Autotrophic and heterotrophic metabolism in the surface ocean has a significant influence over global climate through the marine biological carbon pump, a major natural sink for atmospheric carbon dioxide. The biological pump consists of phytoplankton, ‘fixing,’ carbon dioxide into organic carbon and the subsequent transport (export) of this organic carbon into the deep sea, mainly through sinking particles (below). Understanding the strength and efficiency at which this system operates is critical to fully comprehend the ocean’s role in the climate system and how it may respond to future climate forcing.
Over the continental margins, the proportion of total organic carbon that is exported is much greater than that in the open ocean, so even small fluctuations in biological productivity in these regions can have a large impact on the global carbon budget. During my graduate work, I used dissolved oxygen and its isotopes as tracers of export (‘Net’ biological production; red dots in top panel) and export efficiency (‘Net/Gross’ production; blue dots in middle panel) out of the euphotic zone through time in a coastal upwelling region in Southern California.
We found that export efficiency varied by up to an order of magnitude on an annual cycle: lowest in the fall when there is no upwelling and highest just following upwelling initiation in early spring (upwelling velocity shown in bottom panel), presumably as the ecosystem shifts into the spring assemblage. But, as upwelling velocity and photosynthetic production (Gross; black dots) increased through the spring, export efficiency (Net/Gross) decreased. Furthermore, the peak magnitude of export efficiency in spring suggests efficient photosynthetic energy conversion by phytoplankton, the first observation of this in temperate latitudes, which may contribute the dominance of diatom species in the spring assemblage.
Collaborators: Doug Hammond (USC), Maria Prokopenko (Pomona), Rachel Stanley (Wellesley), Will Berelson (USC)