Phytoplankton biomass and production in the southeastern Beaufort Sea in autumn 2002 and 2003

The phytoplankton community of the Mackenzie shelf and the Amundsen Gulf (south- eastern Beaufort Sea) was characterized (e.g. chlorophyll a biomass, primary production and taxon- omy) during autumn 2002 (23 September to 14 October) and 2003 (30 September to 14 November). Spatial differences were evident, particularly in early autumn. Total phytoplankton biomass and the contribution of large cells (>5 µm) to biomass were higher in the Amundsen Gulf than on the Mackenzie shelf. The community of autotrophic cells (>10 µm) was numerically dominated by diatoms in the Amundsen Gulf and by dinoflagellates on the Mackenzie shelf. The abundance of chlorophytes revealed the influence of the Mackenzie River on the Mackenzie shelf. Contrary to 2002, when all measurements were from early October, the phytoplankton community of the Amund- sen Gulf in 2003 presented the characteristics of a late bloom, which presumably peaked in late Sep- tember. In early autumn, however, primary production rates were similar for both years, averaging 75 mg C m -2 d -1 . High primary production-to-biomass ratios and overall dominance of small cells (<5 µm) suggest that pelagic production in the southeastern Beaufort Sea was sustained by active recycling. During autumn 2003, a temporal decrease in phytoplankton biomass and primary produc- tion likely resulted from decreasing light availability. Overall, the autumnal primary production esti- mated in this study, from mid-September to the end of October, could increase the annual primary production previously estimated for the Beaufort Sea by 15%.