the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Recent inorganic carbon increase in a temperate estuary driven by water quality improvement and enhanced by droughts
Louise C. V. Rewrie
Burkard Baschek
Justus E. E. Beusekom
Arne Körtzinger
Gregor Ollesch
Abstract. Estuaries are an important contributor to the global carbon budget, facilitating carbon removal, transfer and transformation between land and coastal ocean. Estuaries are also susceptible to global climate change and anthropogenic perturbations. We find that a long-term significant increase in dissolved inorganic carbon (DIC) of 6–21 µmol kg-1 yr-1 (1997–2020) in a temperate estuary in Germany (Elbe Estuary), was driven by an increase in upper estuary particulate organic carbon (POC) content of 8–14 µmol kg-1 yr-1. The temporal POC increase was due to an overall improvement in water quality observed in the form of high rates of primary production and a significant drop in biological oxygen demand. The magnitude of mid-estuary DIC gain was equivalent to the increased POC production in the upper estuary, suggesting that POC is efficiently remineralized and retained as DIC in the mid-estuary, with the estuary acting as an efficient natural filter for POC. In the context of the significant DIC increase, the impact of a recent extensive drought period (2014–2020) significantly lowered the annual mean river discharge (468 ± 234 m3 s-1) compared to the long-term mean (690 ± 441 m3 s-1, 1960–2020). During the drought period, the late spring internal DIC load in the estuary doubled. This suggests that the drought induced a longer dry season, starting in May (earlier than normal), increased the residence time in the estuary and allowed for a longer remineralization period for POC. Annually, 77–94 % of the total DIC export was laterally transported to coastal water, reaching 89 ± 4.8 Gmol C yr-1, and thus only a maximum of 23 %, at 10 Gmol C yr-1, was released via carbon dioxide (CO2) evasion, between 1997 and 2020. Export of DIC to coastal waters decreased significantly during the drought (2014–2020: 38 ± 5.4 Gmol C yr-1), on average by 24 % compared to the non-drought period. In addition, we have identified that seasonal changes in DIC processing in an estuary require consideration in order to understand both the long-term and future changes in air-water CO2 flux, DIC export to coastal waters, as well as the impacts of prolonged droughts on the land-ocean carbonate system.
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Louise C. V. Rewrie et al.
Status: open (until 21 Jun 2023)
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RC1: 'Comment on egusphere-2023-961', Anonymous Referee #1, 26 May 2023
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Comments to Authors
This is an interesting dataset describing longitudinal gradients and long-term trends in POC and DIC for the Elbe Estuary. The findings from an analysis of these data would be of interest to estuarine ecologists and to the broader community studying the global C cycle. My main concern with the paper is that there are many aspects that are either not well explained, or insufficiently explained. This issue pervades key components of the paper including conceptualization of the study, description of methods and inferences made from the data.
Methods: the level of detail in explaining methods is highly uneven among the various components of the study. For example, the means for calculating air-water CO2 fluxes is highly detailed, whereas other equally important elements (e.g., POC loads and DIC mineralization) are hardly described at all in the main body of the paper. These fluxes, and how they are derived should be explained more fully and at an early stage. For example, in the Abstract there is reference to the “spring internal DIC load” (line 30), but at this stage, the leader is not likely to understand what this is (remineralization of organic C) or how it was determined. Also, I did not see an explanation of how POC was measured (perhaps I missed this).
Introduction: the first paragraph focuses on C cycling in estuaries, with a mention of eutrophication. The second paragraph focuses of climate change, and specifically, the occurrence of drought. The third (final) paragraph does not link the ideas presented in the first and second paragraphs in such a way as to provide a clear direction for the paper. There should be some consideration (prediction) of how estuarine C cycles may be affected by drought. My initial reaction was that drought would reduce external (watershed) inputs of POC and DOC to the estuary. I was surprised that there was no consideration here, or elsewhere, of the importance of allochthonous organic matter inputs, particularly as this is the driving mechanism accounting for the excess of C mineralization relative to autotrophic fixation in estuaries (see e.g., Hoellein et al. 2013). The third paragraph should provide some clear expectations of the direction of the paper from a conceptual point of view, which the stated objectives fail to do. Also, there should be some consideration of allochthonous C inputs and how these may have changed over time (and in response to drought).
The reliance on Rewrie et al. in review, here, and at many points throughout the paper, is not helpful, particularly as these are vague references to “ecosystem recovery”, “major shifts in ecosystem state” and “amelioration of water quality”. It is not clear to the readers of this paper what these changes are, and what implications they may have for C cycling in the Elbe. There is a subsequent statement that these changes include a reduction in BOD and an increase in NPP. The former implies that the changes may have to do with improved wastewater treatment resulting in reduced organic matter inputs to the estuary. But if that is the case, should these fluxes not be accounted for in this paper describing trends in POC and DIC? Also, if wastewater treatment practices have been improved, this should bring about a reduction in nutrient inputs, and potentially diminish, not enhance NPP. I short, I found it difficult to understand the long-term trends presented in this paper while not understanding what are the changes occurring in this system that seem to be the focus of a different paper.
Results: throughout the paper, loads are presented as mass per unit of time (e.g., the total mass of CO2 leaving the estuary), which is not very helpful to facilitating cross-system comparisons (vs. presenting these as values per unit area of the estuary). By analogy, river loads (watershed export) are more commonly normalized to watershed area (i.e., as a yield per square meter) to allow comparisons among watersheds of different size. Readers could take the values provided in this paper and divide by the specified area of the estuary to obtain estimates for comparisons to other estuaries. But the potential for making inter-system comparisons would be enhanced if the authors were to present their data as per unit area of the estuary.
Other Comments:
The Abstract lacks a ‘big picture’ perspective. The overall findings of the study are difficult to discern among the details of the results.
Site Description: it would be helpful if this included an indication of salinity levels along the length of the estuary (perhaps add data to Figure 1, or at least delineate polyhaline, mesohaline, etc.).
DOC: I was surprised that in a paper on estuarine C dynamics there was virtually no mention of DOC. Is it the case that internal production of POC and subsequent remineralization of POC are the dominant C fluxes in this system? At a minimum, it would be useful to report the proportions of total C represented by DIC, POC and DOC in river inputs to the estuary vs. relative contributions in export to the sea (and for drought vs. non-drought conditions).
Results (line 240): there is frequent use of indirect metrics (AOU, pH) to make inferences about autotrophic activity. Are there no primary data that can be used to support these inferences (e.g., CHLa measurements)?
Results (line 260): the statement “significant POC increases occurred...” is followed by some specified values, but it is unclear what these numbers represent (the mean concentration? the increase in concentration? If the latter, increase relative to what?).
Results (line 278): I did not understand why the TA:DIC ratio should be of interest, or what is the significance of this ratio being <1.
Discussion (line 425): do you mean to say that mineralization rates increase linearly with POC concentrations, or that mineralization efficiency increases (i.e., that the proportion of POC that is remineralized increases)?
Table 3.2: it would be helpful to include the standard error of the slope.
Figure 4: it is somewhat confusing to use the designation “m-1” as this is much more commonly used to indicate per meter. Perhaps “mon-1” would be the better abbreviation for monthly values?
Citation: https://doi.org/10.5194/egusphere-2023-961-RC1
Louise C. V. Rewrie et al.
Louise C. V. Rewrie et al.
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