Plankton Respiration in the Ultradeep Waters over the Atacama Trench: Implications for Particulate Organic Carbon Sequestration

The deep ocean, the marine waters below 200 m depth, is the largest ecosystem on Earth. It makes up approximately 90% of the global ocean volume and, even though it is too dark for photosynthesis, represents the largest reservoir of organic matter in the biosphere. Hidden from satellite observations and hardly accessible by conventional methods, the contribution of this major component of carbon cycle to the ocean metabolism remains virtually unknown, especially when it comes to the abyssal and hadal realms. Here we present unique depth profiles of microplankton and zooplankton respiration over the Atacama Trench that served to calculate the minimum carbon flux needed to sustain the heterotrophic requirements in these ultradeep waters during the Austral summer 2018. According to the high plankton respiration rates measured at depth, which were mostly associated with the microplankton community, around 30 mmol C m^-2 d^-1 were necessary to satisfy the carbon demands below 4000 m, a value much higher than the one found in other less productive deep environments. Based on these carbon flux models, we further derived other biogeochemical variables such as the nutrient retention efficiency (NRE), a metric that quantifies the ability of an ocean layer to retain its nutrients. The estimated NRE values through the water column showed a more effient use of the available resources at depth (up to 80% in the hadalpelagic waters). Altogether our data show evidence of a dynamic environment with high metabolic activities in the deep water column over the Atacama Trench, which points to a great export of organic material into the ocean interior of this marine ecosystem.