Distribution and bacterial availability of dissolved neutral sugars in the South East Pacific

Abstract

Abstract. The distribution and bacterial availability of dissolved neutral sugars were studied in the South East Pacific from October to December 2004 during the BIOSOPE cruise. Four contrasted sites were investigated: Marquesas Islands (MAR), the hyper-oligotrophic South Pacific Gyre (GYR), the eastern part of the Gyre (EGY), and the coastal waters associated to upwelling of Chile (UPW). Total (free and combined) dissolved neutral sugar (TDNS) concentrations were higher in UPW (149–329 nM) and MAR (111–540 nM), than in GYR (79–390 nM) and EGY (58–492 nM). Nevertheless, their contribution to dissolved organic carbon (TDNS-C/DOC%) was generally low for all sites varying from 0.5% to 4% indicating that our South East Pacific surface waters were relatively poor in neutral sugars. Free dissolved neutral sugar (FDNS; e.g. sugars analyzed without hydrolysis) concentrations were very low within the detection of our method (5–10 nM) accounting <5% of the TDNS. In general, the predominant sugars within the TDNS pool were glucose, xylose, arabinose, and galactose while in the FDNS pool only glucose was present. TDNS stock to bacterial production ratios (integrated values from the surface to the deep chlorophyll maximum) were relatively high in GYR with respect to the low primary production, whereas the opposite trend was observed in the highly productive area of UPW. Intermediate situations were observed for MAR and EGY. Bioavailability of dissolved organic matter (DOM) exposed to natural solar radiation was also experimentally studied and compared to dark treatments. Our results showed no or little detectable effect of sunlight on DOM bacterial assimilation in UPW and in GYR while a significant stimulation was found in MAR and EGY. The overall results clearly suggest the semi-labile character of DOM in GYR compared to the labile of UPW and are consistent with dissolved organic carbon accumulation and the elevated C/N ratios reported by Raimbault et al. (2007).