scholarly journals The Roles of Resuspension, Diffusion and Biogeochemical Processes on Oxygen Dynamics Offshore of the Rhone River, France: A Numerical Modeling Study

2016 ◽  
Author(s):  
Julia M. Moriarty ◽  
Courtney K. Harris ◽  
Christophe Rabouille ◽  
Katja Fennel ◽  
Marjorie A. M. Friedrichs ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Organic Matter ◽  
Oxygen Consumption ◽  
Water Column ◽  
Bottom Boundary Layer ◽  
Water Interface ◽  
Bottom Boundary ◽  
Rhône Delta ◽  
Oxygen Dynamics

Abstract. Observations indicate that seabed resuspension of organic material and the associated entrainment of porewater into the overlying water can alter biogeochemical fluxes 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; entrainment of this material into the water column via resuspension 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 Delta, France. To isolate the role of resuspension on biogeochemical dynamics, this model implementation was run for a two-month period that included three resuspension events; also, the supply of organic matter, oxygen and nutrients to the water column was held constant in time. Consistent with time-series observations from the Rhone Delta, model results showed that resuspension 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 allowed seabed oxygen consumption to increase, primarily through nitrification. Resuspension of POM into the water column, and the associated increase in remineralization, also increased oxygen consumption in the bottom boundary layer. During these resuspension events, modeled rates of oxygen consumption increased by up to factors of ~ 2 and ~ 8 in the seabed and bottom boundary layer, respectively. When averaged over two months, the intermittent cycles of erosion and deposition led to a 20 % increase of oxygen consumption in the seabed, as well as a larger increase of ~ 200 % in the bottom boundary layer. 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 Subaqueous Delta. Local resuspension likely has the most pronounced effect on oxygen dynamics at study sites with a high oxygen concentration in the bottom boundary layer, only a thin seabed oxic layer, and abundant labile organic matter.

scholarly journals The roles of resuspension, diffusion and biogeochemical processes on oxygen dynamics offshore of the Rhône River, France: a numerical modeling study

2017 ◽  
Vol 14 (7) ◽  
pp. 1919-1946 ◽  
Author(s):  
Julia M. Moriarty ◽  
Courtney K. Harris ◽  
Katja Fennel ◽  
Marjorie A. M. Friedrichs ◽  
Kehui Xu ◽  
...  
Keyword(s):  
Organic Matter ◽  
Oxygen Consumption ◽  
Water Column ◽  
Water Interface ◽  
Regional Ocean Modeling System ◽  
Biogeochemical Models ◽  
Rhône Delta ◽  
Rhone Delta ◽  
Oxygen Dynamics

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.

Effects of Fe addition on sediment P dynamics in a eutrophic lake

2021 ◽  
Author(s):  
Melanie Münch ◽  
Rianne van Kaam ◽  
Karel As ◽  
Stefan Peiffer ◽  
Gerard ter Heerdt ◽  
...  
Keyword(s):  
Water Quality ◽  
Organic Matter ◽  
Surface Water ◽  
Water Column ◽  
Sediment Cores ◽  
Surface Water Quality ◽  
Water Interface ◽  
Eutrophic Lakes ◽  
Fe Addition

<p>The decline of surface water quality due to excess phosphorus (P) input is a global problem of increasing urgency. Finding sustainable measures to restore the surface water quality of eutrophic lakes with respect to P, other than by decreasing P inputs, remains a challenge. The addition of iron (Fe) salts has been shown to be effective in removing dissolved phosphate from the water column of eutrophic lakes. However, the resulting changes in biogeochemical processes in sediments as well as the long-term effects of Fe additions on P dynamics in both sediments and the water column are not well understood.</p><p>In this study, we assess the impact of past Fe additions on the sediment P biogeochemistry of Lake Terra Nova, a well-mixed shallow peat lake in the Netherlands. The Fe-treatment in 2010 efficiently reduced P release from the sediments to the surface waters for 6 years. Since then, the internal sediment P source in the lake has been increasing again with a growing trend over the years.</p><p>In 2020, we sampled sediments at three locations in Terra Nova, of which one received two times more Fe during treatment than the other two. Sediment cores from all sites were sectioned under oxygen-free conditions. Both the porewaters and sediments were analysed for their chemical composition, with sequential extractions providing insight into the sediment forms of P and Fe. Additional sediment cores were incubated under oxic and anoxic conditions and the respective fluxes of P and Fe across the sediment water interface were measured.</p><p>The results suggest that Fe and P dynamics in the lake sediments are strongly coupled. We also find that the P dynamics are sensitive to the amount of Fe supplied, even though enhanced burial of P in the sediment was not detected. The results of the sequential extraction procedure for P, which distinguishes P associated with humic acids and Fe oxides, as well as reduced flux of Fe(II) across the sediment water interface in the anoxic incubations, suggest a major role of organic matter in the interaction of Fe and P in these sediments.</p><p>Further research will include investigations of the role of organic matter and sulphur in determining the success of Fe-treatment in sequestering P in lake sediments. Based on these data in combination with reactive transport modelling we aim to constrain conditions for successful lake restoration through Fe addition.</p>

scholarly journals NUMERICAL MODELING OF A TURBULENT BOTTOM BOUNDARY LAYER UNDER SOLITARY WAVES ON A SMOOTH SURFACE

2018 ◽  
pp. 26
Author(s):  
Ahmad Sana ◽  
Hitoshi Tanaka
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Solitary Waves ◽  
Bottom Boundary Layer ◽  
Stream Velocity ◽  
Bottom Boundary ◽  
Sediment Movement ◽  
Bottom Boundary Layers ◽  
Turbulent Models

A number of studies on bottom boundary layers under sinusoidal and cnoidal waves were carried out in the past owing to the role of bottom shear stress on coastal sediment movement. In recent years, the bottom boundary layers under long waves have attracted considerable attention due to the occurrence of huge tsunamis and corresponding sediment movement. In the present study two-equation turbulent models proposed by Menter(1994) have been applied to a bottom boundary layer under solitary waves. A comparison has been made for cross-stream velocity profile and other turbulence properties in x-direction.

Physical limitations of dissolved methane fluxes: The role of bottom-boundary layer processes

2010 ◽  
Vol 272 (1-4) ◽  
pp. 209-222 ◽  
Author(s):  
Peter Linke ◽  
Stefan Sommer ◽  
Lorenzo Rovelli ◽  
Daniel F. McGinnis
Keyword(s):  
Boundary Layer ◽  
Bottom Boundary Layer ◽  
Dissolved Methane ◽  
Bottom Boundary ◽  
Boundary Layer Processes ◽  
Methane Fluxes ◽  
Physical Limitations

scholarly journals Bottom RedOx Model (BROM, v.1.0): a coupled benthic-pelagic model for simulation of seasonal anoxia and its impact

2016 ◽  
Author(s):  
E. V. Yakushev ◽  
E. A. Protsenko ◽  
J. Bruggeman ◽  
R. G. J. Bellerby ◽  
S. V. Pakhomova ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Organic Matter ◽  
Redox State ◽  
Nutrient Loading ◽  
Chemical Elements ◽  
Bottom Layer ◽  
Redox Conditions ◽  
Bottom Boundary Layer ◽  
Bottom Boundary ◽  
Benthic Sediments

Abstract. Interaction between seawater and benthic sediments plays an important role in global biogeochemical cycling. Benthic fluxes of chemical elements (C, N, P, O, Si, Fe, Mn, S) directly affect redox state and acidification (i.e. pH and carbonate saturation), which in turn determine the functioning of the benthic and pelagic ecosystems. The redox state of the near bottom layer can change and oscillate in many regions responding to the supply of organic matter, physical regime and coastal discharge. The goal of this work was to develop a model that captures key biogeochemical processes occurring at the bottom boundary layer and sediment–water interface and analyze the changes that result from seasonal variability in redox conditions in the water column. We used a modular approach allowing the model to be coupled to existing hydrophysical models in 1-D, 2-D or 3-D. The model is capable to simulate seasonality in production and respiration of organic matter as well as in mixing, that leads to variation of redox conditions in the bottom boundary layer. Production and reduction of organic matter and varying redox conditions in the bottom boundary layer affect the carbonate system and lead to changes in pH and alkalinity. Bacteria play a significant role in the fate of organic matter due to chemosynthesis (autotrophs) and consumption of organic matter (heterotrophs). Changes in the bottom boundary layer redox conditions modify the distribution of nutrients (N and P) and redox metals (Mn and Fe). The model can be used for analyzing and interpreting data on sediment-water exchange, and estimating the consequences of forcing such as climate change, external nutrient loading, ocean acidification, carbon storage leakages, and point-source metal pollution.

A Study of the Bottom Boundary Layer of the Florida Current

1972 ◽  
Vol 2 (1) ◽  
pp. 54-72 ◽  
Author(s):  
Georges L. Weatherly
Keyword(s):  
Boundary Layer ◽  
Current System ◽  
Bottom Boundary Layer ◽  
Bottom Current ◽  
Frictional Stress ◽  
Florida Current ◽  
Bottom Boundary ◽  
Straits Of Florida ◽  
Logarithmic Layer

This is a report of an experiment designed to study the bottom boundary layer of the Florida Current at a representative site in the Straits of Florida. The objectives of the experiment were 1) to determine the bottom frictional stress τ0, and 2) to determine whether the bottom boundary layer is a turbulent Ekman layer. A typical value of the bottom stress τ0 was found to be ~0.2 dyn cm−2. A mean veering of ~10° in the correct sense was observed in the logarithmic layer. No mean veering was observed above the logarithmic layer; this is believed to be a consequence of the strong modulation of the bottom current by the diurnal tide. The implication of τ0≈0.2 dyn cm−2 is considered in a simplified model of the Gulf Stream current system; this analysis suggests that, dynamically, the role of bottom friction is rather small.

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