Nitrogen transformations along a shallow subterranean estuary

Couturier, Mathilde; Tommi-Morin, Gwendoline; Sirois, Maude; Rao, Alexandra; Nozais, Christian; Chaillou, Gwénaëlle

doi:10.5194/bg-14-3321-2017

Cited by 41 publications

(31 citation statements)

References 75 publications

(107 reference statements)

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“…The nutrient flux can be calculated by multiplying flow velocity and porosity to the integrated concentration of each nutrient species at L1, L2, and L3, which can be described as follows (Couturier et al, ; Gonneea & Charette, ):

F_{i} = v \cdot \int_{0}^{z} C_{i} ((), z) normald z,

where F i is the flux of nutrient species i , C i is the concentration of nutrient species i , which is a function of depth, z . According to water level data at this sampling transection, the horizontal velocity at shallow aquifer is calculated to be 0.18 ± 0.01 m/d according to Darcy's law.…”

Section: Discussionmentioning

confidence: 99%

“…The coastal aquifer may act as a source of marine nutrient pool or a barrier preventing terrestrial nutrients discharging to the sea. Therefore, a better understanding of the spatial distribution and biogeochemical transformation of nutrients in coastal aquifer is essential for appropriately selecting groundwater end‐members and quantifying global nutrient fluxes through SGD (Couturier et al, ; Moore, ).…”

Section: Introductionmentioning

confidence: 99%

“…Nitrogen transformations in coastal aquifers contain many processes such as nitrification (NTR), denitrification (DNTR), nitrogen fixation (NFIX), dissimilatory nitrate reduction to ammonium (DNRA), remineralization (REM), and anammox (ANAM) (Joye & Anderson, ; Santoro, ). The investigations on cycling or the fate of N in intertidal sediments lead to many insightful findings (Couturier et al, ; Kroeger & Charette, ; Loveless & Oldham, ; Schlesinger, ; Schutte et al, ; Spiteri et al, ; Talbot et al, ). For example, Talbot et al () indicated that NH 4 + moved conservatively while the loss of terrestrial NO 3 − was observed in CGMZ of Waquoit Bay, Massachusetts.…”

Section: Introductionmentioning

confidence: 99%

“…They also suggested SGD as a significant source of NO 3 − . However, Couturier et al () found that NO 3 − delivered by terrestrial freshwater was transformed in shallow intertidal aquifer. The nitrogen discharged to ocean in a form of NH 4 + .…”

Section: Introductionmentioning

confidence: 99%

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Tidal Pumping‐Induced Nutrients Dynamics and Biogeochemical Implications in an Intertidal Aquifer

et al. 2017

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Tidal pumping is a major driving force affecting water exchange between land and sea, biogeochemical reactions in the intertidal aquifer, and nutrient loading to the sea. At a sandy beach of Tolo Harbour, Hong Kong, the nutrient (NH4+, NO2−, NO3−, and PO43−) dynamic in coastal groundwater mixing zone (CGMZ) is found to be fluctuated with tidal oscillation. Nutrient dynamic is mainly controlled by tidal pumping‐induced organic matter that serves as a reagent of remineralization in the aquifer. NH4+, NO2−, and PO43− are positively correlated with salinity. Both NH4+ and PO43− have negative correlations with oxidation/reduction potential. NH4+ is the major dissolved inorganic nitrogen species in CGMZ. The adsorption of PO43− onto iron oxides occurs at the deep transition zone with a salinity of 5–10 practical salinity unit (psu), and intensive N‐loss occurs in near‐surface area with a salinity of 10–25 psu. The biogeochemical reactions, producing PO43− and consuming NH4+, are synergistic effect of remineralization‐nitrification‐denitrification. In CGMZ, the annual NH4+ loss is estimated to be ~ 4.32 × 105 mol, while the minimum annual PO43− production is estimated to be ~ 2.55 × 104 mol. Applying these rates to the entire Tolo Harbour, the annual NH4+ input to the harbor through the remineralization of organic matters is estimated to be ~ 1.02 × 107 mol. The annual NH4+ loss via nitrification is 1.32 × 107 mol, and the annual PO43− production is ~ 7.76 × 105 mol.

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F_{i} = v \cdot \int_{0}^{z} C_{i} ((), z) normald z,

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Tidal Pumping‐Induced Nutrients Dynamics and Biogeochemical Implications in an Intertidal Aquifer

et al. 2017

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“…The biogeochemical transformation in the coastal aquifer is the last process of groundwater‐borne nutrients before discharging to the sea, through which both the chemical form and amount of nutrients would shift significantly (Couturier et al, ; Erler et al, ; Gonneea & Charette, ; Heiss et al, ; Kroeger & Charette, ; Y. Liu, Jiao, Liang, & Luo, ; Loveless & Oldham, ; Marchant et al, ; Santos et al, ; Schutte et al, ; Spiteri et al, ; Talbot et al, ; Xiao et al, ). For example, Loveless and Oldham () found the nitrogen attenuation in a sandy aquifer and pointed out that previous estimates of nutrient loading were overestimated due to the neglect of natural biogeochemical process of nutrients in coastal aquifers.…”

Section: Introductionmentioning

confidence: 99%

Seasonality of Nutrient Flux and Biogeochemistry in an Intertidal Aquifer

2018

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Biogeochemical reactions in coastal aquifers highly affect the nutrient flux associated with submarine groundwater discharge (SGD) to the ocean, which subsequently influences the oceanic environment and ecology. This study investigates a seasonal variation of SGD-derived nutrient flux to the ocean and nutrient biogeochemistry in an intertidal aquifer of Tolo Harbor, Hong Kong. The results show that the inventory of NO x À and PO 4 3À in the intertidal aquifer has a clear seasonality with a large inventory in summer, a small inventory in spring, and a median inventory in autumn and winter, respectively. Differently, the inventory of NH 4 + is large in winter and summer and small in spring and autumn, which results from the coupled effects of seasonal change of both production and removal of NH 4 + in the aquifer. The SGD-derived nutrient (NH 4 + , NO x À , and PO 4 3À ) flux is the highest in summer (271.71, 24.86, and 116.66 mmol·day À1 ·(m coastline) À1 ) and is the lowest in spring (114.83, 1.70, and 20.26 mmol·day À1 ·(m coastline) À1 ). The majority of SGD-derived nutrient flux is supported by the local remineralization of organic matter along with seawater infiltration. In autumn, the recharge of seawater induced by tidal pumping significantly shifts the biogeochemical balance of nutrients and is the major source of groundwater nutrients in the intertidal aquifer. Among the various nutrient fluxes (SGD, river discharge, atmospheric deposition, and benthic sediment diffusion) to Tolo Harbor, SGD-derived PO 4 3À flux is the second major source of seawater PO 4 3À in addition to benthic sediment diffusion. The PO 4 3À loading via SGD is of significance to the primary production in the phosphorus-limited environment in Tolo Harbor. After considering the natural attenuation of nutrients in a sandy/silty beach aquifer, this study suggests the overestimation of SGD-derived nutrient loading estimated previously that simply use average nutrient concentration of fresh SGD endmember and saline SGD endmember as the nutrient concentration of total SGD endmember.Plain Language Summary The biogeochemical reactions in the intertidal aquifer are the crucial influencing factors affecting the groundwater nutrients that are essential for oceanic environment and ecology. Seasonal hydrologic variations are of significance to the biogeochemical reactions in the aquifer and nutrient speciation and concentrations in coastal groundwater. The results of this study indicate that both nutrient inventories in the aquifer and nutrient fluxes to the sea are higher in summer and autumn than winter and spring. By comparing the nutrient flux carried by the submarine groundwater discharge with other pathways, nutrient flux carried by coastal groundwater is a major source of phosphorus in the sea and supports the primary production in the harbor, which can be demonstrated by the positive trend between the nutrient flux and chlorophyll-a concentration in seawater. However, the red tide occurrence in the harbor does not have a similar seasonal tr...

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Biogeochemical attenuation of nitrate in a sandy subterranean estuary: Insights from two stable isotope approaches

Wong

Applegate

Poh

et al. 2020

Limnology & Oceanography

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The role of permeable sediments and subterranean estuaries as coastal nutrient filters is a question of key interest, particularly in areas with high nitrogen loadings. Here, we evaluated the effectiveness of a sandy subterranean estuary in cycling and removing nitrate using stable isotopes of N and O at natural (δ15N‐NO3− and δ18O‐NO3−) and enriched levels (15N). Isotopes were used in conjunction with flow through reactors under anoxic conditions to quantify (1) the overall enrichment factor (15ε) of nitrate removal processes which was then applied to estimate the in situ percentage of nitrate removal within the subterranean estuary and (2) the potential rates of denitrification, dissimilatory nitrate reduction to ammonium, and anammox. We found that 15ε varied between −24 and −34‰ and were positively correlated with nitrate concentrations and the percentage of organic carbon added to the sediments. Using these 15ε values in a Rayleigh distillation model resulted in an estimated average of 34% ± 14% nitrate removal within the subterranean estuary, less than half of the percentage estimated using the nitrate‐salinity mixing model (66% ± 28%). Denitrification was the most dominant nitrate removal pathway within the subterranean estuary with potential rates among the highest denitrification rates reported for both permeable and cohesive sediments. The contribution of dissimilatory nitrate reduction to ammonium showed significant seasonal variation while the rates of anammox were consistent throughout the study. We suggest that the spatial shift of the subterranean estuary is the most likely explanation for the seasonal differences in the rates of denitrification and dissimilatory nitrate reduction to ammonium.

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Nitrogen transformations along a shallow subterranean estuary

Cited by 41 publications

References 75 publications

Tidal Pumping‐Induced Nutrients Dynamics and Biogeochemical Implications in an Intertidal Aquifer

Tidal Pumping‐Induced Nutrients Dynamics and Biogeochemical Implications in an Intertidal Aquifer

Seasonality of Nutrient Flux and Biogeochemistry in an Intertidal Aquifer

Biogeochemical attenuation of nitrate in a sandy subterranean estuary: Insights from two stable isotope approaches

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