Abstract
Abstract. 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 Rhône 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 Rhône 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 ∼ 2 and ∼ 8 in the seabed and below the pycnocline, respectively. When averaged over 2 months, the intermittent cycles of erosion and deposition led to a ∼ 16 % increase of oxygen consumption in the seabed, as well as a larger increase of ∼ 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 Rhône 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.
Highlights
Understanding and quantifying the role that physical processes play on coastal water quality remains a scientific and management concern
This paper presents a model called HydroBioSed that can reproduce the millimeter-scale changes in seabed profiles of oxygen, nitrogen, and carbon, as well as the resuspensioninduced changes in seabed–water-column fluxes observed on the Rhône River subaqueous delta, by coupling hydrodynamic, biogeochemical, and sediment transport modules
During quiescent conditions when bed shear stress was low, modeled and observed oxygen concentrations decreased with depth into the seabed, falling from about 250 mmol O2 m−3 in the bottom water column to 0 mmol O2 m−3 within 1–2 mm below the seabed surface
Summary
Understanding and quantifying the role that physical processes play on coastal water quality remains a scientific and management concern. Temporal lags between these reductions and water quality improvements (Kemp et al, 2009), and increased cycling of nutrients within coastal systems (e.g., Testa and Kemp, 2012), indicate that temporary storage of nutrients in the seabed and subsequent release to the water column via diffusion and/or resuspension can affect water quality in some coastal environments Neglecting these processes impairs managers’ ability to develop and evaluate strategies for improving coastal water quality (e.g., Artioli et al, 2008). Toussaint et al (2014) collected high-resolution time series of microelectrode oxygen profiles on the Rhône River subaqueous delta that showed resuspension may increase oxygen consumption in the seabed. This experiment revealed increases in diffusive fluxes of oxygen from the water column to the seabed during erosional events. It remains difficult to distinguish and quantify the relative influences of different biogeochemical (e.g., remineralization, oxidation) and physical (e.g., diffusion, resuspension) processes on oxygen dynamics in both the seabed and bottom-waters
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