Nutrient-Poor, Slowly Sinking Organic Matter in the Sargasso Sea - A Hypothesis and Supporting Evidence

Despite slow nutrient supply to the subtropical surface ocean, the rates of annual inorganic carbon uptake and net oxygen production are similar to those of nutrient-rich high-latitude waters. This surprisingly high subtropical carbon uptake cannot be fully accounted for by sinking particles collected in sediment traps and the downward mixing of suspended and dissolved organic carbon. We propose an explanation for these paradoxical observations: gel-like organic matter rich in carbon but poor in nutrients, akin to transparent exopolymer particles, is produced by phytoplankton under nutrient limitation, and a portion sinks into the shallow subsurface, where it is respired by heterotrophic bacteria. This organic matter would evade detection by sediment traps, effectively representing an additional (sinking-driven) source of dissolved organic carbon to the subsurface. Building on existing evidence for the production of such nutrient-poor organic matter in surface waters, we describe evidence for its decomposition in the shallow subsurface of the Sargasso Sea. First, oxygen at these depths is consumed over the summer without comparable production of nitrate. Second, a seasonal change in the 18O/16O of subsurface nitrate suggests summertime nitrate assimilation down to 400 m, likely by heterotrophic bacteria during remineralization of nitrogen-poor organic matter. Third, incubation of unfiltered subsurface seawater leads to nitrate drawdown and heterotrophic bacterial growth, indicating that the organic matter being remineralized encourages heterotrophic nitrate assimilation. Nutrient-independent organic matter export from subtropical surface waters would contribute little to fisheries, deep ocean carbon dioxide storage, or organic carbon burial in sediments. Given its first-order implications, this hypothesis warrants further investigation.