Quantifying the time lag between organic matter production and export in the surface ocean: Implications for estimates of export efficiency

The ocean's potential to export carbon to depth partly depends on the fraction of primary production (PP) sinking out of the euphotic zone (i.e., the e‐ratio). Measurements of PP and export flux are often performed simultaneously in the field, although there is a temporal delay between those parameters. Thus, resulting e‐ratio estimates often incorrectly assume an instantaneous downward export of PP to export flux. Evaluating results from four mesocosm studies, we find that peaks in organic matter sedimentation lag chlorophyll a peaks by 2 to 15 days. We discuss the implications of these time lags (TLs) for current e‐ratio estimates and evaluate potential controls of TL. Our analysis reveals a strong correlation between TL and the duration of chlorophyll a buildup, indicating a dependency of TL on plankton food web dynamics. This study is one step further toward time‐corrected e‐ratio estimates. Plain Language Summary In times of rising atmospheric carbon dioxide concentrations, it is important to understand how efficient our oceans are in transporting organic matter produced in the surface to depth. In order to estimate this efficiency, surface production and organic matter arrival at depth are monitored simultaneously during field expeditions. Unfortunately, the produced organic matter does not sink out immediately and when it does, it may be transported horizontally by currents. Without the ability to track this organic matter on its way through the water column, we may connect collected organic matter at depth to production measurements at the surface that are completely unrelated. In this paper, we address the temporal part of the problem: the time lag between production of organic matter at the surface and the initiation of its sinking process. We find that this time lag differs substantially throughout the ocean. We argue that this is due to differences in the plankton community. The large range of time lag we observed shows that we have to adapt our methodology in estimating the efficiency of organic matter transport to depth. Otherwise, we may under‐ or overestimate the oceans potential to store carbon. Key Points We calculated time lags between peaks in chlorophyll a and organic matter sedimentation for four mesocosm studies in different regions Time lags varied between 2 to 15 days at the surface and increased with depth depending on sinking velocities Time lag correlated with chlorophyll a buildup rate, indicating a dependency of time lag on th