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

A one-dimensional model incorporating separate labile, semilabile, and refr actory fractions of dissolved organic carbon (DOC) is used to study the ver tical 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 pred icted lifetimes of 0.4 years at the ocean surface increasing to 6.3 years a t 1000 m, owing to diminishing bacterial numbers. Although predicted fluxes of DOC and sinking particles exiting the upper mixed layer are similar, la bile 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 b ecause it does not capture three-dimensional processes such as subduction. The inclusion of a biologically inert refractory pool is a first step to tr y and incorporate the growing awareness that photochemical processes play a n important role in the dynamics of organic carbon in the ocean. However, t he predicted rate of change of refractory material in response to altered c limatic forcing (e.g., ultraviolet radiation at the ocean surface) is so sl ow that it may not be necessary to include it dynamically in ocean models u sed to examine climatic change within the next 200 years.