Program

Carbon-rich transparent exopolymer particles (TEP) are highly concentrated in surface waters, and their high stickiness promotes particle aggregation (biological glue). TEP are thus seen as a key factor driving the downward particulate organic carbon (POC) flux, which is a major component of the ocean’s biological C pump. However, TEP density is lower than that of seawater, which causes them to remain in surface waters and even move upwards. Hence TEP often accumulate in the surface microlayer.

Here we question the generally accepted view that TEP always increase the downward POC flux via gravitational settling. We examine how the presence of the non-sinking TEP pool in surface waters influences the cycling of organic C in the upper ocean. We focus on the role of TEP in retaining organic C in surface waters versus downward export. We show that TEP sink only when ballasted with enough high-density particles to compensate their low density. Hence the TEP fraction of POC determines the degree of retention and remineralization of POC in surface waters versus its downward export, and high TEP volume concentration may result in decoupling the timing of primary production and downward export. The contribution of TEP to POC in surface waters depends on TEP stickiness and the balance between TEP production and degradation. Because these two parameters vary with environmental conditions, the role of TEP in the balance of retention versus export, and hence the drawdown of atmospheric CO₂ by the biological C pump, can be highly variable, and is likely to be affected by climate change.