The composition and distribution of semi-labile dissolved organic matter across the southwest Pacific

The distribution and dynamics of dissolved organic carbon (DOC) and dissolved combined neutral sugars (DCNS) were studied across an increasing oligotrophic gradient (18 to 22^∘ S latitude) in the tropical South Pacific Ocean, spanning from the Melanesian Archipelago (MA) area to the western part of the South Pacific gyre (WGY), in austral summer as a part of the OUTPACE project. Our results show that DOC and DCNS concentrations exhibited no statistical differences between the MA and WGY areas (0–200 m: 47–81 µM C for DOC and 0.2-4.2 µM C for DCNS). However, due to a deepening of the euphotic zone, a deeper penetration of DOC was noticeable at 150 m of depth at the WGY area. Excess DOC (DOC_EX) was determined as the difference between surface and deep-sea DOC values, and euphotic zone integrated stocks of both DOC and DOC_EX were higher in the WGY than the MA area. Considering DOC_EX as representative of semi-labile DOC (DOC_SL), its residence time was calculated as the ratio of DOC_SL to bacterial carbon demand (BCD). This residence time was 176±43 days (n=3) in the WGY area, about 3 times longer than in the MA area ($T_{\mathrm{r}}=\mathrm{51}\pm \mathrm{13}$ days, n=8), suggesting an accumulation of semi-labile dissolved organic matter (DOM) in the surface waters of WGY. Average epipelagic (0–200 m) DCNS yields (DCNS × DOC⁻¹) based on volumetric data were roughly similar in both areas, accounting for ∼2.8 % of DOC. DCNS exhibited a longer residence time in WGY ($T_{\mathrm{r}}=\mathrm{91}\pm \mathrm{41}$ days, n=3) than in MA ($T_{\mathrm{r}}=\mathrm{31}\pm \mathrm{10}$ days, n=8), further suggesting that this DCNS pool persists longer in the surface waters of the WGY. The accumulation of DOC_EX in the surface waters of WGY is probably due to very slow bacterial degradation due to nutrient and/or energy limitation of heterotrophic prokaryotes, indicating that biologically produced DOC can be stored in the euphotic layer of the South Pacific gyre for a long period.