the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The influence of mesoscale climate drivers on hypoxia in a fjord-like deep coastal inlet and its potential implications regarding climate change and greenhouse gas production: examining a decade of water quality data
- 1Faculty of Engineering, Computing and Science, Swinburne University of Technology, Kuching 93350, Malaysia
- 2ADS Environmental Services, Kota Kinabalu, Sabah, 88400, Malaysia
- 3Faculty of Resource Science & Technology, University Malaysia Sarawak, Kota Samarahan 94300, Sarawak, Malaysia
- 1Faculty of Engineering, Computing and Science, Swinburne University of Technology, Kuching 93350, Malaysia
- 2ADS Environmental Services, Kota Kinabalu, Sabah, 88400, Malaysia
- 3Faculty of Resource Science & Technology, University Malaysia Sarawak, Kota Samarahan 94300, Sarawak, Malaysia
Abstract. Deep coastal inlets are sites of high sedimentation and organic carbon deposition that account for 11 % of the world's organic carbon burial. Australasia's mid to high latitude regions have many such systems. It is important to understand the role of climate forcings in influencing hypoxia and organic matter cycling in these systems, but many such systems, especially in Australasia, remain poorly described.
We analysed a decade of in-situ water quality data from Macquarie Harbour, Tasmania, a deep coastal inlet with more than 180,000 tons of organic carbon loading per annum. Monthly dissolved oxygen, total Kjeldhal nitrogen, dissolved organic carbon, and dissolved inorganic nitrogen concentrations were significantly affected by rainfall patterns. Increased rainfall was correlated to higher organic carbon and nitrogen loading, lower oxygen concentrations in deep basins, and greater oxygen concentrations in surface waters. Most notably, the Southern Annular Mode (SAM) significantly influenced oxygen distribution in the system. High river flow (associated with low SAM index values) impedes deep water renewal as the primary mechanism driving basin water hypoxia. Climate forecasting predicted increased winter rainfall and decreased summer rainfall, which may further exacerbate hypoxia in this system.
Currently, the Harbour basins experience frequent (up to 36 % of the time) and prolonged (up to 2 years) oxygen-poor conditions with the potential to promote greenhouse gas (CH4, N2O) production. Increased greenhouse gas production will alter the processing of organic matter entering the system. The increased winter rainfall predicted for the area will potentially increase greenhouse gas emissions due to increased spread and duration of hypoxia in the basins. Further understanding of these systems and how they respond to climate change will improve our estimates of future organic matter cycling (burial vs export) and greenhouse gas production.
Johnathan Daniel Maxey et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2022-20', Anonymous Referee #1, 21 Feb 2022
General Comments:
I found this to be a well-structured analysis of a complicated system with clear results to support the authors’ hypotheses. The authors describe the system and all of the potential drivers and contributors of observed low oxygen in detail and leverage a unique, long-term dataset to do so. The figures were of high quality and supported the statements made by the authors well and it was clear that they are well-acquainted with the relevant literature for this system. I found their conclusions relating changes in rainfall to deep hypoxia and deep water renewal event frequency to be very convincing, though I do wonder what role eutrophication from the increased DOM resulting from large rain events might have – potential positive feedbacks? Aside from some more specific comments (see next section), my only note is regarding the connection drawn between the increased hypoxia and outgassing of greenhouse gases. While the authors provide evidence from the literature to support this hypothesis, I think their claims would be better supported with quantitative measurements to show that in this particular system, this outgassing already occurs and might increase. Overall, this paper appears to fill a notable gap in knowledge for this system and sets up the potential for future analyses on additional questions raised.
Specific Comments:
- Citation for the statement on lines 95-96?
- Figure 1: In inset map of Tasmania, put box around area that is zoomed in on in larger figure? Also in right map, it is hard to tell where the river is – can you draw a line or something to highlight its path rather than the two arrows?
- Line 155: I am not clear on how distinct functional groups support that external climatic drivers influence harbour processes.
- Line 162: At this point, I was curious to know how many basins there were in the harbor, how deep they were, etc. and was curious if there was a map or drawing of them. I see later in Figure 10 this is shown, but it may be good to have another figure earlier showing this since these deep basins are a large part of your story.
- Line 165: Please add the accuracy/precision of your YSI
- Table 1: Perhaps add maximum depth of each station?
- Figure 2: Why are there not groupings provided above A and B?
- For the final publication, note that Figures 5 and 8 are a bit blurry.
- Figure 7 was really nicely done – good way to display many different variables
- Figure 8: Because you have the y-axis crossing at 0 it becomes somewhat hard to tell where one plot ends and the next begins, and also hard to read the axes on plots that cross the y-axis. Perhaps have the y-axis cross at a negative x-value to avoid this and add a dotted line to indicate where 0 is?
- Figure 9: same comment about crossing the y-axis as in Figure 8
- Figure 10: Really informative figure, curious here about feedbacks of the increased OM loading under high flow – if this will also work to exacerbate low oxygen in combination with the lack of DWR?
- One other thing to consider is that deoxygenation of the deep waters outside the harbor will also decrease the O2 available in the water coming up during these DWR events, so this may also further inhibit relief from low oxygen?
- Data Availability: Will the dataset be made available following publication? For transparency and ethical scientific practices, the data used should be made public.
Technical Corrections:
- Title: “its” should be “the” or “their”
- Line 22 : “predicts”
- Line 62: “it” should be “they” or “these factors”
- Line 90: “it’s” should be “the”
- Line 95: Please define DWR before using acronym
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AC1: 'Reply on RC1', johnathan maxey, 01 Mar 2022
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2022-20/bg-2022-20-AC1-supplement.pdf
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RC2: 'Reply on AC1', Anonymous Referee #1, 01 Mar 2022
Thank you for your responses to my previous comments.
In regards to my comment about deoxygenation of the deeper waters, I was referring to global deoxygenation patterns as a result of increased stratification, decreased solubility, etc. due to global warming (see, for example, "Ocean Deoxygenation in a Warming World" by Keeling et al. in Annual Rev in Mar Sci 2011 or "Linking coasts and seas to address ocean deoxygenation" by Levin and Breitburg in Nature Climate Change 2015). So even in the pristine waters off of Tasmania, oxygen levels are predicted to decrease in the future also as a result of climate change.
Given that you state that the measurements of greenhouse gas emissions will be presented in a forthcoming publication, I would still consider not leading with the claims of increased GHG emissions (as this is a hypothesized result and you do not present the data to support it in this particular study). This idea is presented in the title of the paper and in the abstract, but I got the impression that your main findings revolve more around how past rainfall and predicted changes in rainfall due to climate change will affect hypoxia and anoxia in the Harbour as well as DWR events - which is still a really interesting result. However, you do not show directly that these changes have already or will in the future lead to increased GHG emissions. Leaving that in the discussion as something that could potentially occur and later demonstrating this hypothesis with supportive data in the next MS you mention seems more appropriate.
Overall I am satisfied with the responses as long as the noted changes are made prior to publication. Perhaps other reviewer(s) or the Editor can comment on my second comment above.
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AC3: 'Reply on RC2', johnathan maxey, 04 Apr 2022
Thank you for your responses to my previous comments.
In regards to my comment about deoxygenation of the deeper waters, I was referring to global deoxygenation patterns as a result of increased stratification, decreased solubility, etc. due to global warming (see, for example, "Ocean Deoxygenation in a Warming World" by Keeling et al. in Annual Rev in Mar Sci 2011 or "Linking coasts and seas to address ocean deoxygenation" by Levin and Breitburg in Nature Climate Change 2015). So even in the pristine waters off of Tasmania, oxygen levels are predicted to decrease in the future also as a result of climate change.
Thank you for pointing out those references. It does make for an interesting question especiaily regarding the magnitudes of basin reoxygenation during future DWR events.Given that you state that the measurements of greenhouse gas emissions will be presented in a forthcoming publication, I would still consider not leading with the claims of increased GHG emissions (as this is a hypothesized result and you do not present the data to support it in this particular study). This idea is presented in the title of the paper and in the abstract, but I got the impression that your main findings revolve more around how past rainfall and predicted changes in rainfall due to climate change will affect hypoxia and anoxia in the Harbour as well as DWR events - which is still a really interesting result. However, you do not show directly that these changes have already or will in the future lead to increased GHG emissions. Leaving that in the discussion as something that could potentially occur and later demonstrating this hypothesis with supportive data in the next MS you mention seems more appropriate.
This seems to be an opinion shared by both reviewers and we are in the process of making changes to the title of the manuscript to ensure that we do not mislead potiential readers. We will move those points of discussion to a more appropriate section of the MS.Overall I am satisfied with the responses as long as the noted changes are made prior to publication. Perhaps other reviewer(s) or the Editor can comment on my second comment above.
Again thank you for your time and effort spent reviewing and commenting on this MS. The feedback provided has been valuable and the suggested changes will make for a much improved paper once incorporated.
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AC3: 'Reply on RC2', johnathan maxey, 04 Apr 2022
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RC2: 'Reply on AC1', Anonymous Referee #1, 01 Mar 2022
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RC3: 'Comment on bg-2022-20', Anonymous Referee #2, 11 Mar 2022
The major challenge of the authors’ work is to tease apart seasonal and inter-annual climate variations affecting the organic matter (OM) loading and hypoxia formation in a deep coastal inlet. Considerable amount of observational data is acquired and statistically processed to address three issues (in line 76 – 86): (1) effects of rainfall on OM loading and oxygen distribution; (2) effects of climate forcing on rainfall patterns and associated hypoxia formation; (3) implications on greenhouse gas emissions in this seasonally hypoxic system. Overall, I find issue #1 is well demonstrated, #2 is logically sound; and #3 is loosely based on current dataset. Nevertheless, the topic is interesting and, once the manuscript is improved, it will be suitable for publication in Biogeosciences. The following major issues are suggested for the authors to consider in the next round of revision.
(1) I am not sure whether the rainfall pattern shows seasonal variation? I am very confused with the 8 panels in figure 2, because the authors did not describe any panel (A through H) at all. Is it possible to have a simpler version of figure 2, and demonstrate the rainfall pattern?
(2) In figure 3, what is the meaning of x-axis? Does higher values represent more rainfall? My intuition is that, more rainfall results in higher river flow; but why would the Pearson corr. different towards the left of the two panels (at low rainfall and low river flow)?
(3) In figure 4, the upper panel show no significant seasonal variations in organic carbon loading; in figure 10, why OM loading is low during positive SAM? Can the authors show correlation between SAM and OM loading to support this claim? In addition, the daily average farm carbon load is much lower than riverine input; I would suggest the upstream dams are a much more important factor to consider because dams may dampen seasonal variabilities of river flow and OM loading.
(4) This manuscript does not present any greenhouse gas data; with these data the manuscript would have been more convincing by linking the greenhouse gas formation to SAM and further to climate variation. The aim #3 of this manuscript remains unresolved.
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AC2: 'Reply on RC3', johnathan maxey, 16 Mar 2022
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2022-20/bg-2022-20-AC2-supplement.pdf
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AC2: 'Reply on RC3', johnathan maxey, 16 Mar 2022
Johnathan Daniel Maxey et al.
Johnathan Daniel Maxey et al.
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