How do changes in plankton community structure influence the biological pump? A mesocosm study

Strength and efficiency of the biological pump have a profound influence on the marine carbon cycle and depend on the plankton community structure of the ocean. Unraveling this control will significantly increase our understanding of the biological pump and help predicting changes in the future. However, investigating this relationship with traditional methods is challenging in the open ocean.

During autumn 2014, we conducted a mesocosm experiment off the coast of the Canary Islands (CE Atlantic). We closely followed the development of a plankton community under different CO₂ levels for eight weeks, while simultaneously measuring the mass flux, sinking velocity and remineralization rates of sinking detritus. After two weeks of low productivity we added deep-water (collected from 500 m depth) to each mesocosm to mimic eddy-induced upwelling. The addition induced a diatom bloom, characterized by an increased abundance of large particles and high mass flux. However, sinking velocities decreased by up to 30% and remineralization rates remained rather constant, indicating that higher mass flux is associated with lower export efficiency during the bloom. Our first results highlight the importance of community-related changes on both export strength and efficiency of the biological pump. We show that high mass flux events are potentially less efficient in sequestering carbon from the atmosphere.

Ultimately, changes in plankton community structure have the potential to significantly affect oceanic carbon sequestration in a changing environment.