3.1. Hydrochemical Parameters
The salinity, pH of water and sediments, Eh, concentrations of total phosphorus, and dissolved oxygen are presented in
Table 1.
In the previous studies [
5], it was detected that algal mats can create specific conditions in the coastal zone, affecting the metal distribution in the bottom sediments; we also measured the parameters under the algal layer on the sites with algal mats.
Eh and pH are very important for the water bodies because they reflect oxidation–reduction reactions. In our study, most sites were characterized by the alkaline reaction of water. The exception was site 8, where the water had an acid reaction (
Table 1). Unlike the water, the sediments from most sites were characterized by an acid reaction. The Eh potential had very low and negative values on all study sites, which shows the prevalence of reduction reactions on the border water sediments [
24].
The salinity increased from east to west, with a decrease under the influence of the Neva river.
3.2. Metal Concentrations in the Water and Sediments
The metal concentration in the water in May on the majority of sites was distributed in the following order:
The distribution of metals on the different study sites in May is shown in
Figure 2.
As seen from
Figure 2, on the northern shore (Sites 1–5), the highest concentrations of all four metals were observed at Site 4. It is remarkable that at this site, the concentration of cadmium exceeded that of lead and was significantly higher than that on other sites. The concentrations of Zn were higher at Sites 1–4 and significantly lower at Site 5, which is the closest point to the Neva Bay (
Figure 2). The concentrations of copper increased in the direction from Site 1 (the most distant site) to Sites 4 and 5, where they were significantly higher.
On the southern shore (Sites 6–10), the maximum metal concentration was observed at Site 6, which is situated in Neva Bay. The concentrations of all metals (Zn, Cu, Pb), excluding cadmium, were significantly higher at this site. In the direction to the west, from Neva Bay to Koporskaya Bay, the concentrations of metals decreased. In Luga Bay, we observed some non-significant rises in concentrations, which could be caused by the proximity of the Ust’ Luga port. The maximum Zn concentration was found in Neva Bay (Site 6) and reached 5.91 ± 0.33 μg L−1. The highest concentrations of copper were on the northern shore at Sites 4 and 5 (3.75 ± 0.02 and 3.69 ± 0.16 μ L−1, respectively). The maximum concentration of Pb was found in Neva Bay, reaching 1.98 ± 0.07 μg L−1.
The concentrations of cadmium were at the same level on the majority of sites, excluding Site 4, where Cd concentrations exceeded lead concentrations and reached a value of 0.66 ± 0.06 μg L−1.
In July, the metal distribution differed from that in June (
Figure 3). Significantly higher concentrations of Zn were found at Sites 5 and 8. The maximum Zn concentration was found at Site 8 and reached 4.09 ± 0.12 μg L
−1. Copper particularly repeated the pattern of Zn with significantly higher concentrations at Sites 5, 6 and the maximum concentration at Site 10 (1.92 ± 0.06 μg L
−1). The lead concentration did not change significantly across the sites, excluding Site 8, where the Pb concentration was significantly higher and reached 1.67 ± 0.04 μg L
−1.
The cadmium concentration did not change significantly over the study sites.
The concentration of metals differed significantly during the study period. The copper concentration in July decreased at all study sites, except at Site 10, where its concentration increased but non-significantly (ANOVA, F (14;30) = 303.73, p = 0.000).
The dynamics of Zn concentration varied widely; in July, Zn concentrations decreased significantly at Sites 1, 2, 6, 7 and rose at Sites 5, 8, 10 (F (14;30) = 234.33; p = 0.000).
The lead concentrations in July increased significantly at all sites, except for Site 6, where the lead concentration was halved in comparison with June (F (14;30) = 463.22; p = 0.000).
A significant decrease in Cd concentration was observed at all study sites (F (14;30) = 53.389; p = 0.000).
The metal concentrations in the pore water are presented in
Table 2. The concentrations of metals in the pore water were several times higher than those in the water column. Only Pb was an exception; its concentration in the pore water exceeded that in the water layer, but not by more than twice.
Judging by the concentrations, the metals in the sediments are primarily distributed in the order Zn > Pb > Cu > Cd. Significantly, the highest concentrations of all metals were found at Site 1 (
Figure 4). The highest concentrations of Zn were registered in sediments at Sites 1 and 5 (36.22 ± 2.01 and 35.13 ± 2.26 mg kg
−1, respectively). The highest concentrations of lead, copper, and cadmium were recorded at Site 1 near port Primorsk, reaching 23.9 ± 1.40; 10.33 ± 0.53 and 0.30 ± 0.02 mg kg
−1, respectively.
3.3. Macroalgae Biomass and Metal Accumulation
Figure 5 and
Figure 6 present the measured metal concentrations in the macroalgae at the study sites in May and July, respectively. The biomass of macroalgae was measured in May, but only for Sites 1 and 9. At Site 1, it reached 409.27 ± 263.73 gDWm
−2, or 2 kg of fresh biomass per square meter. At Site 9, the biomass of the macroalgae was 346.36 ± 101.21 gDWm
−2 (or 1.7 kg of fresh biomass per square meter).
The highest macroalgae biomass in July was at Sites 1 (326.17 ± 191.86 gDWm−2 or 1.6 kg of fresh biomass per square meter) and 2 (696.5 ± 143.14 gDWm−2, or 3.5 kg of fresh biomass per square meter). The algae biomass on Sites 3, 5, and 7 was 34.75 ± 18.34, 46.56 ± 15.57 and 19.17 ± 10.62 gDWm−2, respectively. At Sites 8 and 9, besides particular thalli of fresh algae, we observed decaying biomass with a thickness up to 30 cm (Site 8).
In May, the Zn, Cu, and Cd concentrations in the macroalgal biomass repeated the same concentration pattern in the water (
Figure 5).
The maximum metal concentrations were recorded in the algae from Site 6. The Zn content at this site reached 105.20 ± 6.30 mg kg−1 of dry weight, and was significantly higher than that on the other sites. The concentration of copper at Site 6 reached 28.35 ± 1.70 mg kg−1 and was also significantly higher in comparison with the other sites. Cadmium also had the highest concentration in macroalgae at Site 6 (0.56 ± 0.04 mg kg−1). Lead had the highest concentration in macroalgae at Site 1 (31.06 ± 1.77 mg kg−1). At Site 6, the Pb content in macroalgae was 20.70 ± 1.24 mg kg−1. The lead content in macroalgae at Sites 1 and 6 was significantly higher than that at Sites 4, 9, and 10.
In May, the Spearman rank correlations were significant for Cd content in algae and in the water (R = 0.66, p < 0.05), also for Pb content in algae and in the sediments (R = 0.84, p < 0.05), and Pb content in algae and in the water (R = 0.51, p < 0.05).
In July, none of the analyzed metal concentrations in the algal biomass showed a significant difference at Sites 1–5 (on the northern shore) (
Figure 6). A significant sharp rise in Zn and Pb concentrations was recorded in
Cladophora glomerata from Site 7 (southern shore). Even though the concentrations of Zn at most sites, on average, did not exceed 28 mg kg
−1, at Site 7, the concentration reached 47.30 ± 3.70 mg kg
−1. Lead concentrations at the majority of the studied sites did not exceed 7.00 mg kg
−1, but, at Site 7, the concentration was 17.00 ± 1.20 mg kg
−1.
In July, significant Spearman rank correlations were found for Zn in the algae and Zn in the water (R = −0.53).
The ANOVA showed that the Cu concentration in macroalgae in July significantly decreased at all study sites (F (6,14) = 21.566; p = 0.000) compared to that in May. The same pattern was found for Zn (F (6,12) = 5.6357; p = 0.005). The Cd concentration in macroalgal biomass in July was significantly lower at Sites 4 and 9 (F (6,14) = 48.544; p = 0.000), and also the Pb concentration in July was significantly lower at all sites, except Site 10 (F(6,14) = 65.057; p = 0.000), than in May.
3.4. Comparison of Spearman Rank Correlations between Metals in the Pore Water, the Water Column, and Algae
The correlations in May and July are presented in
Table 3 and
Table 4, respectively.
In May, Cu and Cd in the pore water showed the highest correlation with the concentration of these metals in the algal biomass (R = 1). The Zn concentration also showed a very high correlation with the metal concentration in algal biomass (R = 0.98). Also, all of these metals in algal tissues correlated with each other (
Table 3). Zn and Cd in the pore water were negatively correlated with the concentrations of these metals in the sediment. Pb in the pore water was positively correlated with Pb in the sediments, the water, and the algae. Cd and Pb in the water were significantly correlated with Cd and Pb in algal tissues. The Spearman rank correlation was 0.66 for Cd in the water and algae and 0.51 for Pb in the water and algae.
In July, we observed other correlations (
Table 4).
In July, Cu in the pore water was positively correlated with Cu in the water column and with Cd in the water column and algae. However, there was no correlation between Cu concentrations in the pore water and in algae (
Table 4). Zinc in the pore water did not correlate with Zn in the water and algae but had a negative correlation with Cd in the water. A strong positive correlation was observed for Zn in the pore water and copper and lead in the algae. Cadmium in the pore water was strongly negatively correlated with Zn, Cd, and Pb in the water and positively correlated with Cu, Zn, and Pb concentrations in algae (
Table 4). Lead in the pore water had a strong positive correlation with Cu and Pb in algae (
Table 4).
3.5. Results of Principal Component and Classification Analysis
PCCA extracted two main factors that influenced the distribution of metals in the eastern GoF in May (
Figure 7). Factor 1 was positively correlated with Eh and had a negative correlation with salinity. The majority of metals from the water, the pore water, and algae, which are usually regarded as anthropogenic pollution, were positively correlated with Factor 1 and grouped with Eh potential.
Factor 2 is highly positively correlated with Cu, Cd, Zn, and Pb in sediments; this allows us to connect it with the anthropogenic sources of their loading.
In July, the influence of the factor, related to conditions at the water–sediment interface, on metal distribution in the studied environments decreased (
Figure 8, Factor 2) and contributed 25.21% to the total dispersion. However, there were still high correlations with metals in the pore water and metals in the macroalgae. Copper in the pore water had a strong negative correlation with Factor 2. Factor 1 contributed 47.18%; it was positively correlated with oxygen and strongly negatively correlated with water temperature and pH.