A mechanistic model of an upper bound on oceanic carbon export as a function of mixed layer depth and temperature

Li, Zuchuan; Cassar, Nicolas

doi:10.5194/bg-14-5015-2017

Cited by 16 publications

(21 citation statements)

References 70 publications

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“…This can be explained by (1) the shoaling depth of integration Z eu resulting from decreasing light availability with increasing C and [Chl] (equations , , and ) and (2) the balance between phytoplankton physiology ( C : [Chl]) and the package effect on light attenuation (equation ). It is worth noting that the responses of euphotic depth‐integrated NCP, NPP, and HR to variations in C are markedly different from the ones presented in Figure 2 of Li and Cassar (), where the rates were integrated to a fixed depth (e.g., MLD) as opposed to the euphotic depth.…”

Section: Influence Of the Depth Of Measurements On The Export Productmentioning

confidence: 72%

“…By definition, the volumetric NCP at depth z (NCP( z )) is equal to the volumetric NPP (NPP( z )) minus the volumetric heterotrophic respiration (HR( z )) (Li & Cassar, ):

NCP ((), z) = NPP ((), z) - HR ((), z) = N_{m} \times I_{m} ((), z) \times μ_{\max} \times C - r_{HR} \times C

where C , μ max , N m , I m ( z ), and r HR represent the phytoplankton biomass concentration and maximum growth rate, the effects of nutrient concentration and light availability on the phytoplankton growth rate, and the heterotrophic respiration rate, respectively (see Table for a list of acronyms and definitions). N m , μ max , r HR , and C are assumed to be homogeneous above the depth of integration.…”

Section: Model Descriptionmentioning

confidence: 99%

“…In order to evaluate how the depth of integration influences export production and the ef ratio, we derive equations describing these properties at the base of the euphotic layer and at a fixed depth (Table ). To that end, we use equations –(5) and build on the model presented in Li and Cassar (). For simplicity, we assume that μ max , r HR , N ,

k_{m}^{N}

k_{m}^{I}

, [Chl], and C in equations –(5) are well mixed or constant within the depth of integration.…”

Section: Influence Of the Depth Of Measurements On The Export Productmentioning

confidence: 99%

“…Based on equation , NCP integrated over the euphotic layer (NCP(0, Z eu )) may be expressed as follows (Li & Cassar, ):

NCP ((),, 0, Z_{eu}) = NPP ((),, 0, Z_{eu}) - HR ((),, 0, Z_{eu}) = \int_{0}^{Z_{eu}} NPP ((), z) normald z - \int_{0}^{Z_{eu}} HR ((), z) normald z = N_{m} \times I_{m} ((),, 0, Z_{eu}) \times μ_{\max} \times C - r_{HR} \times C \times Z_{eu}

where NPP(0, Z eu ) and HR(0, Z eu ) represent NPP and HR integrated over the euphotic zone, respectively, Z eu denotes the euphotic depth where 1% of surface PAR remains, and I m (0, Z eu ) is calculated from equations and as follows:

I_{m} ((),, 0, Z_{eu}) = \int_{0}^{Z_{eu}} I_{m} ((), z) normald z = - \frac{1}{K_{I}} \times ln ((), \frac{I_{0} \times e^{- K_{I} \times Z_{eu}} + k_{m}^{I}}{I_{0} + k_{m}^{I}})

where

Z_{eu} = - \frac{ln ((), 0.01)}{K_{I}}

The autotrophic carbon to [Chl] ratio ( C : [Chl]) influences how NCP(0, Z eu ), NPP(0, Z eu ), and HR(0, Z eu ) respond to changes in C (Figure and supporting information). Intuitively, one would expect NCP(0, Z eu ), NPP(0, Z eu ), and HR(0, Z eu ) to monotonically increase with C , which is observed when C : [Chl] ratio is a constant.…”

Section: Influence Of the Depth Of Measurements On The Export Productmentioning

confidence: 99%

“…For example, the depth of the measurements (Boyd & Newton, ; Buesseler & Boyd, ; Palevsky & Doney, ), and the depth of the mixed layer in absolute terms and in relation to the depth of measurement of the ef ratio may also introduce noise. As stated by Buesseler (), “since both production and particulate export are strongly depth dependent, the relative ratio of these will depend upon the depth of integration.” The net production of organic matter, also known as net community production (NCP) and which reflects the carbon export potential as presented in Li and Cassar (), is depth dependent because it results from the balance between photosynthesis, which decreases with depth, and respiration. Conversely, POC export is depth dependent because of remineralization and POC attenuation with depth.…”

Section: Introductionmentioning

confidence: 99%

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Theoretical Considerations on Factors Confounding the Interpretation of the Oceanic Carbon Export Ratio

Cassar

2018

Global Biogeochemical Cycles

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The fraction of primary production exported out of the surface ocean, known as the export ratio (ef ratio), is often used to assess how various factors, including temperature, primary production, phytoplankton size, and community structure, affect the export efficiency of an ecosystem. To investigate possible causes for reported discrepancies in the dominant factors influencing the export efficiency, we develop a metabolism‐based mechanistic model of the ef ratio. Consistent with earlier studies, we find based on theoretical considerations that the ef ratio is a negative function of temperature. We show that the ef ratio depends on the optical depth, defined as the physical depth times the light attenuation coefficient. As a result, varying light attenuation may confound the interpretation of ef ratio when measured at a fixed depth (e.g., 100 m) or at the base of the mixed layer. Finally, we decompose the contribution of individual factors on the seasonality of the ef ratio. Our results show that at high latitudes, the ef ratio at the base of mixed layer is strongly influenced by mixed layer depth and surface irradiation on seasonal time scales. Future studies should report the ef ratio at the base of the euphotic layer or account for the effect of varying light attenuation if measured at a different depth. Overall, our modeling study highlights the large number of factors confounding the interpretation of field observations of the ef ratio.

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Section: Influence Of the Depth Of Measurements On The Export Productmentioning

confidence: 72%

“…By definition, the volumetric NCP at depth z (NCP( z )) is equal to the volumetric NPP (NPP( z )) minus the volumetric heterotrophic respiration (HR( z )) (Li & Cassar, ):