the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Nitrogen isotopes reveal a particulate-matter driven biogeochemical reactor in a temperate estuary
- 1Institute of Carbon Cycles, Helmholtz-Zentrum Hereon, Geesthacht, 21502, Germany
- 2Woods Hole Oceanographic Institution, Woods Hole, 360 Woods Hole Rd, MA 02543, USA
- 1Institute of Carbon Cycles, Helmholtz-Zentrum Hereon, Geesthacht, 21502, Germany
- 2Woods Hole Oceanographic Institution, Woods Hole, 360 Woods Hole Rd, MA 02543, USA
Abstract. Estuaries and rivers are important biogeochemical reactors that act to modify nutrient loads and composition in the intensively used coastal zone. In a case study during July 2013, we sampled an 80 km transect along the Elbe estuary under unusually low-oxygen conditions. To better elucidate specific mechanisms of estuarine nitrogen processing, we tracked the evolution of the stable isotopic composition of nitrate, nitrite, particulate matter, and ammonium through the water column.
We used this exceptional summer situation to constrain the in-situ isotope effects of ammonium and nitrite oxidation and of remineralization at the reach scale. The isotope effects of nitrite oxidation and ammonium oxidation are consistent with pure culture assessments. We found that estuarine biogeochemistry is governed by settling, resuspension, and remineralization of particulate matter. We used the stable isotope data to quantify sources and sinks of nitrogen in the Elbe estuary. An isotope mass balance box-model was developed to reproduce internal N-cycling and associated isotope dynamics. The model underscores the role of the delivery and reactivity of particulate matter, but it also allowed us to pinpoint additional sinks of reactive nitrogen, such as the denitrification of water column nitrate in the intensively dredged and deep Hamburg harbour basin.
Kirstin Dähnke et al.
Status: final response (author comments only)
-
RC1: 'Comment on bg-2022-123', Anonymous Referee #1, 28 Jun 2022
General comments
- This paper reports nitrogen transformation along the Elbe estuary and river by tracking concentration and the stable isotope composition of nitrogen compounds and particulate matter. The authors constructed a total nitrogen isotope mass balance to explore potential processes and controls on the evolution of riverine nitrogen along the estuarine reach. They also determine the in-situ nitrogen isotope effect of ammonium and nitrite oxidation and of remineralization.
- The Elbe River is the most important source of reactive nitrogen to the Southern North Sea which regarded as a problem area of eutrophication. The topic of this study is important because they investigate contribution of complicated N recycling processes (e.g. remineralization, nitrification) for nitrate production based on obtained in-situ nitrogen isotope effects. In addition, they provided a novel insight into the potential control process of nitrification in the Elbe estuary.
- Sampling and experimental procedures are clearly explained in detail. To confirm the activity of nitrification, the authors perform not only nutrient and isotope analysis but also nitrification rate measurement. Those combined approach is suitable for determination of contribution of nitrification.
- For the most part, the results are clearly presented, and the biogeochemical data are well illustrated with figures. However, there are some specific comments on determination of in-situ isotope effect of nitrification and the total isotope mass balance modeling.
- In summary, I recommend the publication of this paper if the authors add some discussions and make minor revisions as shown below.
Specific comments
Line: 62
The significance of evaluation of nitrogen transformation along Elbe estuary under “intense summer oxygen depletion” is a little vague. Why do sample the water column under unusual condition? Please revise them for clearer description.
Line: 70
In the explanation of study site, there is a lack of information where an agricultural catchment area and areas of nutrient discharge (Line 71-73) exist. Where is the input of N ? Those information helps us to understand the interpretation of geochemical data along Elbe Estuary.
Line: 128
As for nitrate isotope analysis, how was nitrite removed from the nitrate samples? Some samples contained enough nitrite for isotope measurements. Thus, the presence of nitrite interfered with nitrate isotope measurements.
Line: 150
It seems that both of ammonium oxidation and nitrite oxidation occur between stream km 641 and 656. Isotope compositions of nitrite could be affected by both of ammonium oxidation and nitrite oxidation. Do you consider the influence of ammonium oxidation on nitrite isotopes?
Line:261
How did you calculate and conclude that the drop of oxygen isotope values in the harbor region was due to nitrification? What is the value of oxygen composition of river water (δ18OH2O) ?
Line:320
The authors constructed a total isotope mass balance by modeling. I understood the assumption of the model. However, the equation, parameters and calculation method were not presented in this text. Therefore, it is difficult to understand the conclusion that Total N fluctuations are driven by PN fluctuations. I’m not so familiar with this box model, but it seems better that the authors briefly explain equations and parameters in the box model and a modeling software in the method section or supplemental information.
- AC1: 'Reply on RC1', Kirstin Dähnke, 22 Jul 2022
-
RC2: 'Comment on bg-2022-123', Anonymous Referee #2, 06 Jul 2022
In the manuscript Nitrogen isotopes reveal a particulate-matter driven biogeochemical reactor in a temperate estuary, Dähnke and colleagues present a timely study to assess estuarine nitrogen processing, taking advantage of conditions that enabled the isotopic analysis of nitrate, nitrite, ammonium, and particulate nitrogen along the Elbe estuary. Overall, this is an interesting dataset, with a lot of potential, but additional details are needed in several places to allow the reader to validate your conclusions and place them in context, these areas are outlined below.
Throughout, language such as unusual, exceptional, and unique are used to describe the conditions encountered, so how applicable are your results, just to these conditions? How often do these conditions occur? Are your findings applicable to the conditions seen in Sanders et al, 2018? It would be nice to see more comparisons drawn to this dataset.
Nitrification rates: more details are needed both in the methods and data interpretation.
- Nitrite and nitrate concentrations were stable, meaning all ammonium had been utilized?
- room temperature, was this close to insitu conditions?
- 14 days seems a long time and that bottle effects would be likely, was there any sign of this in the data? Exponential behavior for example? It would be beneficial to show some of this data, maybe in a supplement. How representative are these rates of insitu, as there seem to be a number of caveats, none of which are mentioned and there is also no comparison of the rates determined to those in the literature to put them in context, it is only mentioned that they are high.
- you mention in the methods that ammonia and nitrite oxidation rates were determined but this is not mentioned in the results/discussion.
Isotope mass balance box model: it is difficult to assess the outcomes of the model as no details are provided, equations, parameters etc, please provide this in the methods or supplement.
Specific comments
Line 34 to 36: it is not clear how the second half of the sentence links to the first
Line 106: How was chlorophyll analyzed
Line 126 / Nitrate Isotopes: There is no mention of a nitrite removal step, so are these actually N+N and not nitrate only? Please note the implications of this.
Line 137: You note here that high concentrations were needed for isotope analysis of nitrite and ammonium, please include what concentrations needed to be greater than for isotopic analysis
Line 225: For ammonium you use εamm to represent the isotope effect for ammonium removal and then go on to discuss uptake and oxidation, which is great, but why not the same for nitrite? Here you assume it is just nitrite oxidation (εnitox), but highlight later in the manuscript a potential role for denitrification in this system (e.g. Line 372), which would also consume nitrite, what would be the implications of this for your calculated isotope effect?
Figure 4 and associated text: it would be nice to see some errors on the calculated isotope effects.
Line 294 to 298: Across these lines, you discuss how nitrification scales / correlates with N content (%) and indicators of OM quality, where do I see this, you refer to Figure 4, but this is your isotope effects figure. These relationships need to clearly evident to support your conclusions.
Line 305 to 307 (and throughout this section): more explanation is needed for SPM reactivity, use the literature, for example, why does low C/N suggest its fresh and labile, references and details are needed for the reader to keep up with your line of thinking and confirm your conclusions.
- AC2: 'Reply on RC2', Kirstin Dähnke, 22 Jul 2022
Kirstin Dähnke et al.
Kirstin Dähnke et al.
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 256 | 69 | 12 | 337 | 0 | 3 |
- HTML: 256
- PDF: 69
- XML: 12
- Total: 337
- BibTeX: 0
- EndNote: 3
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1