Seminar: Dr. Curtis Deutsch, UW

The role of phytoplankton size in the biological pump and its response to climate warming (Host: Carly Moreno)  The settling of organic particles into the deep sea sustains complex food webs, sequesters CO₂ from the atmosphere, and constrains marine aerobic habitat.  Despite these profound effects, the factors that regulate the downward transfer of particulate matter are poorly understood, and difficult to directly observe.  I will present a mechanistic model of particle sinking and decomposition through the deep ocean that accounts for a wide range of physical and biological factors, from particle size and composition to microbial colonization and metabolic rates.   The model is tested by its ability to reproduce global nutrient distributions, which reflect the long-term and large-scale variations in the depths of particle decay. Global nutrient data imply a leading role for the plankton size spectrum; regions dominated by large phytoplankton exhibit weaker vertical attenuation of remineralization rates in the upper kilometer of the water column, and thus a higher efficiency of carbon and nutrient transfer to the deep sea.  These results imply that as climate warms and ocean circulation slows, the expected shift toward smaller phytoplankton will result in shallower remineralization of carbon and nutrients with a shorter return path to the photic zone.  We estimate this plankton size feedback will slow the decline of global export production by ~30%.