Assessment of PCBs in Surface Waters at Ultratrace Levels: Traditional Approaches and Biomonitoring (Lake Baikal, Russia)

Round 1

Reviewer 1 Report

The subject is interesting and methodology is in accordance with modern trends in analytical chemistry. . Sampling and analysis of samples were done with knowledge  of good laboratory practice.

Author Response

The authors are very grateful for your high estimate of our manuscript. 

Author Response File: Author Response.docx

Reviewer 2 Report

I consider this is a good research that should be published, but a few major comments would improve the quality of the research.

The manuscript would benefit from adding some statistics by repeating some of the experiments. E.g., what is the standard deviation on the performed analysis? 

Another important question is that usually the PCB content, in samples such as omul, is expressed per unit of fat content, i.e., per gram of fat. The fat in fish specimens is determined by extraction in different solvents. 

I find it extremely striking the very large amounts of PCBs found in these fish. European legislation indicates that the content should not exceed 2-10 pg/g fat, counting the equivalent toxicity of dioxins and PCBs. However, only with PCBs the authors find values 1000-10000 times higher. This must be explained in detail. 

Minor comment: In Figure 2, there is a mistake '202' should be '2020'

Author Response

The authors are very grateful for your high estimate of our study. The Reviewer’s suggestions on taking into account statistical data are very useful. We have included two tables in Supplementary:  

Table S1. Statistical parameters of the averaged values of the studied PCB concentrations for all samples from Lake Baikal.

Table S2. Relative error statistics for repeated measurements of PCB concentrations in all samples from Lake Baikal.

The data shown in the tables allowed us to estimate the average value and range of PCB concentrations in Baikal water in terms of statistical significance.

To the second important question of the Reviewer concerning the concentrations of PCBs in Baikal omul, we should answer the following. According to the Legally binding documents for the European Union (Eurofins. Dioxins and PCBs in food products: Maximum levels and action levels), the total concentration of indicator PCB congeners (Ʃ₆PCBs, except for indicator congener No 118) in fish muscles as food products should not exceed the level of 75 ng/g ww. In the fish caught in the Polish Baltic fishing area, the concentrations of Ʃ₆PCBs ranged from 1.1 to 36 ng/g (ww), and in the fish from European alpine lakes – from 0.68 to 17 ng/g (ww).

In C. migratorius from Lake Baikal, the average concentration of Ʃ₇PCBs was 5.6 ng/g (ww), ranging from 4.9 to 6.3 ng/g (ww). These values do not exceed the maximum permissible concentrations for Ʃ₆PCBs established for fish as food products. The concentrations of dl-PCBs in the muscles of C. migratorius in the WHO-TEQ values ranged from 0.03 to 0.06 pg/g (ww), which is by two orders of magnitude lower than their maximum permissible concentrations in the fish used as food products. Legally binding documents for the European Union regulate the concentrations of PCB dl-congeners in fish with a maximum permissible concentration of 3.0 pg WHO-TEQ/g (ww).    

It should be noted that in the EU regulations, the concentrations of PCBs are shown for the wet weight of fish muscle tissue (ww), corresponding to the presented data on C. migratorius. C. migratorius belongs to the species of Baikal fish characterised by low lipid content in muscle tissues, less than 4-5%. Therefore, the concentrations of PCBs calculated per the content of lipids in tissues (wl) are characterised by higher values. Nevertheless, the concentrations of Ʃ₇PCBs in C. migratorius calculated per the content of lipids ranged from 68 to 260 ng/g wl, which is lower than in fish from the European alpine lakes with Ʃ₆PCBs ranging from 175 to 1300 ng/g (wl).

The error in Figure 2 has been corrected.

Author Response File: Author Response.docx

Reviewer 3 Report

The present paper reported the PCB under ultratrace level of concentrations in an important regional lake. The study area is of geographical importance. The results are practical for local-scale water management and protection. The methodology is interesting, with both conventional method and bio-indicators applied. The structure and format can be further improved. It is also recommended to polish the language. There's so many long sentences used in the present form, which are hard to read and may somehow lead to confusion for the readers. Specific comments for each part are provided as follows.

Abstract: 

Abstract should be concise and within 200 words (see journal instruction for authors). The authors provided detailed information about the geographical background, sampling, measuring methods and data but did not emphasize their novelty and main findings. It is suggested to re-organize this part and highlight the most important findings. 

Introduction:

Lines 69-71: why do the studies of PoPs in atmosphere cited as an example? As the paper focused on surface water, this example is not necessary to provide.

Line 77: I don't think "According to [3, 42]" is a right format for quotation. Please correct it.

Methods and materials:

The contexts in this section should be divided under several sub-headings, such as "study area", "sampling design", "instrumental analysis" and "statistical methods", etc...

Lines 120-121: Why were these months selected for sampling? It is suggested to describe the sampling design in detail.

Line 180: " from zero to 1" should be written as " from 0 to 1".

Results and discussion:

Figure 2: what are years for the 7-th and 8-th columns? Please complete it in x axis. What are the meanings of different colors? Add a legend to note it. Keep the font and size the same in the figure caption. It is also suggested to classify the ranges into "pelagic zone" and "deep layers" for a clearer comparison.

Lines 212-213: "range from" instead of "ranging from".

Lines 245-251: it's more suitable to move the reasons for fish species to the "M & M" section.

Lines 254-256, 262-268: Keep the font and size the same and follow the journal template.

Lines 264-266: The authors mentioned that " age of fish is a critical biomonitoring factor ", but it's not clear now because only the maximum values were mentioned. Please show the comparison between different ages in a figure or table to make it more straightforward.

Lines 425 & 430: I think the range of r value should be 0.3<r<0.6.

Line 503: Please provide the full name of "EEC".

Conclusion:

Please add some insights or suggestions for PCB monitoring in surface water based on the findings of the study.

Author Response

The authors are very grateful for your high estimate of our study and a thorough review of the manuscript.

English in the text of the manuscript has been professionally proofread and corrected.

All noted small errors have been amended.

The text of the Abstract has been edited and does not exceed 200 words. The new version of the Abstract has been included in the manuscript.

Introduction:

Lines 69-71. In [30-34], fish were used as indicators of PCBs in surface waters with a background level of pollution, which was associated with global atmospheric transport. We have edited this paragraph.

The dependence of the levels of PCB accumulation in fish on the concentrations of these substances in the water was used to assess temporal and spatial variations in surface waters, investigate vertical transport of pollutants from the surface to deep water layers [19, 28, 29] and estimate the content of pollutants at the background level of concentrations [30- 34].

Line 77. The text has been corrected.

As shown in [3, 42], the number of PCBs in the environment is currently not decreasing but even increasing in some regions of the world.

The Materials and Methods section has been divided into several sub-headings.

Lines 120-121. The following reasons determined the choice of the seasons of samplings of water and C. migratorius. In spring, after ice melt, the influx of PCBs in the lake water is associated with the melting of the ice on the water surface of the lake and the snow on the coast, in which PCBs have been accumulated during winter. Water samples collected in autumn reflect the accumulation of PCBs in the water, which have come from the atmosphere during summer. These details have been highlighted in the text.      

Line 180. Corrected.

Results and discussion:

Figure 2: Figure caption and the figure itself have been edited.

Lines 212-213. Corrected.

Lines 245-251: This paragraph has been edited.

Lines 254-256, 262-268. Corrected.

Lines 264-266. The age of fish is a critical biomonitoring factor because specimens of the older age group have higher PCB concentrations [31]. During the biomonitoring, we chose individuals of C. migratorius aged from three to seven years. In this group of fish, the level of concentration of Ʃ₇PCBs changes insignificantly: ∑₇PCBs were 3.9 (3 year), 4.9 (5 year) and 5.4-5.6 (6-7 years), ng/g (ww), and the proportion of congeners with a high degree of chlorination (Nos, 153, 138, 180) increased.

The accumulation level and the composition of PCB congeners in fish depend on their age: Vives et al., 2005, [31]. For the species C. migratorius this dependence is also the same. During four years of the life of C. migratorius, there could be up to a three-fold increase in the concentrations of indicator PCB congeners in muscle tissue: ∑₇PCBs were 4.7-5.8 (3-4 years), 6.0-9.9 (5-6 years) and 14-15 (7 years), and the proportion of congeners with a high degree of chlorination increased.

Lines 425 & 430: Corrected.

Line 503: Corrected.

The text of Conclusions has been edited, and sub-heading 3.4. Opportunities for monitoring has been added to Results and Discussion.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

All my previous suggestions have been included.