Size as a Trait for Understanding the Role of Zooplankton in the Biological Carbon Pump

Living organisms impact carbon transport between the atmosphere and the ocean through the biological carbon pump. Some plankton communities augment carbon export from the ocean’s surface, and are thought to have a major role in global climate. These export communities are often characterized by larger organisms that sink to depths where the carbon they contain is sequestered from the atmosphere. Zooplankton can enhance export by aggregating prey into larger sinking fecal pellets; however fecal pellet flux is a highly variable component of the biological carbon pump. Relating plankton trophic dynamics to changes in particulate carbon flux is an important step in understanding the ocean’s carbon cycle. This research aims to connect plankton ecology with variability in zooplankton fecal pellet carbon flux, using body size as an organizing trait. A copepod fecal pellet carbon flux model is presented and applied to 25 years of copepod data from the Gulf of Maine. This model uses size-based metabolic rates to estimate fecal pellet production, and sinking and decay functions to estimate flux. The results show that copepod community size structure determines fecal pellet carbon flux efficiency, but that flux itself is determined by copepod abundance and size. A second iteration of this model, which includes a temperature-dependent pellet decay function and diel vertical migration behavior, is applied to 55 years of copepod data from the North Atlantic Ocean. Analysis shows that fecal pellet carbon flux is decreasing as a result of declining copepod biomass, coincident with ocean warming. However, these changes vary from region to region, highlighting the importance of local dynamics. A study of local-scale trophic dynamics in the Gulf of Maine tests whether feeding and zooplankton fecal pellet production increases particle size, and therefore flux potential, in plankton communities. These experiments show tight coupling between microplankton and mesozooplankton, and demonstrate the importance of fecal pellet production as a mechanism for aggregating smaller particles into larger, sinking fecal pellets. Collectively this work shows that organism body size can be used as an organizing trait to connect individual-scale biology with variability in the biological carbon pump