Meridional variability in dissolved organic matter stocks and diagenetic state within the euphotic and mesopelagic zone of the North Atlantic subtropical gyre

Concentrations of dissolved organic carbon (DOC), bulk carbohydrates (TCHO), and dissolved combined neutral sugars (DCNS) were measured during a meridional U.S. CO2 — Climate Variability (CLIVAR) Repeat Hydrography Project cruise (A20) from 7–43°N in the North Atlantic. Maxima in DOC, TCHO, and DCNS concentrations and carbohydrate yields (TCHO and DCNS as a % of DOC) were observed within the most stratified surface waters south of 27°N. Concentrations and yields of TCHO and DCNS decreased to the north and south of their respective maxima, indicating significant variability in the apparent diagenetic state of dissolved organic matter (DOM) along the transect. Changes in apparent diagenetic state to the north appeared to be largely related to the previous entrainment of recalcitrant DOM during deep winter/spring convective mixing. Deep mixing was less of a factor in the more stratified waters to the south where the decrease in carbohydrate yields indicated diagenetic alteration of the surface-accumulated pool of DOM. Overall, diagenesis decreased both carbohydrate yields and the mol% of galactose and mannose + xylose content and in deep waters resulted in a significant enrichment in the mol% of glucose. TCHO yields and the mol% of glucose revealed significant differences in DOM quality between subtropical mode water (STMW) and upper Antarctic Intermediate Water (uAAIW) that were related to the extent of water mass ventilation.