Variations of the abundance and nucleic acid content of heterotrophic bacteria in Beaufort Shelf waters during winter and spring Claude Belzile a, ⁎ , Sonia Brugel a , Christian Nozais b , Yves Gratton c , Serge Demers a a Institut des sciences de la mer de Rimouski, Université du Québec à Rimouski, 310 Allée des Ursulines, Rimouski, Québec, Canada G5L 3A1 b Département de biologie et centre d'études nordiques, Université du Québec à Rimouski, 300 Allée des Ursulines, Rimouski, Québec, Canada G5L 3A1 c Institut national de la recherche scientifique, Centre Eau, Terre et Environnement, 490 de la Couronne, Québec, Québec, Canada G1K 9A9 Received 15 May 2007; received in revised form 11 December 2007; accepted 17 December 2007 Available online 17 January 2008 Abstract Depth profiles of heterotrophic bacteria abundance were measured weekly over a 6-month period from December to May in Franklin Bay, a 230 m-deep coastal Arctic Ocean site of the southeastern Beaufort Sea. Total bacteria, low nucleic acid (LNA) and high nucleic acid (HNA) bacteria abundances were measured using flow cytometry after SYBR Green I staining. The HNA bacteria abundance in surface waters started to increase 5–6 weeks after phytoplankton growth resumed in spring, increasing from 1×10 5 to 3×10 5 cells mL - 1 over an 8-week period, with a net growth rate of 0.018 d - 1 . LNA bacteria response was delayed by more than two months relative to the beginning of the phytoplankton biomass accumulation and had a lower net growth rate of 0.013 d - 1 . The marked increase in bacterial abundance occurred before any significant increase in organic matter input from river discharge (as indicated by the unchanged surface water salinity and DOC concentrations), and in the absence of water temperature increase. The abundance of bacteria below the halocline was relatively high until January (up to 5 × 10 5 cells mL - 1 ) but then decreased to values close to 2 × 10 5 cells mL - 1 . The three-fold bacterial abundance increase observed in surface waters in spring was mostly due to HNA bacteria, supporting the idea that these cells are the most active. © 2007 Elsevier B.V. All rights reserved. Keywords: Heterotrophic bacteria; Flow cytometry; Beaufort Sea; Arctic shelves; Arctic Ocean. 1. Introduction A very large amount of dissolved and particulate organic matter enters the Arctic Ocean from the numerous, large rivers discharging into this compara- tively small, strongly stratified ocean (Anderson 2002; Dittmar and Kattner 2003; Benner et al., 2005). The fate of this organic material of terrestrial origin, whether quickly re-mineralized or preserved for a long period of time, has a significant impact on the global carbon cycle. Many biogeochemical studies have suggested a mini- mum extent of photochemical and biological reminer- alization of dissolved organic matter (DOM) in shelf waters, resulting in the export of 25–50% of this material to the North Atlantic via the East Greenland current (Benner et al., 2005 and references therein). In the Arctic Ocean, the low amount of incident photo- chemically active irradiance resulting from the high Available online at www.sciencedirect.com Journal of Marine Systems 74 (2008) 946 – 956 www.elsevier.com/locate/jmarsys ⁎ Corresponding author. Tel.: +418 723 1986(poste)1332. E-mail address: claude_belzile@uqar.qc.ca (C. Belzile). 0924-7963/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2007.12.010