Shedding light on processes that control particle export and flux attenuation in the twilight zone of the open ocean

Pelagic food webs drive a flux of >10 x 10¹² kg C yr^-1 that exits surface waters, mostly via sinking particles through the ocean’s "biological pump." Much of this particle flux is remineralized in the poorly studied waters of the twilight zone, i.e., the layer underlying the euphotic zone and extending to 1000 m. We present a reanalysis of selected upper-ocean studies of particulate organic carbon (POC) flux and relate these observations to a simple one-dimensional biological model to shed light on twilight zone processes. The ecosystem model first predicts particle flux from the base of the euphotic zone, and then its attenuation below based on transformations by heterotrophic bacteria and zooplankton, and active downward transport of surface-derived particles by zooplankton. Observations and simulations both suggest that future sampling strategies for the twilight zone should take regional variability of the euphotic zone depth (Ez) into account. In addition, conventional curvefitting of particle flux data (i.e., power law or exponential) skews our interpretation of twilight zone processes. To overcome these artifacts, we introduce two new terms: the Ez-ratio (POC flux at Ez relative to net primary production [NPP]) and T₁₀₀ (the ratio of POC flux 100 m below Ez to POC flux at Ez). A comparison of NPP, Ez-ratios, and T₁₀₀ provides a new set of metrics to classify the ocean into different regimes, representing high and low surface export and subsurface flux attenuation.