0.02
Oi−0.13−0.140.10−0.23−0.180.00
AREA IIIHgtot1.000.520.440.930.910.50
HgF10.521.000.540.660.700.46
HgF20.440.541.000.570.560.28
HgF30.930.660.571.000.800.60
HgF40.910.700.560.801.000.52
HgF50.500.460.280.600.521.00
Eu−0.01−0.22−0.35−0.11−0.19−0.01
Meso0.03−0.030.18−0.07−0.01−0.13
Oh−0.40−0.070.17−0.32−0.17−0.36
Oi0.310.19−0.120.390.220.44
Table 4. Correlation coefficients determined for Hgtot, labile and stable fractions of mercury with diatoms species in distinguished areas. Statistically significant correlations are marked in bold.
Table 4. Correlation coefficients determined for Hgtot, labile and stable fractions of mercury with diatoms species in distinguished areas. Statistically significant correlations are marked in bold.
HgtotHgF1HgF2HgF3HgF4HgF5
AREA IIHgtot1.000.770.990.110.270.04
HgF10.771.000.700.560.520.05
HgF20.990.701.00−0.030.16−0.01
HgF30.110.56−0.031.000.69−0.04
HgF40.270.520.160.691.00−0.07
HgF50.040.05−0.01−0.04−0.071.00
Chaetoceros diadema RS0.520.370.52−0.080.09−0.02
Chaetoceros seiracanthus RS0.03−0.010.01−0.010.030.44
Chaetoceros spp. RS0.540.310.56−0.090.030.03
Epithemia adnata0.480.320.49−0.100.040.00
Epithemia frickei0.470.200.50−0.07−0.12−0.03
Epithemia goeppertiana0.620.530.61−0.060.21−0.01
Staurosirella martyi0.700.340.73−0.16−0.01−0.03
Fragilaria inflata var. instvantfyi0.620.530.61−0.060.21−0.01
Navicula cryptocephala0.540.240.57−0.06−0.09−0.02
Navicula phyllepta0.120.640.030.750.350.00
Pseudostaurosira perminuta0.620.530.61−0.060.21−0.01
Rhabdonema adriaticum0.620.530.61−0.060.21−0.01
Rhabdonema minutum0.490.340.49−0.050.110.01
Surirella brebissonii0.490.340.49−0.050.110.01
AREA IIIHgtot1.000.750.700.950.920.58
HgF10.751.000.540.660.700.46
HgF20.700.541.000.570.560.28
HgF30.950.660.571.000.800.60
HgF40.920.700.560.801.000.52
HgF50.580.460.280.600.521.00
Chaetoceros seiracanthus RS0.380.250.270.440.250.27
Staurosirella martyi0.440.580.330.360.440.19
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Hetko, D.; Witak, M.; Bełdowska, M. The Relationship between Total Mercury, Its Fractions and Species Diversity of Diatom Taphocoenoses Deposited in Surface Sediments (Southern Baltic Sea). Water 2023, 15, 3907. https://doi.org/10.3390/w15223907

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Hetko D, Witak M, Bełdowska M. The Relationship between Total Mercury, Its Fractions and Species Diversity of Diatom Taphocoenoses Deposited in Surface Sediments (Southern Baltic Sea). Water. 2023; 15(22):3907. https://doi.org/10.3390/w15223907

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Hetko, Dominika, Małgorzata Witak, and Magdalena Bełdowska. 2023. "The Relationship between Total Mercury, Its Fractions and Species Diversity of Diatom Taphocoenoses Deposited in Surface Sediments (Southern Baltic Sea)" Water 15, no. 22: 3907. https://doi.org/10.3390/w15223907

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