Carbon dynamics at the river-estuarine transition: a comparison among tributaries of Chesapeake Bay
- Center for Environmental Studies, Virginia Commonwealth University
- Center for Environmental Studies, Virginia Commonwealth University
Abstract. Sources and transformation of C were quantified using mass balance and ecosystem metabolism data for the upper segments of the James, Pamunkey and Mattaponi Estuaries. The goal was to assess the role of external (river inputs & tidal exchange) vs. internal (metabolism) drivers in influencing the forms and fluxes of C. C forms and their response to river discharge differed among the estuaries based on their physiographic setting. The James, which receives the bulk of inputs from upland areas (Piedmont and Mountain), exhibited a higher ratio of inorganic to organic C, and larger inputs of POC. The Pamunkey and Mattaponi receive a greater proportion of inputs from lowland (Coastal Plain) areas, which were characterized by low DIC and POC, and elevated DOC. We anticipated that transport processes would dominate during colder months when discharge is elevated and metabolism is low, and that biological processes would predominate in summer, leading to attenuation of C through-puts via de-gassing of CO2. Contrary to expectations, highest retention of OC occurred during periods of high through-put, as elevated discharge resulted in greater loading and retention of POC. In summer, internal cycling of C via production and respiration was large in comparison to external forcing despite the large riverine influence in these upper estuarine segments. The estuaries were found to be net heterotrophic based on retention of OC, export of DIC, low GPP relative to ER, and a net flux of CO2 to the atmosphere. In the James, greater contributions from phytoplankton production resulted in a closer balance between GPP and ER, with autochthonous production exceeding allochthonous inputs. Combining the mass balance and metabolism data with bioenergetics provided a basis for estimating the proportion of C inputs utilized by the dominant metazoan. The findings suggest that invasive catfish utilize 15 % of total OM inputs and up to 40 % of allochthonous inputs to the James.
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Paul Bukaveckas
Status: final response (author comments only)
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RC1: 'Comment on bg-2021-209', Anonymous Referee #1, 15 Nov 2021
General Comments
The author presents a study in which he uses mass balance and ecosystem metabolism data to generate carbon (C) sources and transformations in the James, Mattaponi, and Pamunkey River Estuaries. He found that the C inputs differed between rivers and season based on watershed characteristics and discharge. Contrary to his prediction, highest retention of organic C occurred during periods of relatively high discharge. These systems were net heterotrophic, though there was some contribution from autotrophy that varied by river and season. Finally, the author applied a bioenergetics model to estimate the proportion of organic C removed by catfish, bald eagles, and osprey.
This is a nice study that will be well received by the readers of this journal. The study is a thorough examination of the C cycle in terms of external (river inputs, tidal exchange) versus internal (metabolism) drivers in influencing the forms and fluxes of C in the study systems. The manuscript is well written, clear, and well organized, and I think this is a very strong and interesting dataset. For the James River, they have a relatively complete, impressive C budget dataset that spans 10 years. For the Mattaponi and Pamunkey Rivers, the dataset is less complete and spans only 2 years. But the systems are different enough that it is worth including the analysis of the less-sampled Mattaponi and Pamunkey Rivers for comparison. There are a lot of display items (2 tables + 11 figures + supplemental material), but they all seem to serve a purpose, so I don’t recommend dropping any. Overall, I am comfortable with the conclusions and support publication of this manuscript with minor edits, as detailed below.
Specific Comments
Lines 70-89 Other studies to consider for this section of tidal freshwater zones:
Xu, X., H. Wei, T. Light, S. Melton, K. Holt, G. Barker, A. Salamanca, B. Hodges, K. Moffett, J. McClelland, A.K. Hardison. 2021. Tidal freshwater zones as hotspots for biogeochemical cycling. Estuaries and Coasts 44:722-733. DOI: 10.1007/s12237-020-00791-4.
Jones, A.E., A.K. Hardison, B. R. Hodges, J.W. McClelland, K. B. Moffett. 2019. An expanded rating curve model to estimate river discharge during tidal influences across the progressive-mixed-standing wave spectrum. PLoS ONE 14(12):e0225758, doi:10.1371/journal.pone.0225758.
Jones, A.E., B.R. Hodges, J.W. McClelland, A.K. Hardison, and K.B. Moffett. 2017. Residence time-based classification of surface water systems. Water Resources Research, 53:5567-5584, doi:10.1002/2016WR019928.
Line 82 and elsewhere Is there a reason why you refer to your systems as the James Estuary and not the James River Estuary? (Similar for the Mattaponi and Pamunkey River Estuaries)
Line 157 How did you determine the “constant fraction” the ungauged discharge was relative to the Fall Line discharge?
Line 214 Define GPP and ER abbreviations.
Line 234 Define PQ and RQ abbreviations.
Line 312-325 Refer more often to Fig. 4 and Table 2 throughout this text. (Also, please do this in the subsequent paragraphs explaining Figs. 5, 6.)
Line 435 Replace “reveled” with “revealed”
Lines 438-440 Explain briefly which rocks in the Mountain and Piedmont regions contribute substantially to DIC runoff.
Line 446 and elsewhere Since you are the sole author of this manuscript, you may not want to use the “we” pronoun.
Lines 484-487 Your findings suggest the inland waters function as pipes during high discharge periods. This is counterintuitive, as one would expect particulates to not be able to settle during high discharge relative to lower discharge. Can you expand on this concept? Are your data an exception to a relatively well-established rule established from other systems? What mechanism in your system might be at play?
Lines 490-500 Your data suggested use of a lower exchange coefficient (1 to 1.5 m/d; section 2.7), and you ended up using a value ~4x higher (4.3 m/d) based on values published by Raymond and colleagues. But in this section of the discussion, you refer to another study where Raymond used a value closer to the low value (1.1 m/d), so you then suggested that might be more appropriate to get your values closer to the Raymond et al. 2000 air-water fluxes. It seems to me like you should have stuck with your data-driven value (1 to 1.5 m/d) in the first place? This issue warrants further explanation in the methods and discussion.
Lines 533-535 What characteristics of the Susquehanna River and Chesapeake Bay mainstem make them net autotrophic?
Line 548 Insert “times” after “residence”
Line 584 Insert “of” before “POC”
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AC1: 'Reply on RC1', Paul Bukaveckas, 19 Nov 2021
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2021-209/bg-2021-209-AC1-supplement.pdf
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AC1: 'Reply on RC1', Paul Bukaveckas, 19 Nov 2021
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RC2: 'Comment on bg-2021-209', Anonymous Referee #2, 07 Dec 2021
Review of " Carbon dynamics at the river-estuarine transition: a comparison among tributaries of Chesapeake Bay" by Paul A. Bukaveckas
The paper discussed Sources and transformation of C to understand external (river inputs & tidal exchange) vs. internal (metabolism) in upper segments of the James, Pamunkey and Mattaponi Estuaries. The contrast in the qualitative and quantitative capacities of different carbon pools in the three studied estuaries, despite that they flow adjacent to each other and share almost similar carbon sources in their catchment, is unique and essential considering the modified carbon cycle under changing global climate condition. The manuscript provides new insight to the modified carbon cycling along the tidal freshwater regions of selected tributaries of Chesapeake Bay, is well-written and the data quality is good. I think the readers of this journal will benefit from the information contained in this paper. I therefore recommend publication of the paper after minor revision listed below.Introduction: The relative fraction of area covered under each estuaries during the study is not clear, whether it represent the entire estuarine contribution?
Methods: Information on the data collection frequency and use for the model is missing.
Summary: The relevance and global significance of the study in terms of tropical and non-tropical context.
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AC2: 'Reply on RC2', Paul Bukaveckas, 10 Dec 2021
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2021-209/bg-2021-209-AC2-supplement.pdf
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AC2: 'Reply on RC2', Paul Bukaveckas, 10 Dec 2021
Paul Bukaveckas
Paul Bukaveckas
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