Continental Shelf Hypoxia: Some Nagging Questions

Rowe, Gilbert T.; Chapman, Piers

doi:10.18785/goms.2002.08

Cited by 38 publications

(48 citation statements)

References 24 publications

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“…Such DIN/P i and APA relationships have been previously reported by Sylvan et al (2006Sylvan et al ( , 2007. These observations are supported by various models used to explain the biological and biogeochemical dynamics in this region (e.g., Lohrenz et al 1999, Rowe andChapman 2002;Green et al 2008;Eldridge and Roelke 2010;Fennel et al 2011). That is, there are distinct zones which can be defined based on salinity, total suspended particulates and nutrient (concentrations and/or ratios) profiles, which, in turn, influence chlorophyll and primary productivity in relatively predictable ways.…”

Section: Discussionsupporting

confidence: 80%

“…Arrows suggest the possible progression of resource limitation as river water mixes with coastal waters and is subject to both dilution and biological processes in the NGOM materials, collected along the Louisiana shelf, reveals that concentrations decrease from 3 to 200 mg L -1 in low-salinity waters (0-18) to 0.5-20 mg L -1 at intermediate salinities (18-32) and 0.3-6 mg L -1 at salinities [32 (Table 3). Dilution and settling improve the light penetration in the water column and facilitates the initial consumption of the riverine N and P in the plume waters (Rowe and Chapman 2002;Lehrter et al 2009;Fennel et al 2011). The highest chlorophyll concentrations and primary productivity were measured at intermediate salinities (18-32) ( Table 3).…”

Section: Discussionmentioning

confidence: 99%

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Going West: Nutrient Limitation of Primary Production in the Northern Gulf of Mexico and the Importance of the Atchafalaya River

et al. 2011

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To investigate controls on phytoplankton production along the Louisiana coastal shelf, we mapped salinity, nutrient concentrations (dissolved inorganic nitrogen (DIN) and phosphorus (P i ), silicate (Si)), nutrient ratios (DIN/P i ), alkaline phosphatase activity, chlorophyll and 14 C primary productivity on fine spatial scales during cruises in March, May, and Electronic supplementary material The online version of this article (. Additionally, resource limitation assays were undertaken in a range of salinity and nutrient regimes reflecting gradients typical of this region. Of these, seven showed P i limitation, five revealed nitrogen (N) limitation, three exhibited light (L) limitation, and one bioassay had no growth. We found the phytoplankton community to shift from being predominately N limited in the early spring (March) to P limited in late spring and summer (May and July). Light limitation of phytoplankton production was recorded in several bioassays in July in water samples collected after peak annual flows from both the Mississippi and Atchafalaya Rivers. We also found that organic phosphorus, as glucose-6-phosphate, alleviated P limitation while phosphono-acetic acid had no effect. Whereas DIN/P i and DIN/Si ratios in the initial water samples were good predictors of the outcome of phytoplankton production in response to inorganic nutrients, alkaline phosphatase activity was the best predictor when examining organic forms of phosphorus. We measured the rates of integrated primary production (0.33-7.01 g C m -2 d -1 ), finding the highest rates within the Mississippi River delta and across Atchafalaya Bay at intermediate salinities. The lowest rates were measured along the outer shelf at the highest salinities and lowest nutrient concentrations (\0.1 lM DIN and Pi). The results of this study indicate that P i limitation of phytoplankton delays the assimilation of riverine DIN in the summer as the plume spreads across the shelf, pushing primary production over a larger region. Findings from water samples, taken adjacent the Atchafalaya River discharge, highlighted the importance of this riverine system to the overall production along the Louisiana coast.

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Section: Discussionsupporting

confidence: 80%

Section: Discussionmentioning

confidence: 99%

Going West: Nutrient Limitation of Primary Production in the Northern Gulf of Mexico and the Importance of the Atchafalaya River

et al. 2011

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“…The assumption is that N : C follows Redfield (Redfield et al, 1963) in the labile fraction (N : C = 0.15), whereas the proportion of carbon increases in the refractory fraction (N : C = 0.1). Since deposited OC mainly originates from local primary production on the shallow Louisiana Shelf (Redalje et al, 1994;Justić et al, 1996;Rowe and Chapman, 2002), labile OC is assumed to repre- sent 74 % of total OC in deposited material. This value was used by Soetaert et al (1996a) to represent the fraction of labile organic matter in surface waters and is in line with previous modeling investigations of the Louisiana Shelf (Justić et al, 1996;Eldridge and Morse, 2008).…”

Section: Diagenetic Modelmentioning

confidence: 99%

Parameterization of biogeochemical sediment–water fluxes using in situ measurements and a diagenetic model

et al. 2016

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Abstract. Diagenetic processes are important drivers of water column biogeochemistry in coastal areas. For example, sediment oxygen consumption can be a significant contributor to oxygen depletion in hypoxic systems, and sedimentwater nutrient fluxes support primary productivity in the overlying water column. Moreover, nonlinearities develop between bottom water conditions and sediment-water fluxes due to loss of oxygen-dependent processes in the sediment as oxygen becomes depleted in bottom waters. Yet, sedimentwater fluxes of chemical species are often parameterized crudely in coupled physical-biogeochemical models, using simple linear parameterizations that are only poorly constrained by observations. Diagenetic models that represent sediment biogeochemistry are available, but rarely are coupled to water column biogeochemical models because they are computationally expensive. Here, we apply a method that efficiently parameterizes sediment-water fluxes of oxygen, nitrate and ammonium by combining in situ measurements, a diagenetic model and a parameter optimization method. As a proof of concept, we apply this method to the Louisiana Shelf where high primary production, stimulated by excessive nutrient loads from the Mississippi-Atchafalaya River system, promotes the development of hypoxic bottom waters in summer. The parameterized sediment-water fluxes represent nonlinear feedbacks between water column and sediment processes at low bottom water oxygen concentrations, which may persist for long periods (weeks to months) in hypoxic systems such as the Louisiana Shelf. This method can be applied to other systems and is particularly relevant for shallow coastal and estuarine waters where the interaction between sediment and water column is strong and hypoxia is prone to occur due to land-based nutrient loads.

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“…However, a number of findings and ideas have been articulated recently that suggest the classic concept is too simplistic and that other factors are important as well (Rowe and Chapman, 2002;Hetland and DiMarco, 2008;Sylvan et al, 2006;Bianchi et al, 2010;Lehrter et al, 2009). For example, terrestrial organic matter load probably contributes significantly to oxygen consumption (Bianchi et al, 2010(Bianchi et al, , 2009, stratification is important for hypoxia formation in preventing supply of oxygen below the pycnocline (Wiseman et al, 1997), sediment oxygen demand is not directly related to river nutrient load (Morse and Rowe, 1999;Rowe and Chapman, 2002), and spatially varying rates of macrozooplankton grazing affect the rate of phytoplankton accumulation and the amount of organic matter reaching the bottom (Dagg, 1995).…”

Section: Introductionmentioning

confidence: 99%

“…For example, terrestrial organic matter load probably contributes significantly to oxygen consumption (Bianchi et al, 2010(Bianchi et al, , 2009, stratification is important for hypoxia formation in preventing supply of oxygen below the pycnocline (Wiseman et al, 1997), sediment oxygen demand is not directly related to river nutrient load (Morse and Rowe, 1999;Rowe and Chapman, 2002), and spatially varying rates of macrozooplankton grazing affect the rate of phytoplankton accumulation and the amount of organic matter reaching the bottom (Dagg, 1995). Walker and Rabalais (2006) found no significant relationship between satellite-derived surface chlorophyll and hypoxia development.…”

Section: Introductionmentioning

confidence: 99%

A coupled physical-biological model of the Northern Gulf of Mexico shelf: model description, validation and analysis of phytoplankton variability

et al. 2011

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Abstract. The Texas-Louisiana shelf in the Northern Gulf of Mexico receives large inputs of nutrients and freshwater from the Mississippi/Atchafalaya River system. The nutrients stimulate high rates of primary production in the river plume, which contributes to the development of a large and recurring hypoxic area in summer, but the mechanistic links between hypoxia and river discharge of freshwater and nutrients are complex as the accumulation and vertical export of organic matter, the establishment and maintenance of vertical stratification, and the microbial degradation of organic matter are controlled by a non-linear interplay of factors. Unraveling these interactions will have to rely on a combination of observations and models. Here we present results from a realistic, 3-dimensional, physical-biological model with focus on a quantification of nutrient-stimulated phytoplankton growth, its variability and the fate of this organic matter. We demonstrate that the model realistically reproduces many features of observed nitrate and phytoplankton dynamics including observed property distributions and rates. We then contrast the environmental factors and phytoplankton source and sink terms characteristic of three model subregions that represent an ecological gradient from eutrophic to oligotrophic conditions. We analyze specifically the reasons behind the counterintuitive observation that primary production in the light-limited plume region near the Mississippi River delta is positively correlated with river nutrient input, and find that, while primary production and phytoplanktonCorrespondence to: K. Fennel (katja.fennel@dal.ca) biomass are positively correlated with nutrient load, phytoplankton growth rate is not. This suggests that accumulation of biomass in this region is not primarily controlled bottom up by nutrient-stimulation, but top down by systematic differences in the loss processes.

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Continental Shelf Hypoxia: Some Nagging Questions

Cited by 38 publications

References 24 publications

Going West: Nutrient Limitation of Primary Production in the Northern Gulf of Mexico and the Importance of the Atchafalaya River

Going West: Nutrient Limitation of Primary Production in the Northern Gulf of Mexico and the Importance of the Atchafalaya River

Parameterization of biogeochemical sediment–water fluxes using in situ measurements and a diagenetic model

A coupled physical-biological model of the Northern Gulf of Mexico shelf: model description, validation and analysis of phytoplankton variability

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