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Comparing benthic biogeochemistry at a sandy and a muddy site in the Celtic Sea using a model and observations

Comparing benthic biogeochemistry at a sandy and a muddy site in the Celtic Sea using a model and observations
Comparing benthic biogeochemistry at a sandy and a muddy site in the Celtic Sea using a model and observations
Results from a 1D setup of the European Regional Seas Ecosystem Model (ERSEM) biogeochemical model were compared with new observations collected under the UK Shelf Seas Biogeochemistry (SSB) programme to assess model performance and clarify elements of shelf-sea benthic biogeochemistry and carbon cycling. Observations from two contrasting sites (muddy and sandy) in the Celtic Sea in otherwise comparable hydrographic conditions were considered, with the focus on the benthic system. A standard model parameterisation with site-specific light and nutrient adjustments was used, along with modifications to the within-seabed diffusivity to accommodate the modelling of permeable (sandy) sediments. Differences between modelled and observed quantities of organic carbon in the bed were interpreted to suggest that a large part (>90%) of the observed benthic organic carbon is biologically relatively inactive. Evidence on the rate at which this inactive fraction is produced will constitute important information to quantify offshore carbon sequestration. Total oxygen uptake and oxic layer depths were within the range of the measured values. Modelled depth average pore water concentrations of ammonium, phosphate and silicate were typically 5–20% of observed values at the muddy site due to an underestimate of concentrations associated with the deeper sediment layers. Model agreement for these nutrients was better at the sandy site, which had lower pore water concentrations, especially deeper in the sediment. Comparison of pore water nitrate with observations had added uncertainty, as the results from process studies at the sites indicated the dominance of the anammox pathway for nitrogen removal; a pathway that is not included in the model. Macrofaunal biomasses were overestimated, although a model run with increased macrofaunal background mortality rates decreased macrofaunal biomass and improved agreement with observations. The decrease in macrofaunal biomass was compensated by an increase in meiofaunal biomass such that total oxygen demand remained within the observed range. The permeable sediment modification reproduced some of the observed behaviour of oxygen penetration depth at the sandy site. It is suggested that future development in ERSEM benthic modelling should focus on: (1) mixing and degradation rates of benthic organic matter, (2) validation of benthic faunal biomass against large scale spatial datasets, (3) incorporation of anammox in the benthic nitrogen cycle, and (4) further developments to represent permeable sediment processes.
0168-2563
155-182
Aldridge, J. N.
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Lessin, G.
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Amoudry, L. O.
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Hicks, N.
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Hull, T.
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Klar, J. K.
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Kitidis, V.
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McNeill, C. L.
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Ingels, J.
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Parker, E. R.
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Silburn, B.
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Silva, T.
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Sivyer, D. B.
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Smith, H.E.K.
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Widdicombe, S.
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Woodward, E. M. S.
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Van Der Molen, J.
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Garcia, L.
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Kröger, S.
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Aldridge, J. N.
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Lessin, G.
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Amoudry, L. O.
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Hicks, N.
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Hull, T.
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Klar, J. K.
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Kitidis, V.
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McNeill, C. L.
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Ingels, J.
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Parker, E. R.
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Silburn, B.
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Silva, T.
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Sivyer, D. B.
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Smith, H.E.K.
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Widdicombe, S.
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Woodward, E. M. S.
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Van Der Molen, J.
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Garcia, L.
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Kröger, S.
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Aldridge, J. N., Lessin, G., Amoudry, L. O., Hicks, N., Hull, T., Klar, J. K., Kitidis, V., McNeill, C. L., Ingels, J., Parker, E. R., Silburn, B., Silva, T., Sivyer, D. B., Smith, H.E.K., Widdicombe, S., Woodward, E. M. S., Van Der Molen, J., Garcia, L. and Kröger, S. (2017) Comparing benthic biogeochemistry at a sandy and a muddy site in the Celtic Sea using a model and observations Biogeochemistry, 135, (1-2), pp. 155-182. (doi:10.1007/s10533-017-0367-0).

Record type: Article

Abstract

Results from a 1D setup of the European Regional Seas Ecosystem Model (ERSEM) biogeochemical model were compared with new observations collected under the UK Shelf Seas Biogeochemistry (SSB) programme to assess model performance and clarify elements of shelf-sea benthic biogeochemistry and carbon cycling. Observations from two contrasting sites (muddy and sandy) in the Celtic Sea in otherwise comparable hydrographic conditions were considered, with the focus on the benthic system. A standard model parameterisation with site-specific light and nutrient adjustments was used, along with modifications to the within-seabed diffusivity to accommodate the modelling of permeable (sandy) sediments. Differences between modelled and observed quantities of organic carbon in the bed were interpreted to suggest that a large part (>90%) of the observed benthic organic carbon is biologically relatively inactive. Evidence on the rate at which this inactive fraction is produced will constitute important information to quantify offshore carbon sequestration. Total oxygen uptake and oxic layer depths were within the range of the measured values. Modelled depth average pore water concentrations of ammonium, phosphate and silicate were typically 5–20% of observed values at the muddy site due to an underestimate of concentrations associated with the deeper sediment layers. Model agreement for these nutrients was better at the sandy site, which had lower pore water concentrations, especially deeper in the sediment. Comparison of pore water nitrate with observations had added uncertainty, as the results from process studies at the sites indicated the dominance of the anammox pathway for nitrogen removal; a pathway that is not included in the model. Macrofaunal biomasses were overestimated, although a model run with increased macrofaunal background mortality rates decreased macrofaunal biomass and improved agreement with observations. The decrease in macrofaunal biomass was compensated by an increase in meiofaunal biomass such that total oxygen demand remained within the observed range. The permeable sediment modification reproduced some of the observed behaviour of oxygen penetration depth at the sandy site. It is suggested that future development in ERSEM benthic modelling should focus on: (1) mixing and degradation rates of benthic organic matter, (2) validation of benthic faunal biomass against large scale spatial datasets, (3) incorporation of anammox in the benthic nitrogen cycle, and (4) further developments to represent permeable sediment processes.

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Accepted/In Press date: 22 July 2017
e-pub ahead of print date: 7 September 2017
Published date: September 2017

Identifiers

Local EPrints ID: 414229
URI: http://eprints.soton.ac.uk/id/eprint/414229
ISSN: 0168-2563
PURE UUID: 58b72acc-5174-4896-89a3-de36fd35ca8f

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Date deposited: 20 Sep 2017 16:31
Last modified: 01 Nov 2017 17:31

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Contributors

Author: J. N. Aldridge
Author: G. Lessin
Author: L. O. Amoudry
Author: N. Hicks
Author: T. Hull
Author: J. K. Klar
Author: V. Kitidis
Author: C. L. McNeill
Author: J. Ingels
Author: E. R. Parker
Author: B. Silburn
Author: T. Silva
Author: D. B. Sivyer
Author: H.E.K. Smith
Author: S. Widdicombe
Author: E. M. S. Woodward
Author: J. Van Der Molen
Author: L. Garcia
Author: S. Kröger

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