Virginia Institute of Marine Science
Observations indicate that resuspension and associated fluxes of organic material and porewater between the seabed and overlying water can alter biogeochemical dynamics in some environments, but measuring the role of sediment processes on oxygen and nutrient dynamics is challenging. A modeling approach offers a means of quantifying these fluxes for a range of conditions, but models have typically relied on simplifying assumptions regarding seabed-water-column interactions. Thus, to evaluate the role of resuspension on biogeochemical dynamics, we developed a coupled hydrodynamic, sediment transport, and biogeochemical model (HydroBioSed) within the Regional Ocean Modeling System (ROMS). This coupled model accounts for processes including the storage of particulate organic matter (POM) and dissolved nutrients within the seabed; fluxes of this material between the seabed and the water column via erosion, deposition, and diffusion at the sediment-water interface; and biogeochemical reactions within the seabed. A one-dimensional version of HydroBioSed was then implemented for the Rhone subaqueous delta in France. To isolate the role of resuspension on biogeochemical dynamics, this model implementation was run for a 2-month period that included three resuspension events; also, the supply of organic matter, oxygen, and nutrients to the model was held constant in time. Consistent with time series observations from the Rhone Delta, model results showed that erosion increased the diffusive flux of oxygen into the seabed by increasing the vertical gradient of oxygen at the seabed-water interface. This enhanced supply of oxygen to the seabed, as well as resuspension-induced increases in ammonium availability in surficial sediments, allowed seabed oxygen consumption to increase via nitrification. This increase in nitrification compensated for the decrease in seabed oxygen consumption due to aerobic remineralization that occurred as organic matter was entrained into the water column. Additionally, entrainment of POM into the water column during resuspension events, and the associated increase in remineralization there, also increased oxygen consumption in the region of the water column below the pycnocline. During these resuspension events, modeled rates of oxygen consumption increased by factors of up to similar to 2 and similar to 8 in the seabed and below the pycnocline, respectively. When averaged over 2 months, the intermittent cycles of erosion and deposition led to a similar to 16% increase of oxygen consumption in the seabed, as well as a larger increase of similar to 140% below the pycnocline. These results imply that observations collected during quiescent periods, and biogeochemical models that neglect resuspension or use typical parameterizations for resuspension, may underestimate net oxygen consumption at sites like the Rhone Delta. Local resuspension likely has the most pronounced effect on oxygen dynamics at study sites with a high oxygen concentration in bottom waters, only a thin seabed oxic layer, and abundant labile organic matter.
NW MEDITERRANEAN SEA; GULF-OF-MEXICO; ORGANIC-CARBON MINERALIZATION; LOUISIANA CONTINENTAL-SHELF; EARLY DIAGENETIC PROCESSES; IN-SITU MEASUREMENTS; MARINE-SEDIMENTS; SUSPENDED SEDIMENT; REDOX OSCILLATION; COASTAL SEDIMENTS
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Dataset Available: Moriarty, J. M., Harris, C. K., Fennel, K., Xu, K., Rabouille, C., and Friedrichs, M.A.M. 2017. A Model Archive for a Coupled Hydrodynamic-Sediment Transport-Biogeochemistry Model for the Rhône River Sub-aqueous Delta, France. Virginia Institute of Marine Science, College of William and Mary. https://doi.org/10.21220/V53P4Q
Observations from the Rhone River delta observatory (Mesurho) were provided by F. Toussaint (Laboratoire des Sciences du Climat et de l'Environnement). R. Wilson (formerly Dalhousie University) provided model code from Wilson et al. (2013). Feedback from E. Canuel, C. Friedrichs (Virginia Institute of Marine Science; VIMS), two anonymous reviewers, and Biogeosciences Associate Editor Jack Middelburg improved this paper. A. Miller, D. Weiss (VIMS), and E. Walters (the College of William & Mary; W&M) provided computational support and access to W&M's computing facilities, which are funded by the National Science Foundation, the Commonwealth of Virginia Equipment Trust Fund, and the Office of Naval Research. Funding was provided by the US National Oceanic and Atmospheric Administration Center for Sponsored Coastal Ocean Research (NA09NOS4780229, NA09NOS4780231, NGOMEX contribution 217) (Moriarty, Harris, Fennel, Xu), VIMS student fellowships (Moriarty), and MISTRALS/MERMEX-River and ANR-11-RSNR-0002/ AMORAD (Rabouille). This is contribution 3618 of the Virginia Institute of Marine Science.
Moriarty, Julia; Harris, Courtney K.; Fennel, Katja; Friedrichs, Marjorie A.M.; Xu, Kehui; and Rabouille, Christophe, The roles of resuspension, diffusion and biogeochemical processes on oxygen dynamics offshore of the Rhone River, France: a numerical modeling study (2017). BIOGEOSCIENCES, 14(7), 1919-1946.