) decreased markedly from west to east, with minima in NO 3 2 and NH 4 1 in surface BS-CB waters, but relatively invariant urea-N concentrations across the entire region. In the BS-CB domain, low uptake rates of nitrate (qNO 32 ) and ammonium (qNH 4 1 ) were exceeded by uptake of urea (qUrea-N). Whereas average qNO 3 2 was highest in the BE-CH domain, qUrea-N was maximal in BB-LS. Average depth-integrated f-ratios ranged from 0.27 in the BS-CB domain to 0.57 in BE-CH, while chlorophyll a (chl a) and primary productivity (qC) were highest in BE-CH and BB-LS, and consistently low in the BS-CB domain. The >5 mm phytoplankton fraction dominated qC and qNO 3 2 in the BE-CH and CAA domains, whereas ESNP and BS-CB were dominated by the <5 mm fraction. In the BB-LS domain, the larger cells were responsible for $50% of qC, qNO 3 2 , and qUrea-N. This study highlights the contrast in ice-corrected average new production between the BE-CH (396 mg C m 22 d 21 ) and BS-CB (5.50 mg C m 22 d 21) domains in summer, and the larger contribution of urea-N uptake to total N uptake in central and eastern regions where NO 3 2 concentrations were lower.

Suspended biogenic particles were examined across Subarctic and Arctic Seas during the Canada‐Three‐Oceans (C3O) program in the summers of 2007 and 2008. Particulate organic carbon (POC) and nitrogen (PON), biogenic silica (bSiO2), and size‐fractionated chlorophyll a (chl a) concentrations were measured within the euphotic zone throughout five domains: Eastern Subarctic North Pacific (ESNP), Bering and Chukchi Seas (BE‐CH), Southern Beaufort Sea and Canada Basin (BS‐CB), Canadian Arctic Archipelago (CAA), and Baffin Bay and Labrador Sea (BB‐LS). Despite large variability along the sampling transect and vertically throughout the euphotic zone, domain averages of depth‐integrated POC and PON were relatively invariant across the ESNP, BE‐CH, CAA, and BB‐LS, ranging from 546 to 670 mmol m−2 (POC) and 101 to 154 mmol m−2 (PON), but were much lower in BS‐CB (199 and 78 mmol m−2, respectively). In contrast, depth‐integrated bSiO2 averages were highest in BE‐CH and CAA (>100 mmol m−2), intermediate in ESNP and BB‐LS (34–48 mmol m−2), and low in BS‐CB (6 mmol m−2). Similarly to the bSiO2 distribution, the biomass of the >5 μm chl a size‐fraction averaged 68–69% of total chl a in BE‐CH and CAA, 56% in BB‐LS, and 22–23% in ESNP and BS‐CB. Ratios of bSiO2:PON averaged 1.10–1.30 in BE‐CH and CAA, 0.30–0.34 in ESNP and BB‐LS, and 0.08 in BS‐CB. This study highlights the relative importance of siliceous phytoplankton such as diatoms in BE‐CH and CAA, smaller nonsiliceous cells in BS‐CB and ESNP, and a mixed assemblage of variably sized phytoplankton with a less siliceous component in BB‐LS.

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During the Canada's Three Oceans and Joint Ocean Ice Study projects in the summers of 2007 and 2008, we measured particulate organic carbon to nitrogen ratios (POC:PON) throughout the euphotic zone in Subarctic and Arctic waters. Average depth‐integrated values (2.65 ± 0.19) in the Beaufort Sea and Canada Basin (BS‐CB domain) were much lower than both the Redfield ratio (6.6) and the average ratios (3.9 to 5.6) measured across other Arctic‐Subarctic domains. Average uptake ratios of C and N (ρC:ρN) were also lower (0.87 ± 0.14) in BS‐CB than in the other four domains (2.10 to 3.51). Decreasing POC:PON ratios were associated with low concentrations of phytoplankton C, reduced abundance of biogenic silica (bSiO2), a smaller relative contribution of the >5 µm fraction to total chlorophyll a and a larger relative contribution of small flagellates (<8 µm) to total phytoplankton C. In the subsurface chlorophyll a maximum (SCM) within the BS‐CB domain, phytoplankton C represented only ~13% of POC; and therefore, the presence of heterotrophic microbes may have decreased POC:PON. These ratios are supported by data obtained during other Arctic programs in 2006, 2008, and 2009. Previous work has suggested a link between freshening of surface waters and increasing dominance of picophytoplankton and bacterioplankton in the Canada Basin, and the low POC:PON ratios measured during this study may be a consequence of this shift. Our results have ramifications for the conversion between C‐ and N‐based estimates of primary productivity, and for biogeochemical modeling of marine Arctic waters.

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