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

R T Letscher,Y.-C Teng,F Primeau,J K Moore

doi:10.5194/bg-12-209-2015

Abstract

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 yr−1, 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.

Highlights

Dissolved organic matter (DOM) is an important pool linking nutrient cycles of nitrogen (N) and phosphorus (P) to the ocean’s carbon cycle
Results are similar for Dissolved organic carbon (DOC) when comparing the DOM OPT and REDFIELD simulations, which is to be expected as the REDFIELD simulation used the same DOC cycling parameters as the DOM OPT simulation
Large positive mean biases were found for DON within the REDFIELD and EZRAPID simulations when compared to the DOM OPT (Table 4)

Summary

Introduction

Dissolved organic matter (DOM) is an important pool linking nutrient cycles of nitrogen (N) and phosphorus (P) to the ocean’s carbon cycle. The concept of the Redfield ratio (Redfield, 1958; Redfield et al, 1963) has been a unifying paradigm in ocean biogeochemistry linking the stoichiometry of biological production and phytoplankton cellular material to that of the remineralization of detrital organic matter (OM) and inorganic nutrient ratios in the water column. Hopkinson and Vallino (2005) found DOM production and decomposition to follow a stoichiometry of 199 : 20 : 1, indicating the more efficient export of C within DOM per mole of N and P relative to sinking POM This finding is significant in light of evidence that future perturbations to the ocean from global climate change may favor enhanced partitioning of production to DOM (Wohlers et al, 2009; Kim et al, 2011).

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