Sinking biological aggregates in the ocean - a modeling study

Carbon dioxide is transported from the atmosphere to the deep ocean via sinking aggregates of phytoplankton cells in a process called the biological pump. Aggregate sinking velocity and respiration rate of organic carbon determine how much organic carbon is removed from the upper ocean and atmosphere on a timescale longer than a few years. The sinking velocity of particles depends on their size and density, and respiration rates depend to a large degree on temperature.;To understand mechanistically how the environment affects the flux of organic carbon we present a new approach: a stochastic particle-resolved model which explicitly resolves the composition and sinking velocity of particles in a column of water. The model tracks sinking particles from when phytoplankton cells are produced at the sea-surface, as they aggregate and are consumed by biology, to when they hit the seafloor. Bacterial respiration and zooplankton consumption of aggregates are included in the model.;The model is able to reproduce regional variations in organic carbon export efficiency and rain ratio (organic carbon:calcium carbonate) to the sea floor by varying sea surface temperature, primary production and seasonality. It also captures the increasing rain ratio with high-latitude surface conditions.;The flux is sensitive to sea surface temperatures. Higher temperatures result in greater organic carbon respiration by zooplankton and bacteria which works to decrease the flux. We find that minerals work as ballast for the aggregate; they counteract the low density of organic carbon and TEP and increase the sinking velocity of aggregates. TEP play a dual role in transporting organic carbon. They provide stickiness that increases coagulation and aggregate size but their density is low thus decreasing the overall density of particles.;When we apply predicted future climate change: an increase in temperature, a decrease in calcification and an increase in organic carbon and TEP production, the response of the biological pump is in all cases a decrease in efficiency. Our results therefore suggest that the biological pump works as a positive feedback in the carbon cycle.