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.

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.

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 Sediment-water column fluxes of carbon, oxygen and nutrients in Bedford Basin, Nova Scotia, inferred from <sup>224</sup>Ra measurements

2013 ◽  
Vol 10 (1) ◽  
pp. 53-66 ◽  
Author(s):  
W. J. Burt ◽  
H. Thomas ◽  
K. Fennel ◽  
E. Horne
Keyword(s):  
Organic Matter ◽  
Nova Scotia ◽  
Water Column ◽  
Explicit Knowledge ◽  
Dissolved Inorganic Carbon ◽  
Chemical Constituents ◽  
Water Interface ◽  
Benthic Fluxes ◽  
Pore Waters ◽  
Overlying Water

Abstract. Exchanges between sediment pore waters and the overlying water column play a significant role in the chemical budgets of many important chemical constituents. Direct quantification of such benthic fluxes requires explicit knowledge of the sediment properties and biogeochemistry. Alternatively, changes in water-column properties near the sediment-water interface can be exploited to gain insight into the sediment biogeochemistry and benthic fluxes. Here, we apply a 1-D diffusive mixing model to near-bottom water-column profiles of 224Ra activity in order to yield vertical eddy diffusivities (KZ), based upon which we assess the diffusive exchange of dissolved inorganic carbon (DIC), nutrients and oxygen (O2), across the sediment-water interface in a coastal inlet, Bedford Basin, Nova Scotia, Canada. Numerical model results are consistent with the assumptions regarding a constant, single benthic source of 224Ra, the lack of mixing by advective processes, and a predominantly benthic source and sink of DIC and O2, respectively, with minimal water-column respiration in the deep waters of Bedford Basin. Near-bottom observations of DIC, O2 and nutrients provide flux ratios similar to Redfield values, suggesting that benthic respiration of primarily marine organic matter is the dominant driver. Furthermore, a relative deficit of nitrate in the observed flux ratios indicates that denitrification also plays a role in the oxidation of organic matter, although its occurrence was not strong enough to allow us to detect the corresponding AT fluxes out of the sediment. Finally, comparison with other carbon sources reveal the observed benthic DIC release as a significant contributor to the Bedford Basin carbon system.

scholarly journals Tracing the origin of the oxygen-consuming organic matter in the hypoxic zone in a large eutrophic estuary: the lower reach of the Pearl River Estuary, China

2017 ◽  
Vol 14 (18) ◽  
pp. 4085-4099 ◽  
Author(s):  
Jianzhong Su ◽  
Minhan Dai ◽  
Biyan He ◽  
Lifang Wang ◽  
Jianping Gan ◽  
...  
Keyword(s):  
Organic Matter ◽  
Oxygen Consumption ◽  
Water Column ◽  
Algal Blooms ◽  
Dissolved Inorganic Carbon ◽  
River Estuary ◽  
Pearl River Estuary ◽  
Pearl River ◽  
Terrestrial Organic Matter ◽  
Hypoxic Zone

Abstract. We assess the relative contributions of different sources of organic matter, marine vs. terrestrial, to oxygen consumption in an emerging hypoxic zone in the lower Pearl River Estuary (PRE), a large eutrophic estuary located in Southern China. Our cruise, conducted in July 2014, consisted of two legs before and after the passing of Typhoon Rammasun, which completely de-stratified the water column. The stratification recovered rapidly, within 1 day after the typhoon. We observed algal blooms in the upper layer of the water column and hypoxia underneath in bottom water during both legs. Repeat sampling at the initial hypoxic station showed severe oxygen depletion down to 30 µmol kg−1 before the typhoon and a clear drawdown of dissolved oxygen after the typhoon. Based on a three endmember mixing model and the mass balance of dissolved inorganic carbon and its isotopic composition, the δ13C of organic carbon remineralized in the hypoxic zone was −23.2 ± 1.1 ‰. We estimated that 65 ± 16 % of the oxygen-consuming organic matter was derived from marine sources, and the rest (35 ± 16 %) was derived from the continent. In contrast to a recently studied hypoxic zone in the East China Sea off the Changjiang Estuary where marine organic matter dominated oxygen consumption, here terrestrial organic matter significantly contributed to the formation and maintenance of hypoxia. How varying amounts of these organic matter sources drive oxygen consumption has important implications for better understanding hypoxia and its mitigation in bottom waters.

scholarly journals Mineralization of organic matter in boreal lake sediments: rates, pathways, and nature of the fermenting substrates

2020 ◽  
Vol 17 (18) ◽  
pp. 4571-4589
Author(s):  
François Clayer ◽  
Yves Gélinas ◽  
André Tessier ◽  
Charles Gobeil
Keyword(s):  
Organic Matter ◽  
Lake Sediments ◽  
Dissolved Inorganic Carbon ◽  
Biogeochemical Models ◽  
Mineralization Of Organic Matter ◽  
Carbohydrate Fermentation ◽  
Mass Balancing ◽  
Reaction Transport ◽  
Lake Basins

Abstract. The complexity of organic matter (OM) degradation mechanisms represents a significant challenge for developing biogeochemical models to quantify the role of aquatic sediments in the climate system. The common representation of OM by carbohydrates formulated as CH2O in models comes with the assumption that its degradation by fermentation produces equimolar amounts of methane (CH4) and dissolved inorganic carbon (DIC). To test the validity of this assumption, we modelled using reaction-transport equation vertical profiles of the concentration and isotopic composition (δ13C) of CH4 and DIC in the top 25 cm of the sediment column from two lake basins, one whose hypolimnion is perennially oxygenated and one with seasonal anoxia. Furthermore, we modelled solute porewater profiles reported in the literature for four other seasonally anoxic lake basins. A total of 17 independent porewater datasets are analyzed. CH4 and DIC production rates associated with methanogenesis at the five seasonally anoxic sites collectively show that the fermenting OM has a mean (± SD) carbon oxidation state (COS) value of -1.4±0.3. This value is much lower than the value of zero expected from carbohydrate fermentation. We conclude that carbohydrates do not adequately represent the fermenting OM in hypolimnetic sediments and propose to include the COS in the formulation of OM fermentation in models applied to lake sediments to better quantify sediment CH4 outflux. This study highlights the potential of mass balancing the products of OM mineralization to characterize labile substrates undergoing fermentation in sediments.

Role of Bioroughness, Bioirrigation, and Turbulence on Oxygen Dynamics at the Sediment‐Water Interface

2019 ◽  
Vol 55 (10) ◽  
pp. 8061-8075
Author(s):  
Y. Liu ◽  
D. Reible ◽  
F. Hussain ◽  
H. Fang
Keyword(s):  
Water Interface ◽  
Oxygen Dynamics
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