In the Sphagnum mosses sampled in Germany and Slovenia in 2019 and 2020, the contents of the major elements are below 1%. They are highest for Ca with a mean value of 0.3%, followed by Al and Fe, both with similar ranges of values and mean values close to 0.05%. The contents of Ba, Pb, Sr, Ti and Zn range from a few mg/kg to over 100 mg/kg for Pb. On average, the contents of Ba, Sr and Ti are about 10 mg/kg, Pb is 20 mg/kg and Zn is 40 mg/kg. The contents of Cr and Cu average around 5 mg/kg, Ni around 2 mg/kg. The mean values of Cd, Co, La, Mo, Nd and Sc are less than 0.5 mg/kg. Average Hg content is around 30 μg/kg. Due to the generally low absolute values, relative comparisons of the contents may give the misleading impression of large differences, even if the absolute differences are only a few mg/kg.
4.1. Within bog variability
Sphagnum species generally behave similarly with respect to the uptake of atmospheric trace metal inputs, but measured contents are often highly variable due to the influence of various factors. The large variability within bogs can make it difficult to confirm statistically significant differences among bogs, areas, and regions. We first investigated the variability due to different moss species and visually checked the variability within peatlands using Box-Whisker diagrams (Fig. 2).
In Germany, we took two parallel samples of the same moss patches at two sites in Acker bog (Ge-A01-20 and Ge-A02-20). In Slovenia, at two sampling sites (Sl-A01-19 and SI -A02-19) in Veliko Blejsko barje on Pokljuka, we sampled two Sphagnum species in close proximity, distinguished only by different colours - green and red.
Parallel samples from the same moss patches have similar chemical composition in terms of the contents of Al, Ca, Ba, Hg, Mo, Sc and Sr. In the case of sampling site Ge-A01-2020, Cd, Zn and Pb have quite different contents. Sampling site Ge-A02-20 is much more variable in terms of the list of elements and their degree of variability. The measured contents of Ba, Cd, Cr, Cu and Zn show less differences, but for Fe, La, Nd, Ni, Ti and Pb the variability is high.
Comparison of chemical composition of two different Sphagnum species shows quite low variability of Cr, Cu, Hg, Mo, Ni, Pb and Sr. For Al, Fe, Ba, Co and Ti the variability is low at the sampling site Sl-A01-19 but higher at the other one. At the sampling site SI -A02-19, the variability is low for Ca, Cd and Zn and higher for other elements.
A comparison of the ranges of element contents (Fig. 2) shows that the variability within the bogs is basically larger in 2019 than in 2020. The elements reaching the maximum content ranges are also different. In general, larger ranges of element contents are observed in Germany. The variability for most elements is always highest in Broken (all elements except Ba, Hg and Sc in 2019 and Ca, Hg, Sc and Sr in 2020). In the bogs at Acker, variability was high only for Co, Pb and Zn in 2019, but in 2020 for most elements except Ca, Ba, Hg, La, Mo, Sc and Sr. In Slovenia, variability was high in 2019 for Cr in Veliko Blejsko barje, for Ca and Mo in Šijec, for Fe in Goreljek and for Sr in Črno jezero. In 2020, variability is high for Ca, Cu and Hg in Veliko Blejsko barje, for Fe, Co and Mo in Šijec, for Cr, Hg, Sr and Zn in Črno jezero and for Fe, Cr, Cu, Mo and Ni in Lovrenška jezera. The variability of Sc is low, but greater in 2020, where, unlike the majority of other elements, the contents are also higher.
4.2 Spatial comparison
Comparison of peatlands using Kruskal-Wallis analysis of variance showed different results for 2019 and 2020. Statistically significant differences between bogs were found in both years for Ba and Zn contents, mainly due to high values of Ba in both German bogs and in 2020 also in Lovrenška jezera, and Zn in both German bogs. In 2019, the bogs also differed statistically significantly in the contents of Al, Cd, Co, Cr, Cu, La, Nd, Ni, Pb and Ti. Higher contents are observed in the two German bogs, especially in bogs around mount Brocken (Fig. 2). In 2020, differences were additionally confirmed only for Hg, which was low in the Acker and Brocken bogs and high in both Pohorje bogs. In contrast to 2019, in 2020 the levels of most elements were higher in Acker than in Brocken. In addition, although not statistically significant, the contents of Cr, Mo and Ni were highest in Pohorje bogs.
The Kruskal-Wallis analysis of variance comparing bogs within the three areas in 2019 and 2020 (Harz, Pokljuka, Pohorje) shows statistically different contents of certain elements only in Harz. In 2019, the contents of Al, Cr, La, Nd and Ti are significantly higher on the Brocken. In 2020, only Ba and Co show significantly higher contents in the Acker bogs.
The general comparison between German and Slovenian bogs performed with the Mann-Whitney test gives different results for the years considered. Again, the countries differ in many more elements in 2019 than in 2020. In both years, differences are indicated only for Ba, Cd, Cu, Nd and Zn, which are higher in German mosses. In 2019, German mosses also show significantly higher contents of Al, Co, La, Mo, Ni, Pb and Ti. In 2020, Slovenian mosses are significantly enriched in Hg compared to German mosses.
4.3 Effect of the COVID -19 lockdown on the chemical composition of mosses
Comparison of the chemical composition of Sphagnum mosses sampled in Germany and Slovenia in 2019 and 2020, performed with the Mann-Whitney test for the whole dataset, showed a statistically significant decrease in Cu (1.5-fold for the mean and 2.6-fold for the median) and an increase in the lithophilic elements, Ni (2-fold for the mean and 2.4-fold for the median) and Sc (2.8-fold for the mean, 2.3-fold for the median). Comparisons for some potentially highly toxic elements are shown in Fig. 3.
The results calculated separately for each country are different, except for Sc. In Harz decrease is confirmed for Cu (2.4-fold for the mean and 6.2-fold for the median), Hg (3-fold for the mean and 3.8-fold for the median) and Pb (about 7.7-fold for the mean and 11.5-fold for the median). In Slovenia, only Fe (about 1.6-fold for mean and median), Cr (2.8-fold for mean and 2-fold for median), Ni (3.1-fold for mean and 2.4-fold for median), Sc (about 2.85-fold for mean and median), and Ti (about 1.75-fold for mean and median) were found to increase. Although not statistically confirmed, the contents of all elements except Co, Ni, Sc and Ti also decreased at least slightly in Germany, and Ca, Ba, Cd, Pb, Sr and Zn in Slovenia. Despite lower median values for PTE in 2020, the variability and maximum contents of Co, Cr, Ni, Sc and Ti are higher in both countries, and additionally Cu, Hg, La, Mo and Nd in Slovenia.
Comparison of Sc-normalised contents of the most potentially hazardous elements for the whole data set (Fig. 4) and for each sampling site generally shows a decrease in the ratio for all elements, indicating a decrease in pollution, which is also confirmed by Mann-Whitney tests for the whole data set (except Cr/Sc and Ni/Sc), Germany (except Fe/Sc, Co/Sc, Cr/Sc, La/Sc, Nd/Sc, Ni/Sc and Ti/Sc) and Slovenia (except Cr/Sc and Ni/Sc). Although not statistically significant, the medians in 2020 are lower than in 2019 in all cases, but in particular for Cr and Ni the ranges and maximum values are higher in 2020. The only elevated ratios in 2020 are Co, Cr and Pb in one sample from Acker and Cr and Ni in all four Pohorje samples.
From enrichment factors in Table 1 it can be seen that Al, Ca and Fe are strongly depleted in Sphagnum mosses compared to the upper continental crust (UCC); neglecting the maximum values, which are often due to some outliers, Ba, Co, La and Nd are only slightly enriched compared to the UCC, and Cr, Ni, Sc and Sr differ up to 30-fold. Extremely large deviations from the composition of the UCC are characteristic for Cd, Hg, Mo, Pb, and Zn, and to a lesser extent for Cu. In 2020, the mean, median, minimum, and maximum contents for most elements are lower than those observed in 2019. The exceptions are higher mean values of Co, Cr, Ni, Sc, and Sr. For Cr, Ni, and Sc, the median and maximum values are also higher in 2020. At the level of the whole dataset comparing only samples from the same sites, the decrease in EFs from 2019 to 2020 is statistically significant for Ba, Cd, Cu, La, Mo, Nd, Pb and Zn, and the increase for Sc. For the German mosses, EFs of all elements except Sc and Ti show a decrease, but it is not statistically significant. For the Slovenian mosses EF values of Ba, Cu, La, Nd and Zn are significantly lower and Sc higher in 2020 compared to 2020.
Table 1. Mean, median, minimum and maximum enrichment factors (EF) for German and Slovenian Sphagnum moss samples, sampled at the same locations in 2019 and 2020.
|
Year
|
Average
|
Median
|
Min
|
Max
|
|
|
Year
|
Average
|
Median
|
Min
|
Max
|
Al
|
2019
|
0.00024
|
0.00034
|
0.00012
|
0.00235
|
|
La
|
2019
|
4
|
5
|
2
|
78
|
|
2020
|
0.00021
|
0.00022
|
0.00004
|
0.00106
|
|
|
2020
|
4
|
4
|
1
|
39
|
Ca
|
2019
|
0.0052
|
0.0092
|
0.0007
|
0.0262
|
|
Mo
|
2019
|
198
|
249
|
39
|
453
|
|
2020
|
0.0035
|
0.0048
|
0.0006
|
0.0154
|
|
|
2020
|
190
|
126
|
13
|
305
|
Fe
|
2019
|
0.0006
|
0.0007
|
0.0002
|
0.0012
|
|
Nd
|
2019
|
5
|
5
|
2
|
66
|
|
2020
|
0.0006
|
0.0006
|
0.0001
|
0.0011
|
|
|
2020
|
4
|
4
|
1
|
31
|
Ba
|
2019
|
10
|
20
|
2
|
30
|
|
Ni
|
2019
|
12
|
22
|
6
|
39
|
|
2020
|
7
|
7
|
1
|
19
|
|
|
2020
|
43
|
19
|
4
|
69
|
Cd
|
2019
|
3837
|
3368
|
365
|
40928
|
|
Pb
|
2019
|
155
|
599
|
66
|
6562
|
|
2020
|
805
|
841
|
320
|
22063
|
|
|
2020
|
95
|
116
|
35
|
879
|
Co
|
2019
|
5
|
12
|
3
|
39
|
|
Sc
|
2019
|
14
|
14
|
2
|
41
|
|
2020
|
7
|
8
|
4
|
20
|
|
|
2020
|
27
|
23
|
3
|
64
|
Cr
|
2019
|
24
|
25
|
6
|
54
|
|
Sr
|
2019
|
10
|
24
|
2
|
88
|
|
2020
|
56
|
28
|
3
|
103
|
|
|
2020
|
13
|
11
|
1
|
53
|
Cu
|
2019
|
88
|
151
|
37
|
414
|
|
Zn
|
2019
|
312
|
511
|
105
|
888
|
|
2020
|
7
|
24
|
4
|
289
|
|
|
2020
|
188
|
203
|
60
|
663
|
Hg
|
2019
|
460
|
549
|
54
|
1074
|
|
|
|
|
|
|
|
|
2020
|
375
|
351
|
11
|
742
|
|
|
|
|
|
|
|
The decrease in the contents of PTE in 2020 is also evident by directly comparing the levels of EF from 2019 and 2020 at each sampling site (Fig. 5). In 2020, Cd shows an increase of EF in one sample from Veliko Blejsko barje, Cr shows a slight increase in one sample from Brocken and one from Goreljek and a huge increase in all Pohorje samples, Cu in one site Veliko Blejsko barje, Hg and Pb in one site in Veliko Blejsko barje and in one in Šijec. In Veliko Blejsko barje EF of Zn is also slightly elevated in one of the Veliko Blejsko barje replicate sites.