Stable isotope characterization of hypoxia-susceptible waters on the Louisiana shelf: Tracing freshwater discharge and benthic respiration

Strauss, Josiah; Grossman, Ethan L.; DiMarco, Steven F.

doi:10.1016/j.csr.2012.07.020

Cited by 16 publications

(17 citation statements)

References 58 publications

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“…Based on our three end-member model, δ 13 C resp displayed an average of -21.3 ± 2.5 ‰ in hypoxic waters in August 2007 (Table 3). This result was consistent with results in previous studies, which ranged from ~-22 ‰ to -22.7±10.3 ‰ (Strauss et al, 2012;Hu et al, In press), and also in the range of δ 13 C values of -22.5 to -21.2 ‰ for surface sediment on the inner shelf (Bianchi et al, 2002). For the two outliers in shallow waters (-47.4 ‰, -57.4 ‰ at 10 m bottom depth), these two δ 13 C DIC values were probably affected by factors in addition to our assumption (mixing and respiration), such as methane from the marsh and air-sea gas exchange.…”

Section: Bottom Water Respiration In August 2007supporting

confidence: 94%

“…. Lower δ 13 C resp values in 10-m depth compared to values in > 10-m depth were also observed by Strauss et al (2012) in hypoxic waters. In addition, the three end-member model generated results comparable to the salinity normalized method used by Hu et al, (In press) as the water salinity was above 35 in August 2007 and the effect of variations of river end-members on high salinity waters can be neglected.…”

Section: Bottom Water Respiration In August 2007mentioning

confidence: 60%

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The response of inorganic carbon distributions and dynamics to upwelling-favorable winds on the northern Gulf of Mexico during summer

Huang

Cai

Wang

et al. 2015

Continental Shelf Research

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Upwelling-favorable winds and an offshore-distributed Mississippi and Atchafalaya River plume trajectory were observed in summer 2009 in contrast to the mean conditions from 2002 to 2010 (upwelling-unfavorable winds and an alongshore river plume trajectory), a set of conditions which was also observed in summer 2007. The responses of dissolved inorganic carbon (DIC) distributions and dynamics to upwelling-favorable winds are studied by comparing the contrasting conditions between summer 2009 and summer 2007 on the northern Gulf of Mexico. Patterns of surface water partial pressure of CO 2 (pCO 2), DIC, δ 13 C in DIC, and total alkalinity (TA) determined in July 2009 and August 2007 were strongly related to river plume trajectories, and differed between the two summers. The slope of the relationship between dissolved oxygen (DO) and DIC in summer 2007 was comparable to the Redfield O/C ratio of 1.3, which was attributed to respiration of organic matter in the bottom water. The slope of the DO and DIC relationship and δ 13 C DIC values in bottom waters during July 2009 were clearly affected by mixing since their salinities were < 35. A three end-member mixing model was used to remove mixing effects in 1) δ 13 C DIC , to estimate the organic source of respiration, and 2) in DIC concentrations, to calculate DIC removal and release. δ 13 C DIC results in both summers were consistent with an apparent release of DIC in hypoxic waters (DO less than 2 mgL-1) associated with respiration of surface organic matter. The area-weighted surface DIC removal (i.e., biological production) was lower in 2009 than in 2007 on the shelf, as the plume was distributed offshore. The release of DIC in bottom waters was higher over the shelf in 2009 and was surmised to be related to stronger mixing, which was favorable for the DO supply for respiration. Overall, surface waters on the continental shelf in the region of study in July 2009 acted as a weak CO 2 source to the atmosphere, but a weak CO 2 sink in August 2007. We contend that the inorganic carbon distribution and concentrations on the shelf were related to regional wind forcing, through its influence on the distribution of coastal currents and plume trajectories and their subsequent impact on biogeochemical processes.

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Section: Bottom Water Respiration In August 2007supporting

confidence: 94%

Section: Bottom Water Respiration In August 2007mentioning

confidence: 60%

The response of inorganic carbon distributions and dynamics to upwelling-favorable winds on the northern Gulf of Mexico during summer

Huang

Cai

Wang

et al. 2015

Continental Shelf Research

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“…5). Mississippi River discharge is not included in Q s because relatively little Mississippi River flow enters the shelf in summer due to reversal of the westward coastal current (Walker et al 2005, Zhang et al 2012, and stable isotope studies suggest Atchafalaya River discharge constitutes the majority of freshwater in the surface waters of the shelf by midsummer (Strauss et al 2012).…”

Section: Mechanistic Model Formulationmentioning

confidence: 99%

Assessing biophysical controls on Gulf of Mexico hypoxia through probabilistic modeling

Obenour

Michalak

Scavia

2015

Ecological Applications

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A mechanistic model was developed to predict midsummer bottom-water dissolved oxygen (BWDO) concentration and hypoxic area on the Louisiana shelf of the northern Gulf of Mexico, USA (1985-2011). Because of its parsimonious formulation, the model possesses many of the benefits of simpler, more empirical models, in that it is computationally efficient and can rigorously account for uncertainty through Bayesian inference. At the same time, the model incorporates important biophysical processes such that its parameterization can be informed by field-measured biological and physical rates. The model is used to explore how freshwater flow, nutrient load, benthic oxygen demand, and wind velocity affect hypoxia on the western and eastern sections of the shelf, delineated by the Atchafalaya River outfall. The model explains over 70% of the variability in BWDO on both shelf sections, and outperforms linear regression models developed from the same input variables. Model results suggest that physical factors (i.e., wind and flow) control a larger portion of the year-to-year variability in hypoxia than previously thought, especially on the western shelf, though seasonal nutrient loads remain an important driver of hypoxia, as well. Unlike several previous Gulf hypoxia modeling studies, results do not indicate a temporal shift in the system's propensity for hypoxia formation (i.e., no regime change). Results do indicate that benthic oxygen demand is a substantial BWDO sink, and a better understanding of the long-term dynamics of this sink is required to better predict how the size of the hypoxic zone will respond to proposed reductions in nutrient loading.

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“…These two biogeochemical processes (i.e., primary production and respiration) result in highly dynamic DIC patterns. Moreover, DIC is indicative of the sources (i.e., terrestrial and marine) of the organic materials that are decomposing to DIC (i.e., Karlsson et al ., ; Strauss et al ., ], which is important for model prediction of the formation of bottom‐water hypoxia [ Tishchenko et al ., ] and acidification [ Cai et al ., ]. Thus, quantifying spatial and temporal changes in DIC on the continental shelf is vital to our understanding of the carbon cycle and related issues, such as reducing uncertainties in predicting CO 2 air‐sea flux in coastal waters, and therefore estimating global budgets.…”

Section: Introductionmentioning

confidence: 99%

Temporal variation and stoichiometric ratios of organic matter remineralization in bottom waters of the northern Gulf of Mexico during late spring and summer

Xue

Cai

et al. 2015

JGR Oceans

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An improved extended optimum multiparameter (eOMP) analysis was applied to hydrographic (temperature and salinity), and water chemistry data, including dissolved oxygen (O2), nutrients (nitrate plus nitrite, phosphate, and silicate), dissolved inorganic carbon (DIC), and total alkalinity (TAlk) data collected during late spring and summer from 2006 to 2012 in bottom waters off the Louisiana coast, to explore the dynamics and stoichiometry of DIC production during the development and maintenance of summer hypoxia. Our analysis demonstrated that DIC in bottom water was relatively low from April to June, but increased significantly in July, peaked in August, and dropped slightly in September. Furthermore, DIC production resulted from both aerobic organic carbon (OC) respiration and denitrification, as well as substantial loss due to vertical mixing with surface water. The average summer gross OC respiration rate was estimated to be 0.19 g C m−2 d−1, with the highest values occurring in late summer when hypoxic conditions dominated. We also found that Corg/N/P/‐O2 remineralization ratios for aerobic respiration were generally consistent with the classic Redfield ratio (106/16/1/138) except individual C/N and C/P ratios were slightly lower, indicating that marine OC was the major source of the DIC production in the bottom water. This study quantified the role of temporal bottom‐water microbial respiration to seasonal DIC dynamics and provided a means for studying the stoichiometry of biogeochemical processes in coastal waters.

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Stable isotope characterization of hypoxia-susceptible waters on the Louisiana shelf: Tracing freshwater discharge and benthic respiration

Cited by 16 publications

References 58 publications

The response of inorganic carbon distributions and dynamics to upwelling-favorable winds on the northern Gulf of Mexico during summer

The response of inorganic carbon distributions and dynamics to upwelling-favorable winds on the northern Gulf of Mexico during summer

Assessing biophysical controls on Gulf of Mexico hypoxia through probabilistic modeling

Temporal variation and stoichiometric ratios of organic matter remineralization in bottom waters of the northern Gulf of Mexico during late spring and summer

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