Phytoplankton living in the surface ocean are responsible for roughly half of the planet's photosynthesis and hence modulate the ocean's ability to take up anthropogenic CO2. Most of the CO2 taken up by phytoplankton is re-released to the joint surface ocean-atmosphere system due to the respiration of grazers and bacteria. Sequestration of carbon for greater than seasonal time periods requires transport of this organic carbon into the deep ocean, a process that is largely dependent on the gravitational flux of sinking particles.

 

Particles that sink into the deep ocean transport carbon against its concentration gradient in a process known as the 'biological pump'. These particles include fecal pellets, aggregates, and dead phytoplankton. Their sinking rates are dependent on size (large particles sink faster than smaller particles) and excess density (the density difference between the particle and ambient seawater). Processes that degrade particles (e.g. grazing and bacterial remineralization) and re-package particles (e.g. aggregation and fecal pellet production) are thus crucial to determining the strength of the biological pump.

 

I have taken a multi-disciplinary approach to studying the biological pump and employ many different methods. Sediment traps and 234Th measurements allow me to quantify the flux of sinking particles. Analysis of sediment trap contents (microscopic enumeration of fecal pellets and micro-plankton, flow cytometric analyses of picoplankton, and pigment analyses) allow a glimpse into the nature of sinking material. Meanwhile, contemporaneous phytoplankton production and zooplankton grazing measurements allow for a comparison to ecosystem structure and combination of biogeochemical measurements with data assimilative physical circulation models allow quantification of subduction and vertical mixing of organic matter.

 

An additional component of the biological pump that is often neglected stems from the active migrations of mesozooplankton and mesopelagic fish, which feed in the surface during the night and respire at depth during the day. Using paired day-night MOCNESS samples, my collaborators and I have found that diel vertically migrating mesozooplankton contribute an additional carbon flux equivalent to 19% of the total passive (sinking particle) flux.