Variable C : N : P stoichiometry of dissolved organic matter cycling in the Community Earth System Model

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Abstract

Dissolved organic matter (DOM) plays an important role in the ocean's biological carbon pump by providing an advective/mixing pathway for ∼ 20% of export production. DOM is known to have a stoichiometry depleted in nitrogen (N) and phosphorus (P) compared to the particulate organic matter pool, a fact that is often omitted from biogeochemical ocean general circulation models. However the variable C : N : P stoichiometry of DOM becomes important when quantifying carbon export from the upper ocean and linking the nutrient cycles of N and P with that of carbon. Here we utilize recent advances in DOM observational data coverage and offline tracer-modeling techniques to objectively constrain the variable production and remineralization rates of the DOM C : N : P pools in a simple biogeochemical-ocean model of DOM cycling. The optimized DOM cycling parameters are then incorporated within the Biogeochemical Elemental Cycling (BEC) component of the Community Earth System Model (CESM) and validated against the compilation of marine DOM observations. The optimized BEC simulation including variable DOM C : N : P cycling was found to better reproduce the observed DOM spatial gradients than simulations that used the canonical Redfield ratio. Global annual average export of dissolved organic C, N, and P below 100 m was found to be 2.28 Pg C yr -1 (143 Tmol C yr -1, 16.4 Tmol N yrg1, and 1 Tmol P yr -1, respectively, with an average export C : N : P stoichiometry of 225 : 19 : 1 for the semilabile (degradable) DOM pool. Dissolved organic carbon (DOC) export contributed ∼ 25% of the combined organic C export to depths greater than 100 m.

Figures

  • Figure 1. Schematic of organic matter cycling in the CESM BEC. Primary production is carried out by three phytoplankton functional types: small phytoplankton (which also contains a subgroup of calcifying phytoplankton), diatoms, and diazotrophs. Sources to DOM include direct losses from phytoplankton/zooplankton and from zooplankton grazing of phytoplankton. The major sink for DOM is microbial remineralization, parameterized with an assigned lifetime which differs between the euphotic zone and the mesopelagic ocean. A small fraction of phytoplankton production is converted to refractory DOM in the upper ocean with an additional source to DOMr from degradation of sinking POM in the mesopelagic. DOMr is also lost via UV photo-oxidation in the surface layer (< 10 m). The products of organic matter remineralization are dissolved inorganic carbon, nitrate, ammonium, and phosphate.
  • Table 1. Optimized DOM parameters from the DMI-enabled linear DOM model (DMI-DOM solver) and the modified DMI model (MOD DMI-DOM solver) as well as the REF and DOM OPT simulations of the CESM BEC. Euphotic zone: 0–100 m for the DMI-DOM models and depths where PAR > 1 % for REF and DOM OPT. The “flux to DOM” represents the fraction of PP that accumulates as DOM while the “fraction of DOM flux” represents the portion of the DOM production flux that accumulates as semilabile (SL) or refractory (R) DOM. Parameters fi |i = 1· · ·2,κi |i = 1· · ·4 are defined in Eqs. (1)–(4). Surf: surface layer (< 10 m), reminR: remineralization rate, sp: small phytoplankton, diat: diatoms, diaz: diazotrophs, k: half saturation constant for DOP uptake, and NA: not applicable.
  • Figure 2. Configuration of the DOM remineralization scheme and parameter values from the modified DMI-enabled DOM model (solver) and the DOM OPT simulation of the CESM BEC. Note the only minor changes to tracer lifetimes, κ−1 i , between the modified DMI-DOM model and the DOM OPT simulation. The value “%remin” represents the percentage of the DOM production flux that is remineralized within each depth horizon on an annual basis and is common to both models.
  • Table 2. DOM production, export, and stoichiometry metrics for the REF and DOM OPT simulations against observational constraints.
  • Table 3. DOM mean bias and correlation coefficient in relation to the DOM observations within the upper ocean (0–500 m depth) for the REF and DOM OPT 1◦ simulations. Observations of semilabile DOM are calculated as the total observed DOM concentration less the asymptotic concentration below 1000 m in each ocean basin.
  • Figure 3. Plots of simulated semilabile [DOC] (µM; colored contours) with observations (colored dots) for the REF simulation at (a) the surface (EZ) and (b) 200 m (MZ). Total [DOC] (µM; semilabile+ refractory) for the DOM OPT simulation is shown for (c) the surface (EZ) and (d) 200 m (MZ). Note the difference in color scales between plots (a) and (c); (b) and (d) as the REF simulation lacks a DOCr tracer.
  • Figure 4. Plots of simulated total [DON] (µM; colored contours) with observations (colored dots) for the REF simulation at (a) the surface (EZ) and (b) 200 m (MZ), and for the DOM OPT simulation at (c) the surface (EZ) and (d) 200 m (MZ).
  • Figure 5. Plots of simulated total [DOP] (µM; colored contours) with observations (colored dots) for the REF simulation at (a) the surface (EZ) and (b) 200 m (MZ), and for the DOM OPT simulation at (c) the surface (EZ) and (d) 200 m (MZ).

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CITATION STYLE

APA

Letscher, R. T., Moore, J. K., Teng, Y. C., & Primeau, F. (2015). Variable C : N : P stoichiometry of dissolved organic matter cycling in the Community Earth System Model. Biogeosciences, 12(1), 209–221. https://doi.org/10.5194/bg-12-209-2015

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