The Seasonal Mixed-Layer Pump is a Major Source of Energy for the Mesopelagic Ocean

The “mesopelagic” is the region of the ocean that extends from the bottom of the euphotic zone to a depth of about 1000 m. This region harbours one of the largest ecosystems and fish stocks on the planet, is involved in the global cycling of key elements, and ultimately controls the Earth's climate. Due to its remoteness, however, much remains to be learned on how this ecosystem functions. Because at these depths solar radiation is missing, the main source of energy for mesopelagic organisms is fast-sinking organic carbon produced at the surface. This energy supply, however, appears to be insufficient to satisfy the energetic requirements of mesopelagic community. A disregarded, but potentially important supply of energy to the mesopelagic is the mixed-layer pump. At the seasonal scale, this pump exports the organic carbon produced before the summer stratification, due to the formation of ephemeral shallow mixed layers. We quantify the spatial extent and global magnitude of the seasonal mixed-layer pump, and demonstrate that it supplies a significant amount of energy to the mesopelagic ocean. We estimate that this pump exports a flux of about 0.5 Pg of particulate organic carbon annually (range 0.3 – 1.0 Pg C yr⁻¹) and show that in high-latitude regions this flux is on average 60%, but can be greater than 100%, of the currently estimated carbon export flux. Our results stress that the interaction between high-frequency physical and biological processes is a fundamental driver of ocean ecosystems.