Nitrogen fixation-enhanced carbon sequestration in low nitrate, low chlorophyll seascapes

The magnitude of fluxes in the carbon cycle of subtropical and tropical marine habitats is determined by the supply of inorganic nutrients. These habitats have low sea-surface concentrations of nitrate (NO3) and chlorophyll (dubbed LNLC regions), sustain relatively low rates of organic matter production and export, and represent global ocean minima in carbon sequestration potential. The low NO3 resupply should select for nitrogen (N2)-fixing bacteria, termed diazotrophs, provided all other growth-limiting nutrients are available. Several recent field efforts have been aimed at enhancing N2 fixation in LNLC regions through mesoscale fertilization with iron and phosphorus (or both) and we hypothesize herein that controlled upwelling of nutrient-enriched deep water may also be effective. Based on a quantitative assessment of the vertical distribution of NO3, phosphate (PO4) and dissolved inorganic carbon (DIC) at Station ALOHA (22° 45’ N, 158° W), we hypothesize that the process of controlled upwelling of low NO3:PO4 seawater may lead to enhanced N2 fixation, organic matter production and net carbon sequestration. Furthermore, based on a long-term (20 yr) data set from Station ALOHA, we predict that the upwelling of water from a depth of 300 to 350 m during summer months will trigger a 2-stage phytoplankton bloom. The first stage will be characterized by a NO3-supported Redfield ratio (e.g. C106:N16:P by atoms) diatom bloom. Following quantitative NO3 removal, the residual PO4 from the low N:P (<16:1) upwelled nutrient pulse will stimulate a N2-fixing bacterial bloom, leading to net sequestration of carbon. However, any strategic benefit of controlled upwelling for enhancing the long-term carbon sequestration will depend on the spatial and temporal uncoupling of organic matter production and remineralization, which is ultimately controlled by the microbial response to these perturbations.