The balance of marine autotrophy and heterotrophy regulates the ocean’s ability to serve as a sink for atmospheric carbon dioxide through carbon export via the biological pump. The efficiency of this process varies, often in conjunction with variability in surface ocean dynamics, but it is still unclear what drives these changes.
In a recent paper in GBC, my colleagues and I determined that euphotic zone export efficiency varied predictably on an annual cycle in a coastal upwelling zone, peaked just following upwelling initiation, but decreased through the spring while upwelling velocities and biological production increased. This finding suggests that shifts community composition and structure associated with transitions in the physical environment ultimately result in fluctuations in export efficiency. Furthermore, the magnitude of the maximum in export efficiency implies that phytoplankton alter their energetic efficiency during this time of year and likely play a more significant role in determining export efficiency than previously thought.
Dynamic ecosystems over the continental margins, especially coastal upwelling regions, account for a disproportionate amount of carbon export, thus efforts to better quantify the impact of physical dynamics on carbon export may significantly advance our understanding of the global carbon cycle.