Late summer phytoplankton distribution along a 3500 km transect in Canadian Arctic waters: strong numerical dominance by picoeukaryotes

A number of recent studies showed that photosynthetic picoeukaryotes are an active and often dominant component of Arctic algal assemblages. In order to place these observations in a large-scale context, samples were collected in the euphotic zone along a 3500 km transect across northern Baffin Bay, the Northwest Passage and the Beaufort Sea during late summer 2005. Picophytoplankton (< 2 mu m) and nanophytoplankton cells (2 to 20 mu m) were enumerated using flow cytometry, and phytoplankton cells > 2 mu m were identified and counted by light microscopy. Pigment composition of the total community was assessed by reverse-phase HPLC to determine the relative contribution of different algal groups. The spatial distribution of phytoplankton was heterogeneous along the transect. The highest abundance of picophytoplankton was observed in the Beaufort Sea/Northwest Passage region, whereas nanophytoplankton increased numerically toward the eastern Canadian Arctic. Picophytoplankton abundance reached a maximum of 18400 cells ml(-1) and accounted for > 70% of total cell counts in two-thirds of the samples. The < 2 mu m size fraction held a similar share of total chl a, which reached a maximum of 6 mu g l(-1). Overall, the picophytoplankton community was strongly dominated by eukaryotes (presumably the Prasinophyceae Micromonas). Maximum abundances of picocyanobacteria (120 cells ml(-1)) were observed in brackish waters of the Beaufort Sea. These results confirm that picophytoplankton can dominate not only in warm oligotrophic waters, but also in a perennially cold ocean during late summer.