Influence of a small submarine canyon on biogenic matter export flux in the Lower St. Lawrence Estuary, eastern Canada

Sharpe, Hannah; Gosselin, Michel; Lalande, Catherine; Normandeau, Alexandre; Montero-Serrano, Jean-Carlos; Baccara, Khouloud; Bourgault, Daniel; Sherwood, Owen; Limoges, Audrey

doi:https://doi.org/10.5194/egusphere-2023-1538

Preprints

https://doi.org/10.5194/egusphere-2023-1538

Preprints

17 Jul 2023

| 17 Jul 2023

Influence of a small submarine canyon on biogenic matter export flux in the Lower St. Lawrence Estuary, eastern Canada

Hannah Sharpe, Michel Gosselin, Catherine Lalande, Alexandre Normandeau, Jean-Carlos Montero-Serrano, Khouloud Baccara, Daniel Bourgault, Owen Sherwood, and Audrey Limoges

Abstract. Submarine canyons enhance shelf-slope sediment exchange and influence hydrodynamic processes, with consequences for biogeochemical cycles. This work documents variations in the vertical export of biogenic matter on the northern shore of the Lower St. Lawrence Estuary (LSLE, Quebec, eastern Canada), which is characterized by the presence of an active submarine canyon system. A total of three moorings were deployed from November 2020 to September 2021. One nearshore mooring was deployed in the main axis of the Pointe-des-Monts (PDM) canyon system and was equipped with an Acoustic Doppler Current Profiler (ADCP), and two moorings equipped with sediment traps were deployed in the distal PDM canyon system and offshore Baie-Comeau (BC). The ADCP data revealed the occurrence of a minor sediment remobilization event (December 2020) and a small turbidity current (February 2021) in the canyon. Concurrent elevated fluxes of total particulate matter, particulate organic carbon, particulate nitrogen, and chloropigments showed that these events left a signature in the distal PDM sediment trap located >2.6 km further offshore. The composition of diatom and dinoflagellate assemblages was similar in the canyon system and offshore BC, but the diatom bloom occurred two weeks earlier (in mid-April) at the PDM site, where annual diatom and dinoflagellate fluxes were almost 2 times lower than at the BC site. A bloom of the potentially toxic diatom Pseudo-nitzschia seriata was also observed during the second half of September 2021 at the BC site. This study notably helps identify the relationship between near-bed canyon processes and biogenic matter export in the water column, thereby directly influencing the regional ecosystem. The study period further covered an anomalously nearly ice-free winter and thus, in the context of climate change, provides valuable insight into future trends of biogenic matter export in the LSLE.

Received: 06 Jul 2023 – Discussion started: 17 Jul 2023

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Journal article(s) based on this preprint

18 Dec 2023

Influence of a small submarine canyon on biogenic matter export flux in the lower St. Lawrence Estuary, eastern Canada

Hannah Sharpe, Michel Gosselin, Catherine Lalande, Alexandre Normandeau, Jean-Carlos Montero-Serrano, Khouloud Baccara, Daniel Bourgault, Owen Sherwood, and Audrey Limoges

Biogeosciences, 20, 4981–5001, https://doi.org/10.5194/bg-20-4981-2023,https://doi.org/10.5194/bg-20-4981-2023, 2023

Short summary

Hannah Sharpe et al.

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1538', Kyung-Hoon Shin, 16 Sep 2023
General comments: The paper investigates the impact of a submarine canyon system on the vertical export of biogenic matter in the Lower St. Lawrence Estuary, finding that sediment remobilization events and turbidity currents in the canyon influenced the flux of particulate matter and chloropigments in the water column. The study also reveals differences in diatom and dinoflagellate assemblages between the canyon system and an offshore site, with a diatom bloom occurring earlier at the canyon site. The presence of a potentially toxic diatom species was observed at the offshore site. I think this paper is a useful contribution to the literature and worthy to be published in Biogeosciences. However, I have some comments mainly about the clarity of the manuscript. I recommend the journal to accept this work after a moderate revision.

Specific comments:
Line 44: (Normandeau et al., 2014, 2020, 2022) à (Normandeau et al., 2015, 2020, 2022)

Line 329: Calvin pathway Meyers, 1994; Macdonal et al., 2004) à Calvin pathway (Meyers, 1994; Macdonal et al., 2004)

Line 347: Are there any other primary production proxies mentioned here besides chloropigments?

Table B2: symbol check! in table B2

Major comments:
What are the conditions for determining trap depth?

What is the reason for the decrease in water temperature and salinity in December at both sites?

Are the large waves that occur every 2-3 years related to global ocean-atmosphere climate phenomena?

What is the time lag for organic matter produced in the surface layer to reach the depth of the sediment trap? Can I consider that the sinking particles obtained in each month fell during that same month?

Why does diatom bloom occur first, followed by the dinoflagellate later?

Lines 386-390: Isn't the depth of the mixed layer greater during the fall and winter months rather than the impact of resuspension? What do you think/ or other possibility?

Why do they exist only as cysts for Brigantedinium spp. and Selenopemphix quanta?
Citation: https://doi.org/10.5194/egusphere-2023-1538-RC1
- AC1:
  'Reply on RC1', Hannah Sharpe, 16 Oct 2023
  General comments:
  1) We thank the reviewer for their positive comments and for contributing to improving the manuscript. Your comments have been considered in the revised version.
  Specific comments:
  1) We have added both 2014 and 2015 to the references, thank you.
  2) We corrected the reference, thank you.
  3) In our study, primary production proxies at the Pointe-des-Monts site include chloropigments, diatoms and dinoflagellates. To avoid confusion, we have clarified this sentence in the revised version of the manuscript, which now reads “[…], but we note no correlation to other primary production proxies (i.e., fluxes of chloropigments, diatoms and dinoflagellates).”
  4) We have changed the * symbol to [].
  Major comments:
  1) Trap depths were determined based on several considerations:
  All traps were located within the deep layer.
  
  The sediment traps PDM-224 and BC-133 were located 58 and 67 m above seafloor, respectively, which is well above the benthic nepheloid layer (i.e., layer of water above the seafloor that contains significant amounts of suspended sediment), which is estimated to be about 10 m above the seafloor in our study area (Bourgault et al., 2014; Casse et al., 2019).
  
  In the main canyon system, the ADCP was located 27 m above the seafloor (156 m water depth) to be consistent with to methodology used in Normandeau et al. (2020). The PDM-154 sediment trap was placed at a similar water depth to the ADCP.
  
  BC-133 was located at a comparable distance above seafloor as PDM-224 while also being at a comparable depth below the surface as PDM-154.
  
  To clarify this point, in the revised version of the manuscript we have added:
  Lines 109-110: “Traps BC-133 and PDM-224 were positioned 67 m and 58 m above seafloor, respectively, above the estimated benthic nepheloid layer (i.e., <250 m of water depth; Bourgault et al., 2014; Casse et al., 2019).”
  Bourgault, D., Morsilli, M., Richards, C., Neumeier, U., and Kelley, D. E.: Sediment resuspension and nepheloid layers induced by long internal solitary waves shoaling orthogonally on uniform slopes, Continental Shelf Research, 72, 21-33. http://dx.doi.org/10.1016/j.csr.2013.10.019, 2014.
  Casse, M., Montero-Serrano, J.-C., St-Onge, G., and Poirier, A.: REE distribution and Nd isotope composition of estuarine waters and bulk sediment leachates tracing lithogenic inputs in eastern Canada, Marine Chemistry, 211, 117-130. https://doi.org/10.1016/j.marchem.2019.03.012, 2019.
  2) The sediment traps BC-133 and PDM-154 are within the deep layer and temperature and salinity fluctuations are not directly associated with surface conditions. A decrease in water temperature and salinity at these depths could be due to:
  A greater contribution of waters from the Labrador Current (lower temperatures and salinities) than from the North Atlantic waters (higher temperatures and salinities).
  
  Vertical mixing of the cold intermediate layer (lower temperatures and salinities) with the deep layer (higher temperatures and salinities).
  
  However, with the data collected as part of the present study, we cannot determine the cause of the decrease in water temperature and salinity in December 2020.
  3) Normandeau et al. (2020) reported that the large turbidity currents that occur every 2-3 years were initiated during storms that exhibited sustained (>7 h) high windspeeds (>60 km h^-1), which caused large storm waves. At or near low tide, these storm waves subsequently triggered a turbidity current. In the context of global climate warming, it can be expected that a shortening of the sea ice season and increased storm frequency may favor the development of turbidity currents.
  4) We thank the reviewer for raising this important point and have added in the revised version of the manuscript a brief explanation at the beginning of section 5.3 to address this:
  “The sinking rate of biogenic matter is highly variable depending on shape, size, density, aggregation, remineralization, etc. For example, sinking rates of individual diatoms range from 0.1 to 10 m d^-1 (Smayda, 1970) while aggregates can sink at rates of 88 to 569 m d^-1 (Iversen et al., 2010). Sinking biogenic matter generally form aggregates (“marine snow”) to facilitate vertical fluxes of pelagic particles, therefore, we can assume that the sinking particles obtained in the sediment traps are representative of their respective sampling time periods.”
  Iversen, M. H., Norwald, N., Ploug, H., Jackson, G. A., and Fischer, G.: High resolution profiles of vertical particulate organic matter export off Cape Blanc, Mauritania: Degradation processes and ballasting effects, Deep-Sea Research I, 57, 771–784, doi:10.1016/j.dsr.2010.03.007, 2010.
  Smayda, T.: The suspension and sinking of phytoplankton in the sea, Oceanography Marine Bulletin, 8, 353–414, 1970.
  5) Diatoms are the most abundant phytoplankton in the Lower St. Lawrence Estuary and almost all diatom taxa are autotrophic, whereas dinoflagellates are the second most abundant phytoplankton and approximately half the dinoflagellate taxa are heterotrophic (micrograzers). Diatoms bloom first due to higher growth rates, lower light requirements (particularly for pennate diatoms which bloom before centric diatoms), and higher nutrient requirements, particularly nitrate and dissolved silicon. The diatom bloom will end when the surface mixed layer becomes limited in nutrients and dinoflagellates will then bloom because the heterotrophic taxa are able to feed on diatoms and other primary producers. We very briefly mention this pattern in the second paragraph of section 5.3. This has been well established in the region therefore we do not believe it is necessary to explain in depth in our manuscript.
  6) We thank the reviewer for raising this point. The mixed layer deepens from fall into winter and then becomes shallower again in the spring. In lines 386-390, our intention was not to address the question of why primary production is lower in the winter, but rather to evaluate if resuspension events could affect the surface mixed layer when primary production would be more impacted. We have rephrased this in the revised manuscript (lines 393-397) to increase clarity:
  “However, our data do not permit to evaluate if lofted sediments also reached the surface layer where they would have decreased primary production. Additionally, turbidity currents and other sediment remobilization events appear to be more frequent from late fall through winter, therefore we cannot determine if sediment lofting would play a key role in this system throughout the entire year, particularly from late spring through early fall, when primary production is greatest.”
  7) Only about 20% of total dinoflagellate taxa are known to produce organic resting cysts. Of the 16 motile dinoflagellate taxa observed in our study, four are currently known to produce resting cysts: Pentapharsodinium dalei, Protoperidinium americanum, Protoperidinium conicum, and Protoperidinium spp. In the present study, we observed cysts of the last three taxa but did not observe cysts of P. dalei, which we know are abundant in the regional surface sediments. We do not believe that preservation is an issue because other sediment trap studies using formalin as a preservative have observed cysts of P. dalei (e.g., Heikkilä et al., 2016). It is possible that we did not observe cysts of P. dalei because they were outnumbered by other dominant taxa (below detection).
  Heikkilä, M., Pospelova, V., Forest, A., Stern, G. A., Fortier, L., and Macdonald, R. W.: Dinoflagellate cyst production over an annual cycle in seasonally ice-covered Hudson Bay, Marine Micropaleontology, 125, 1-24, http://dx.doi.org/10.1016/j.marmicro.2016.02.005, 2016.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1538-AC1
RC2:
'Comment on egusphere-2023-1538', Anonymous Referee #2, 10 Oct 2023
General Comments
Sharpe et al. explored the vertical transport of organic matter in the St. Lawrence Estuary under the influence of a small near-bed submarine canyon system. Submarine canyons are established as important conduits for sediment accumulation through features like episodic turbidity currents, and thus play a key role in material exchange between upper and deep ocean. The authors presented a 1-year continuous record of export fluxes close to the small canyon feature, comparing observations to a distal reference station outside the influence of canyon processes. Results showed that the organic particle fluxes were anomalously greater at the canyon station following sediment remobilization events, providing strong evidence that the canyon system impact existed. In addition, primary productivity appeared to be suppressed by the small canyon processes likely due to sediment lofting causing declined light availability. The authors also speculated that the declining sea ice cover and prolonged ice-free season could cause increased sediment remobilization events but uncertain impact on the ecosystems, pointing to the necessity for more frequent observations. The topic of this study is interesting because the impact of smaller submarine canyon systems on the water column biogeochemistry has not been well documented, and I believe this study would help address this knowledge gap and potentially be of interest to the readers of Biogeosciences. I also think the experiment is nicely designed and the manuscript well written in general. I only have a few minor comments, and I would like to recommend this manuscript for publication if those minor technical comments were properly addressed by the authors.
Specific comments
Line 56: the Baie-Comeau station. If BC-133 is not affected by any sediment remobilization events whatsoever as the “control” of this study, I think the authors should state it somewhere at the beginning.

Line 176: add a “.” after “(Fig. 3)”.

Line 203: delete “slightly” or replace “slightly” with “evidently”. It is problematic to say PDM-224 is only "slightly" higher than PDM-154, at least not in terms of chloropigments in Fig. 5.

Line 289-290: replace “are likely occurring” with “likely occur”.

Line 383: “limiting light availability”: what makes it different in larger submarine canyon systems from smaller ones in terms of phytoplankton growth-limiting factors (such as conditions with light and nutrient)? I.e., Why do turbidity currents enhance productivity in large canyon systems despite reduced light availability?

Line 406: Section 5.5. This section could have been written in a more organized way. It would be less confusing for the readers if the variables in comparison and the corresponding numbers were presented in a dedicated table. For example, Line 413-414, what are “much greater”, and what are the two numbers respective to?
Citation: https://doi.org/10.5194/egusphere-2023-1538-RC2
- AC2: 'Reply on RC2', Hannah Sharpe, 16 Oct 2023
  
  General comments:
  1) We thank the reviewer for their positive comments and for contributing to improving the manuscript. Your comments have been considered in the revised version.
  Specific comments:
  1) We have clarified this in the revised version of the manuscript, which now reads “To identify canyon-specific processes, a sediment trap was also deployed offshore Baie-Comeau (BC), a site not affected by canyon-related sediment remobilization events, to contrast with biogenic matter export in the LSLE (Fig. 1c).”
  2) We corrected the sentence, thank you.
  3) We have removed “slightly” so the sentence now reads “Overall, fluxes of chloropigments exhibit similar patterns at both depths, with higher flux values at PDM-224.”
  4) We corrected the sentence, thank you.
  5) We thank the reviewer for raising this point. In addition to the size of the canyon system, there are other factors that need to be considered.
  Large submarine canyon systems are not located within estuaries, which are typically rich in nutrients. In the Lower St. Lawrence Estuary (LSLE), upwelling of nutrient-rich waters at the head of the Laurentian Channel, subsequent mixing in the surface layer, and strong estuarine circulation provide an important supply and distribution of nutrients year-round. Freshwater inputs from the Saguenay, Outardes, Manicouagan, and St. Lawrence rivers additionally introduce nutrients to the system. Light is therefore the most important variable controlling phytoplankton growth in the LSLE (Therriault & Levasseur, 1985).
  Globally, there are many triggers for turbidity currents, which are almost always triggered in canyon systems with important sediment supply from rivers of longshore drift; thus the “sediment-starved” canyon system at Pointe-des-Monts is an exception (Normandeau et al., 2017 and references therein). The sediment supply and trigger influence the frequency, seasonality, and amplitude of turbidity currents, which will ultimately determine the impact of these events on regional productivity.
  The results presented here do not allow to determine the different impacts of small and large submarine canyon systems, but rather provide insight into a singular annual cycle for a small submarine canyon system in the LSLE.
  6) We have included the annual fluxes of chloropigments and particulate organic carbon measured in the present study to increase clarity and highlight that they are much greater than those measured in Genin et al. (2021), as well as included the trap and water-column depths. Lines 432 to 434 of the revised manuscript now read: “Annual chloropigment (70 to 120 mg m^-2yr^-1) and POC (11 to 19 g m^-2 yr^-1) fluxes measured here were much greater than those measured near Cabot Strait (35 mg m^-2yr^-1and 1.1 g m^-2 yr^-1, respectively; 100 m trap depth, 461 m water-column depth; Genin et al., 2021)”. We would like to avoid including a table with the values, as this section is not a focus of the study, but rather a brief discussion of previous sediment trap studies to provide a regional context to our values.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1538-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1538', Kyung-Hoon Shin, 16 Sep 2023
General comments: The paper investigates the impact of a submarine canyon system on the vertical export of biogenic matter in the Lower St. Lawrence Estuary, finding that sediment remobilization events and turbidity currents in the canyon influenced the flux of particulate matter and chloropigments in the water column. The study also reveals differences in diatom and dinoflagellate assemblages between the canyon system and an offshore site, with a diatom bloom occurring earlier at the canyon site. The presence of a potentially toxic diatom species was observed at the offshore site. I think this paper is a useful contribution to the literature and worthy to be published in Biogeosciences. However, I have some comments mainly about the clarity of the manuscript. I recommend the journal to accept this work after a moderate revision.

Specific comments:
Line 44: (Normandeau et al., 2014, 2020, 2022) à (Normandeau et al., 2015, 2020, 2022)

Line 329: Calvin pathway Meyers, 1994; Macdonal et al., 2004) à Calvin pathway (Meyers, 1994; Macdonal et al., 2004)

Line 347: Are there any other primary production proxies mentioned here besides chloropigments?

Table B2: symbol check! in table B2

Major comments:
What are the conditions for determining trap depth?

What is the reason for the decrease in water temperature and salinity in December at both sites?

Are the large waves that occur every 2-3 years related to global ocean-atmosphere climate phenomena?

What is the time lag for organic matter produced in the surface layer to reach the depth of the sediment trap? Can I consider that the sinking particles obtained in each month fell during that same month?

Why does diatom bloom occur first, followed by the dinoflagellate later?

Lines 386-390: Isn't the depth of the mixed layer greater during the fall and winter months rather than the impact of resuspension? What do you think/ or other possibility?

Why do they exist only as cysts for Brigantedinium spp. and Selenopemphix quanta?
Citation: https://doi.org/10.5194/egusphere-2023-1538-RC1
- AC1:
  'Reply on RC1', Hannah Sharpe, 16 Oct 2023
  General comments:
  1) We thank the reviewer for their positive comments and for contributing to improving the manuscript. Your comments have been considered in the revised version.
  Specific comments:
  1) We have added both 2014 and 2015 to the references, thank you.
  2) We corrected the reference, thank you.
  3) In our study, primary production proxies at the Pointe-des-Monts site include chloropigments, diatoms and dinoflagellates. To avoid confusion, we have clarified this sentence in the revised version of the manuscript, which now reads “[…], but we note no correlation to other primary production proxies (i.e., fluxes of chloropigments, diatoms and dinoflagellates).”
  4) We have changed the * symbol to [].
  Major comments:
  1) Trap depths were determined based on several considerations:
  All traps were located within the deep layer.
  
  The sediment traps PDM-224 and BC-133 were located 58 and 67 m above seafloor, respectively, which is well above the benthic nepheloid layer (i.e., layer of water above the seafloor that contains significant amounts of suspended sediment), which is estimated to be about 10 m above the seafloor in our study area (Bourgault et al., 2014; Casse et al., 2019).
  
  In the main canyon system, the ADCP was located 27 m above the seafloor (156 m water depth) to be consistent with to methodology used in Normandeau et al. (2020). The PDM-154 sediment trap was placed at a similar water depth to the ADCP.
  
  BC-133 was located at a comparable distance above seafloor as PDM-224 while also being at a comparable depth below the surface as PDM-154.
  
  To clarify this point, in the revised version of the manuscript we have added:
  Lines 109-110: “Traps BC-133 and PDM-224 were positioned 67 m and 58 m above seafloor, respectively, above the estimated benthic nepheloid layer (i.e., <250 m of water depth; Bourgault et al., 2014; Casse et al., 2019).”
  Bourgault, D., Morsilli, M., Richards, C., Neumeier, U., and Kelley, D. E.: Sediment resuspension and nepheloid layers induced by long internal solitary waves shoaling orthogonally on uniform slopes, Continental Shelf Research, 72, 21-33. http://dx.doi.org/10.1016/j.csr.2013.10.019, 2014.
  Casse, M., Montero-Serrano, J.-C., St-Onge, G., and Poirier, A.: REE distribution and Nd isotope composition of estuarine waters and bulk sediment leachates tracing lithogenic inputs in eastern Canada, Marine Chemistry, 211, 117-130. https://doi.org/10.1016/j.marchem.2019.03.012, 2019.
  2) The sediment traps BC-133 and PDM-154 are within the deep layer and temperature and salinity fluctuations are not directly associated with surface conditions. A decrease in water temperature and salinity at these depths could be due to:
  A greater contribution of waters from the Labrador Current (lower temperatures and salinities) than from the North Atlantic waters (higher temperatures and salinities).
  
  Vertical mixing of the cold intermediate layer (lower temperatures and salinities) with the deep layer (higher temperatures and salinities).
  
  However, with the data collected as part of the present study, we cannot determine the cause of the decrease in water temperature and salinity in December 2020.
  3) Normandeau et al. (2020) reported that the large turbidity currents that occur every 2-3 years were initiated during storms that exhibited sustained (>7 h) high windspeeds (>60 km h^-1), which caused large storm waves. At or near low tide, these storm waves subsequently triggered a turbidity current. In the context of global climate warming, it can be expected that a shortening of the sea ice season and increased storm frequency may favor the development of turbidity currents.
  4) We thank the reviewer for raising this important point and have added in the revised version of the manuscript a brief explanation at the beginning of section 5.3 to address this:
  “The sinking rate of biogenic matter is highly variable depending on shape, size, density, aggregation, remineralization, etc. For example, sinking rates of individual diatoms range from 0.1 to 10 m d^-1 (Smayda, 1970) while aggregates can sink at rates of 88 to 569 m d^-1 (Iversen et al., 2010). Sinking biogenic matter generally form aggregates (“marine snow”) to facilitate vertical fluxes of pelagic particles, therefore, we can assume that the sinking particles obtained in the sediment traps are representative of their respective sampling time periods.”
  Iversen, M. H., Norwald, N., Ploug, H., Jackson, G. A., and Fischer, G.: High resolution profiles of vertical particulate organic matter export off Cape Blanc, Mauritania: Degradation processes and ballasting effects, Deep-Sea Research I, 57, 771–784, doi:10.1016/j.dsr.2010.03.007, 2010.
  Smayda, T.: The suspension and sinking of phytoplankton in the sea, Oceanography Marine Bulletin, 8, 353–414, 1970.
  5) Diatoms are the most abundant phytoplankton in the Lower St. Lawrence Estuary and almost all diatom taxa are autotrophic, whereas dinoflagellates are the second most abundant phytoplankton and approximately half the dinoflagellate taxa are heterotrophic (micrograzers). Diatoms bloom first due to higher growth rates, lower light requirements (particularly for pennate diatoms which bloom before centric diatoms), and higher nutrient requirements, particularly nitrate and dissolved silicon. The diatom bloom will end when the surface mixed layer becomes limited in nutrients and dinoflagellates will then bloom because the heterotrophic taxa are able to feed on diatoms and other primary producers. We very briefly mention this pattern in the second paragraph of section 5.3. This has been well established in the region therefore we do not believe it is necessary to explain in depth in our manuscript.
  6) We thank the reviewer for raising this point. The mixed layer deepens from fall into winter and then becomes shallower again in the spring. In lines 386-390, our intention was not to address the question of why primary production is lower in the winter, but rather to evaluate if resuspension events could affect the surface mixed layer when primary production would be more impacted. We have rephrased this in the revised manuscript (lines 393-397) to increase clarity:
  “However, our data do not permit to evaluate if lofted sediments also reached the surface layer where they would have decreased primary production. Additionally, turbidity currents and other sediment remobilization events appear to be more frequent from late fall through winter, therefore we cannot determine if sediment lofting would play a key role in this system throughout the entire year, particularly from late spring through early fall, when primary production is greatest.”
  7) Only about 20% of total dinoflagellate taxa are known to produce organic resting cysts. Of the 16 motile dinoflagellate taxa observed in our study, four are currently known to produce resting cysts: Pentapharsodinium dalei, Protoperidinium americanum, Protoperidinium conicum, and Protoperidinium spp. In the present study, we observed cysts of the last three taxa but did not observe cysts of P. dalei, which we know are abundant in the regional surface sediments. We do not believe that preservation is an issue because other sediment trap studies using formalin as a preservative have observed cysts of P. dalei (e.g., Heikkilä et al., 2016). It is possible that we did not observe cysts of P. dalei because they were outnumbered by other dominant taxa (below detection).
  Heikkilä, M., Pospelova, V., Forest, A., Stern, G. A., Fortier, L., and Macdonald, R. W.: Dinoflagellate cyst production over an annual cycle in seasonally ice-covered Hudson Bay, Marine Micropaleontology, 125, 1-24, http://dx.doi.org/10.1016/j.marmicro.2016.02.005, 2016.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1538-AC1
RC2:
'Comment on egusphere-2023-1538', Anonymous Referee #2, 10 Oct 2023
General Comments
Sharpe et al. explored the vertical transport of organic matter in the St. Lawrence Estuary under the influence of a small near-bed submarine canyon system. Submarine canyons are established as important conduits for sediment accumulation through features like episodic turbidity currents, and thus play a key role in material exchange between upper and deep ocean. The authors presented a 1-year continuous record of export fluxes close to the small canyon feature, comparing observations to a distal reference station outside the influence of canyon processes. Results showed that the organic particle fluxes were anomalously greater at the canyon station following sediment remobilization events, providing strong evidence that the canyon system impact existed. In addition, primary productivity appeared to be suppressed by the small canyon processes likely due to sediment lofting causing declined light availability. The authors also speculated that the declining sea ice cover and prolonged ice-free season could cause increased sediment remobilization events but uncertain impact on the ecosystems, pointing to the necessity for more frequent observations. The topic of this study is interesting because the impact of smaller submarine canyon systems on the water column biogeochemistry has not been well documented, and I believe this study would help address this knowledge gap and potentially be of interest to the readers of Biogeosciences. I also think the experiment is nicely designed and the manuscript well written in general. I only have a few minor comments, and I would like to recommend this manuscript for publication if those minor technical comments were properly addressed by the authors.
Specific comments
Line 56: the Baie-Comeau station. If BC-133 is not affected by any sediment remobilization events whatsoever as the “control” of this study, I think the authors should state it somewhere at the beginning.

Line 176: add a “.” after “(Fig. 3)”.

Line 203: delete “slightly” or replace “slightly” with “evidently”. It is problematic to say PDM-224 is only "slightly" higher than PDM-154, at least not in terms of chloropigments in Fig. 5.

Line 289-290: replace “are likely occurring” with “likely occur”.

Line 383: “limiting light availability”: what makes it different in larger submarine canyon systems from smaller ones in terms of phytoplankton growth-limiting factors (such as conditions with light and nutrient)? I.e., Why do turbidity currents enhance productivity in large canyon systems despite reduced light availability?

Line 406: Section 5.5. This section could have been written in a more organized way. It would be less confusing for the readers if the variables in comparison and the corresponding numbers were presented in a dedicated table. For example, Line 413-414, what are “much greater”, and what are the two numbers respective to?
Citation: https://doi.org/10.5194/egusphere-2023-1538-RC2
- AC2: 'Reply on RC2', Hannah Sharpe, 16 Oct 2023
  
  General comments:
  1) We thank the reviewer for their positive comments and for contributing to improving the manuscript. Your comments have been considered in the revised version.
  Specific comments:
  1) We have clarified this in the revised version of the manuscript, which now reads “To identify canyon-specific processes, a sediment trap was also deployed offshore Baie-Comeau (BC), a site not affected by canyon-related sediment remobilization events, to contrast with biogenic matter export in the LSLE (Fig. 1c).”
  2) We corrected the sentence, thank you.
  3) We have removed “slightly” so the sentence now reads “Overall, fluxes of chloropigments exhibit similar patterns at both depths, with higher flux values at PDM-224.”
  4) We corrected the sentence, thank you.
  5) We thank the reviewer for raising this point. In addition to the size of the canyon system, there are other factors that need to be considered.
  Large submarine canyon systems are not located within estuaries, which are typically rich in nutrients. In the Lower St. Lawrence Estuary (LSLE), upwelling of nutrient-rich waters at the head of the Laurentian Channel, subsequent mixing in the surface layer, and strong estuarine circulation provide an important supply and distribution of nutrients year-round. Freshwater inputs from the Saguenay, Outardes, Manicouagan, and St. Lawrence rivers additionally introduce nutrients to the system. Light is therefore the most important variable controlling phytoplankton growth in the LSLE (Therriault & Levasseur, 1985).
  Globally, there are many triggers for turbidity currents, which are almost always triggered in canyon systems with important sediment supply from rivers of longshore drift; thus the “sediment-starved” canyon system at Pointe-des-Monts is an exception (Normandeau et al., 2017 and references therein). The sediment supply and trigger influence the frequency, seasonality, and amplitude of turbidity currents, which will ultimately determine the impact of these events on regional productivity.
  The results presented here do not allow to determine the different impacts of small and large submarine canyon systems, but rather provide insight into a singular annual cycle for a small submarine canyon system in the LSLE.
  6) We have included the annual fluxes of chloropigments and particulate organic carbon measured in the present study to increase clarity and highlight that they are much greater than those measured in Genin et al. (2021), as well as included the trap and water-column depths. Lines 432 to 434 of the revised manuscript now read: “Annual chloropigment (70 to 120 mg m^-2yr^-1) and POC (11 to 19 g m^-2 yr^-1) fluxes measured here were much greater than those measured near Cabot Strait (35 mg m^-2yr^-1and 1.1 g m^-2 yr^-1, respectively; 100 m trap depth, 461 m water-column depth; Genin et al., 2021)”. We would like to avoid including a table with the values, as this section is not a focus of the study, but rather a brief discussion of previous sediment trap studies to provide a regional context to our values.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1538-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Publish subject to minor revisions (review by editor) (19 Oct 2023) by Ji-Hyung Park

Dear Authors,

Both reviewers recognized the novelty of your work and agreed that the manuscript should be conditionally accepted. Therefore, I recommend conditional acceptance with the assumption that some moderate revision will be needed to bring the manuscript up to an acceptable level and clarity. When you revise the manuscript to incorporate the comments and suggestions offered by the two reviewers, please also take into consideration the following editorial points:

- Some of your responses to the reviewer comments are not clear enough as to whether you would incorporate the addressed issues in the revised manuscript. For instance, do you mean in your response to the second major comment by the first reviewer that you would discuss these two potential causes in the manuscript? Please articulate in your responses (by showing the revised text or indicating the line numbers) how you would incorporate the addressed issues in the revised manuscript.
- Line 22: Is this ‘the distal PDM site’? Please use consistent naming for the two PDM sites throughout the manuscript.
- Line 25 “biogenic matter export”: The preceding descriptions on the monitored phytoplankton data (lines 21-24) need to explain how the canyon system and associated local bloom patterns influence this matter export (vertical or longitudinal), but it is difficult to read out the key message from the current sentences. As you indicated in Discussion (lines 376-377), if the original hypothesis (“canyon-related hydrodynamics would stimulate phytoplankton production at the PDM site”) is not valid, I wondered what alternative mechanism you wanted to suggest here. Please make your point clearer here in the abstract and also in the Conclusions (lines 425-428).
- Line 26: The ‘regional ecosystem’ is a vague term. It would be more appropriate if you specify the impact area that the three monitored sites could cover.
- Section 3.2: Given the usual range of estuarine POC concentrations (no concentration data provided in the manuscript), a 1-3 mL sample volume seems too small to ensure an accurate analysis of POC in the filtrate. It would be helpful if you provided more analytical details, including analytical limits of the used method and instruments and any employed QC measures.
- Lines 315-320: Why don’t you compare the POC:PN ratios of sediments with those of the collected phytoplankton biomass?

I would like to ask you to make all the changes easily identifiable in a marked-up manuscript based on your point-by-point responses to the reviewer comments. If possible, please add up your responses to the original reviewer comments and specify the line numbers of the revised parts in your final responses accompanying the revised manuscript

Sincerely,

Ji-Hyung Park
Associate Editor, Biogeosciences

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AR by Hannah Sharpe on behalf of the Authors (26 Oct 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (30 Oct 2023) by Ji-Hyung Park

AR by Hannah Sharpe on behalf of the Authors (31 Oct 2023) Manuscript

Journal article(s) based on this preprint

18 Dec 2023

Influence of a small submarine canyon on biogenic matter export flux in the lower St. Lawrence Estuary, eastern Canada

Hannah Sharpe, Michel Gosselin, Catherine Lalande, Alexandre Normandeau, Jean-Carlos Montero-Serrano, Khouloud Baccara, Daniel Bourgault, Owen Sherwood, and Audrey Limoges

Biogeosciences, 20, 4981–5001, https://doi.org/10.5194/bg-20-4981-2023,https://doi.org/10.5194/bg-20-4981-2023, 2023

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Hannah Sharpe et al.

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We studied the impact of submarine canyon processes within the Pointe-des-Monts system on biogenic matter export and phytoplankton assemblages. Using data from three oceanographic moorings, we show that the canyon experienced two low-amplitude sediment remobilization events in 2020–2021, that led to enhanced particle fluxes in the deep-water column layer >2.6 km offshore. Sinking phytoplankton fluxes were lower near the canyon compared to background values from the Lower St. Lawrence Estuary.


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