Abstract
Abstract. Export production reflects the amount of organic matter transferred from the ocean surface to depth through biological processes. This export is in large part controlled by nutrient and light availability, which are conditioned by mixed layer depth (MLD). In this study, building on Sverdrup's critical depth hypothesis, we derive a mechanistic model of an upper bound on carbon export based on the metabolic balance between photosynthesis and respiration as a function of MLD and temperature. We find that the upper bound is a positively skewed bell-shaped function of MLD. Specifically, the upper bound increases with deepening mixed layers down to a critical depth, beyond which a long tail of decreasing carbon export is associated with increasing heterotrophic activity and decreasing light availability. We also show that in cold regions the upper bound on carbon export decreases with increasing temperature when mixed layers are deep, but increases with temperature when mixed layers are shallow. A meta-analysis shows that our model envelopes field estimates of carbon export from the mixed layer. When compared to satellite export production estimates, our model indicates that export production in some regions of the Southern Ocean, particularly the subantarctic zone, is likely limited by light for a significant portion of the growing season.
Highlights
Photosynthesis in excess of respiration at the ocean surface leads to the production of organic matter, part of which is transported to the deep ocean through sinking and mixing (Volk and Hoffert, 1985)
Export production is frequently assumed to be a function of net community production (NCP), which is defined as the balance between net primary production (NPP) and heterotrophic respiration (HR) or the difference between gross primary production (GPP) and community respiration (CR; HR plus autotrophic respiration, AR; the abbreviations used in this study are presented in Table A; Li and Cassar, 2016): CO2
We build upon Sverdrup (1953) and derive a mechanistic model of an upper bound on carbon export based on the metabolic balance of photosynthesis and respiration in the oceanic mixed layer, in which the metabolic balance is derived from mixed layer depth (MLD), temperature, photosynthetically active radiation (PAR), phytoplankton maximum growth rate, and heterotrophic activity
Summary
Photosynthesis in excess of respiration at the ocean surface leads to the production of organic matter, part of which is transported to the deep ocean through sinking and mixing (Volk and Hoffert, 1985). Field observations confirm that NCP is generally lower at high temperatures and consistently low when mixed layers are deep These patterns have been attributed to the balance between depth-integrated photosynthesis (controlled by the availability of nutrients and light) and respiration as a function of MLD and temperature (Cassar et al, 2011; Eveleth et al, 2017; Huang et al, 2012; Shadwick et al, 2015; Tortell et al, 2015). To decompose the influence of light and nutrient availability on NCP, we define the upper bound on carbon export from the mixed layer (NCPâ) as the maximum export achievable should all limiting factors other than light (taking into account self-shading) be alleviated In his seminal paper, Sverdrup presented an elegant model to demonstrate that vernal phytoplankton blooms (i.e., organic matter accumulation at the ocean surface) may be driven by increased light availability when the MLD shoals above a critical depth (Zc; Sverdrup, 1953). Our key findings are that (1) using parameters available in the literature, the modeled upper bound envelopes field observations of O2 / Ar-derived NCP and export production derived from 234Th and sediment traps, and (2) the model identifies regions of the Southern Ocean where carbon export is likely limited by light during part of the growing season
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