Phytoplankton Community Structure and the Efficiency of the Biological Pump

The sinking of particulate organic matter and the advection/mixing export of dissolved organic matter “pumps” carbon from surface waters into the ocean interior, lowering surface dissolved inorganic carbon and pCO₂concentrations. The sinking particulate flux accounts for most of the sequestration of carbon and nutrients in the deep ocean by the biological pump. The efficiency of the export or carbon and nutrients from surface waters and the efficiency of the downward flux of sinking material at depth depend on a large number of biogeochemical processes. Particle size, porosity, and composition all influence the efficiency of the downward transport and remineralization at depth. These factors in turn are strongly influenced by the ecosystem structure in surface waters. Marine ecosystem models often link the export efficiency of different phytoplankton groups to cell size, with export efficiency increasing with increasing cell size. Often models include only 2-3 phytoplankton groups, which crudely encompass the actual size spectrum with “large phytoplankton” vs. “small phytoplankton” classes. The difference in export efficiencies between large vs. small phytoplankton varies widely across models. The impact of different phytoplankton groups on export efficiency is likely amplified by the associated grazer communities. Export efficiency out of surface waters strongly impacts rates of net primary production by determining the regenerated primary production.

Experiments with the ocean component of the Community Earth System Model, in conjunction with observed nutrient distributions and biogeochemical fluxes, will be used to test hypotheses concerning phytoplankton community structure and the efficiency of the biological pump. The CESM includes key growth-limiting nutrients (N, Fe, P, Si), multiple phytoplankton groups, semi-labile and refractory dissolved organic matter pools (C, N, P), and sinking particulate fluxes (lithogenic, bSi, CaCO₃, POM, Fe). The export efficiency of the different phytoplankton groups will be manipulated in experiments designed to test several hypotheses: 1) All phytoplankton production is exported from surface waters and transported to depth with the same efficiency; 2) Diatoms export organic matter from surface waters more efficiently than other phytoplankton; 3) Phytoplankton blooms export organic matter very efficiently. These hypotheses will be evaluated through comparison of the model output with large-scale observations of nutrients, oxygen, and carbon distributions.