Colloids and the Ocean Carbon Cycle

Abstract

Colloidal organic carbon (COC) is a globally significant fraction of dissolved organic carbon (DOC) in the surface ocean. As a non-living but reactive reservoir of carbon, COC outweighs the living carbon stored in marine biomass by a wide margin. Aggregation, respiration and photooxidation are all involved in the removal of COC from surface waters. Despite these removal mechanisms, bioreactive COC can still build up. The end result of the incorporation of this COC into the repeated diurnal cycling of carbon is the accumulation of old, low molecular weight organic carbon (LMWOC) at the expense of younger, more reactive COC. With respect to the sequestration of carbon in deep water, COC is either caught up into aggregates that can sediment out to fuel the deep ocean ecosystem or is converted to less reactive LMWOC that can accumulate prior to deep transport by winter mixing. An understanding of the degree to which aggregation or accumulation is coupled to the net production of DOC is a key requirement for any quantitative description of the ocean carbon cycle.