A one-dimensional model of dissolved organic carbon cycling in the water column incorporating combined biological-photochemical decomposition

Abstract

A one-dimensional model incorporating separate labile, semilabile, and refractory fractions of dissolved organic carbon (DOC) is used to study the vertical distribution of dissolved organics in the ocean and the downward flux of organic carbon into the water column. The modeled vertical gradient of DOC is generated almost entirely by the semilabile fraction, which has predicted lifetimes of 0.4 years at the ocean surface increasing to 6.3 years at 1000 m, owing to diminishing bacterial numbers. Although predicted fluxes of DOC and sinking particles exiting the upper mixed layer are similar, labile and semilabile DOC are mostly degraded by bacteria before being mixed below 250 m. The simple one-dimensional scheme employed by the modeling may, however, underestimate downward transport of DOC by physical mechanisms because it does not capture three-dimensional processes such as subduction. The inclusion of a biologically inert refractory pool is a first step to try and incorporate the growing awareness that photochemical processes play an important role in the dynamics of organic carbon in the ocean. However, the predicted rate of change of refractory material in response to altered climatic forcing (e.g., ultraviolet radiation at the ocean surface) is so slow that it may not be necessary to include it dynamically in ocean models used to examine climatic change within the next 200 years.