Macroplastic Storage and Remobilization in Rivers

Round 1

Reviewer 1 Report

The authors present a useful and well-written framework for the study of macroplastic storage and mobilization in rivers. I recommend acceptance after a minor revision that responds to the observations below.

Figure 1: Although I like this illustration, it is not immediately clear that the figure regards the cross-section of a river valley rather than the cross-section of a small stream. An additional clarification in the figure caption might help the reader.

Lines 129-131, "their importance as input factors seems to be lower than that of humans, especially in populated areas and where dense riparian vegetation occurs along a river course": Why is dense riparian vegetation mentioned as an input factor? Isn't it rather a storage factor?

Figure 2: "gryones" must be "groynes".

Line 143, "The occurrence and intensity of this phase [= transport phase] depends primarily on fluvial processes (e.g., flood occurrence, local river hydrodynamics) which influence the movement of plastic debris by river water": I disagree, in analogy to considerations regarding sediment transport. The transport of sediment can be divided in supply-limited transport (depending on how much comes into the river from upstream) and capacity-limited transport (depending on flow strength which determines the capacity of the flow to transport sediment). Generally, the transport of clay and silt is supply-limited whereas the transport of sand and gravel is capacity-limited. Translating this insight to macroplastics, I find it hard to believe that the movement of plastic in rivers is capacity-limited, as suggested by the sentence in line 143. I agree that flood occurrence may be a factor, as it determines the duration and frequency of exposure, but mentioning hydrodynamics (flow strength) in this context seems inappropriate.

Lines 180-182, "The zone of potential macroplastic storage can be assumed to be the areal extent of maximum floodplain inundation since the 1960s (Figure 1B), because only in this zone plastic debris might have been transported and deposited by river flow". I find this statement rather bold. After fluvial deposition on floodplains, wind could transport macroplastics outside the zone of maximum floodplain inundation since the 1960s, allowing storage in vegetation. The statement would become more accurate when adding "and covered by sediment deposition".

Lines 198-199, "modifications of riparian vegetation, which change floodplain roughness [e.g., 60], will modify the potential for macroplastic storage": I guess the trapping of macroplastics by vegetation will depend not only on hydraulic roughness but also, as a separate factor, on plant structure. Moreover, for flow over a vegetated floodplain, the relation between bed shear stress and overall hydraulic roughness is not straightforward. This will affect the bedload transport of macroplastics (and sediment) too (and hence the potential for storage).

Line 264, "m3 of the volume of storage zone": This would require insight in the thickness ("active layer") of the storage zone. How could this thickness be determined reliably, at different time scales?

Table 1 is presented as a general quantification method, but actually appears to relate to a single specific lake with a 390 m long shoreline (~ 7.8 km being mentioned as 2% of the total length). The recommended 3 days for surveying surface sediments relates to a specific lake size too; larger lakes might require more days; smaller lakes might require less days. The table needs to be either generalized or presented as a specific case study, as an example.

 

Author Response

The reply to the reviewers is attached in the separate file.

Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript presents a comprehensive framework for studying macroplastics in rivers, covering inputs, outputs, transport, storage, and remobilization processes.  The conceptual framework is scientifically reasonable, and has the advantage of being more holistic than existing approaches which are dominated (to my knowledge) by transport analysis.  The manuscript is quite readable, but contains a lot of small stylistic mistakes in English. Given the timeliness of the topic, and its likely interest to readers of Water, my recommendation is for the manuscript to be accepted as a concept paper, once the authors have addressed the following points:

• The manuscript contains a large number of small grammatical and stylistic errors in English.  
• The authors should discuss how best to incorporate uncertainty propagation in their conceptual framework.
• The manuscript sets out a roadmap for improved representation of the whole life processes affecting macroplastics in fluvial systems.  Could this be incorporated in a holistic sustainability model, perhaps using life cycle assessment?

Minor corrections

P1. L15.  Change to “… (2) transport, (3) storage, …”

P1. L32.  Change to “Knowledge of riverine …

P2. L46.  Change to “Taking into account the long …”

P2. L47. Change “… its storage–remobilization cycles in fluvial systems may …” to “… the storage–remobilization cycles of macroplastic debris in fluvial systems may …”

P2. L49.  Change to “… when input of new plastic debris …”

P2. L50.  Change to “Macroplastic storage and remobilization processes are overlooked in the literature …”

P2. L53.  Change to “… targeting of cleanup actions …”

P2. L53.  Change to “… management of present and future remobilization of plastic, …”

P2. L54.  Change to “… and (v) the influence of macroplastic debris on …”

P2. L55.  Change to “From this perspective, precise information is still required on the spatial …”

P2. L57.  Change to “… and riparian vegetation, and estimation of the of the potential for macroplastic remobilization …”

P2. L60.  Change “… which creates a …” to “… thus creating a …”

P2. L62.  Change to “… scheme for idealising macroplastic storage …”

P2. L64.  Change to “We therefore developed a framework …”

P2. L67.  Change to “We use this model to identify the main controlling factors …”

P2. L69.  Change to “… which may be used for the quantification of the controlling factors in future studies.”

P3. L100.  Change to “However, once macroplastic starts to be transported …”

P3. L103.  Change to “… in a similar way as documented for …”

P3. L110.  Change to “… we demonstrate that changes in main …”

P3. L111.  Change to “… (input of macroplastic) are  mainly controlled by …”

P3. L113.  Change to “Table 1 summarizes methods for measuring microplastic …”

P4. L120.  Change to “… terraces), facilitating their immediate …”

P4. L122.  Change to “Input of plastic debris to fluvial systems is primarily controlled …”

P5. L142.  Change to “… input to an active river zone …”

P5. L148.  Change to “… a given part of the macroplastic storage zone …”

P5. L149.  Change “… similarly as it was previously found for heavy …” to “… as was similarly found for heavy …”

P5. L150.  Change to “The flow hydrodynamics has been suggested to act as an important …”

P6. L154.  Change to “… vegetation, as has also been found for large wood pieces …”

P6. L156.  Change to “… which causes its particles to become mechanically fragmented …”

P6. L157.  Change to “… a higher potential for further transport …”

P6. L168.  Change to “To date, the transport of …”

P6. L169.  Change to “… particles from aerial photographs taken by …” The word ‘photo’ is colloquial in English.

P6. L169.  Change to “Macroplastic debris transport was also measured …”

P6. L172.  Change to “… combination of a visual counting method …”

P6. L172.  Change “Some insight into …” to “Insight into …”

P6. L178.  Change to “… became deposited on the surface of river sediment or in vegetation (surface storage) or accumulated in river sediment below the river bed …”

P6. L182.  Change to “… might plastic debris have been transported …”

P6. L182.  Change “The storage occurrence …” to “Storage occurrence …”

P6. L184.  Change to “… which control deposition.”

P6. L186.  Change to “… or covered with mineral and organic sediments, and become stored without …”

P6. L191.  Change to “… would facilitate the covering of surface-stored macroplastic with mineral and organic sediments, and its transformation into mineral …”

P6. L194.   Change to “… plastic debris influence their transport mode …”

P6. L196.  Change to “From this perspective, …”

P6. L199.  Change “… will modify the potential …” to “… will alter the potential …”

P7. L205.  Change to “… lateral extent of the storage zone should increase because of a wider extent …”

P7. L210.  Change to “Previous works on surface and …”

P7. L212.  Should “quadrats” be “quadrants”?

P7. L219.  Change to “… samples in a way that allows the mass of plastic in a given sediment volume to be established [see …”

P7. L221.  Change to “… should also be useful in future studies utilizing samples …”

P7. L223.  Change to “… may be implemented using existing approaches …”

P7. L224.  Change to “… giving an opportunity to study …” to “… making it possible to study …”

P7. L225.  What is meant by “it” in “… and to compare it with similar older deposits.”?

P7. L226.  Change to “… approach could also enable calculation of the total amount …”

P7. L229.  Change to “… adaption of certain field experiments …”

P7. L230.  Change to “… studies of seed deposition …”

P7. L233.  Change to “… from aerial photographs taken …”

P7. L234.  Wording of “… in non-vegetated parts of river sediments …”

P7. L236.  Change to “During the storage phase, plastic particles are degraded …”

P7. L238.  Change to “… but produce a certain quantity of microplastic which …”

P7. L247.  Change to “The timescale of the …”

P8. L258.  Change to “… (e.g., aerial photographs) …”

P8. L274.  Change to “Similar to the transport phase, …”

P8. L276.  Change to “… a lower possibility of multiple cycles of transport–storage–remobilization occurring …”

P8. L281.  Change to “… In turn, river estuaries have been identified as possible temporary …”

P8. L282.  Change to “Similar to the transport phase, the output phase …”

P8. L286.  Change to “Table 1. Methods used …”

P9. Table 1.  Change “Emerged and …” to “Emergent and …”

P9. Table 1.  Should “quadrats” be “quadrants”?

P9.  L291.  Change to “… which have been overlooked to date in the literature.”

P10. L298.  Change to “The type and number of these controls …”

P10. L300.  What is meant by “those active” in “… additionally influence those active at smaller units …”

P10. L304.  Change to “… (e.g., aerial photographs) …”

P10. L304.  Delete “… since that time …”  This doesn’t help the meaning of the sentence.

P10. L306.  Change to “Information about …”

P10. L306.  Change to “… sediments of the storage zone …”

 

Author Response

The reply to the Reviewer Report is attached in separated file.

Author Response File: Author Response.docx