Resolving seasonal to annual marine net community production using biogeochemical Argo float observations

Abstract

The marine biological carbon pump sequesters dissolved inorganic carbon in the ocean interior, influencing atmospheric carbon dioxide levels and the global climate on long timescales. Changes in the environment impact biogenic carbon production and export on various scales. However, future changes in biological carbon cycling and the resulting impact on the global carbon cycle remain uncertain. Key challenges in understanding the biological carbon cycle include identifying drivers of variability in organic carbon production in the surface ocean and connecting surface organic carbon production to biogenic carbon storage in the ocean interior. To address these challenges, this work leverages biogeochemical observations from robotic profiling floats to quantify net community production (NCP), the net production of organic carbon by phytoplankton minus community respiration, on seasonal to annual timescales and from the surface to the ocean interior. The first study focuses on the Southern Ocean seasonal sea ice zone, where sea ice has recently declined. This study reveals that the timing of initial sea ice breakup, which occurs while sea ice concentrations are high, controls the onset of the seasonal increase in phytoplankton biomass and net organic carbon production. Furthermore, the majority of seasonal NCP is found to occur under sea-ice cover, and the magnitude of seasonal NCP is influenced by sea ice seasonality. The second study investigates how Southern Ocean surface mixed layer NCP during seasonal blooms relates to annual NCP, the annual organic carbon sequestered from the atmosphere (ANCP). This study reveals that a variable fraction of organic carbon produced during seasonal blooms is respired above the winter mixed layer depth and is re-entrained, which decouples seasonal surface NCP and ANCP. This finding highlights the importance of understanding the mechanisms that control variability in respired and re- entrained carbon to identify how changes in surface organic carbon production may influence ANCP in the Southern Ocean. The third study characterizes ANCP and the amount of respired and re-entrained carbon in ten global open ocean biomes. Variability in the fraction of respired and re-entrained carbon is found to be related to the ratio of the euphotic depth to the maximum mixed layer depth. Subpolar biomes have elevated surface seasonal NCP; however, winter mixing extends below the euphotic zone and re-entrains respired carbon, while subtropical biomes have deep euphotic zones and very little respired and re-entrained carbon. This leads to similar ANCP in seasonally stratified and subtropical permanently stratified biomes. Collectively, these studies provide insight into how environmental changes may affect seasonal NCP as well as alter the amount of coupling between surface NCP and annual export, and offer new considerations for connecting surface and interior observations.