Low-nutrient organic matter in the Sargasso Sea thermocline: A hypothesis for its role, identity, and carbon cycle implications

Despite slow nutrient supply to the subtropical surface ocean, its rates of annual inorganic carbon drawdown and net oxygen production are similar to those of nutrient-rich high latitude waters. This surprisingly rapid carbon drawdown, if due to the production and export of marine biomass, cannot be explained in terms of known nutrient supply mechanisms. Moreover, carbon budgets have failed to detect the export of this organic matter. One possible explanation is the export of nutrient-poor organic matter with a composition that avoids detection as sinking particles. We describe three signs of the decomposition of such organic matter in the shallow Sargasso Sea subsurface. First, summertime oxygen consumption at 80–400 m occurs without the rate of nitrate and phosphate production expected from the remineralization of marine biomass, matching the observed summertime mixed layer inorganic carbon drawdown. Second, a seasonal change in the ¹⁸O/¹⁶O of subsurface nitrate suggests summertime heterotrophic bacterial nitrate assimilation down to ~400 m, as may be required for the remineralization of nutrient-poor organic matter. Third, incubation of subsurface seawater leads to nitrate drawdown and heterotrophic bacterial growth, supporting the thermocline nitrate ¹⁸O/¹⁶O evidence for heterotrophic nitrate assimilation. These three pieces of evidence suggest the export of nutrient-poor organic matter from the surface at a rate adequate to explain net community production in the Sargasso Sea. We propose that transparent exopolymer particles or related compounds, generated by a nutrient-limited upper ocean ecosystem, comprise this nutrient-poor export, and that its properties cause its flux out of the euphotic zone to be underestimated by sediment traps. Such nutrient-poor organic matter would contribute little to fisheries, deep ocean carbon dioxide storage, or organic carbon burial, so that it may change our view of the significance of net community production in the subtropical ocean.