Fig. 4 Phaeocystis pouchetii/diatom (P/D) cell carbon ratio is compared suspended (integrated mg C m-2, 0-40 and 0-100 m) and in the corresponding sediment trap material (mg C m-2 d-1) at (a) 40 m and (b) 100 m depth during spring bloom events. The 40 m data includes measurements from North Norwegian fjords (Balsfjord in 1996 and Balsfjord, Ullsfjord and Malangen 1997, n = 26), and the 100 m data includes measurements from information on the contribution from Phaeocystis cell carbon from 15 different ecologically settings in five regions (Antarctica, coastal North Sea, North Norwegian fjords, the Barents Sea and the central Arctic Ocean; Fig. 5). Except for Balsfjord -96, all rates were obtained by short-term (24 h) sediment traps, deployed at or close to 100 m depth ( ± 10 m). In the shallow North Sea, sediment trap sampling was only possible at 20 m and 30 m. The vertical fluxes presented are averages of several measurements (68 observations in total) covering periods during and closely after Phaeocystis spp. blooms (see Table 1 and Fig. 5 legend for details).

The averaged, daily POC fluxes are highly variable between the different regions, with rates on the North Sea shelf and the central Arctic Ocean representing the extremes of 1,500 and 50 mg C m-2 d-1, respectively. The highest and lowest PPC fluxes were also measured in these two regions (430 and 12 mg C m-2 d-1, respectively). The polar region exhibited POC fluxes that were a factor of 10 or more below the North Sea. Within the Barents Sea, there was a distinct decrease in POC fluxes from the station in the deeper-mixed and more-productive Atlantic region, to the Polar Front, and the more-stratified station in the ice-covered Arctic region. In the North Sea, the <40 m depth obviously played an important role

Balsfjord 1996, Ullsfjord 1997, Balsfjord, Malangen and Ullsfjord 2001, Barents Sea 1998 and Amundsen Basin, Arctic Ocean 2001 (n = 19) (for further details, see Table 1). A ratio >1 reflects Phaeocystis dominance, while a ratio <1 reflects diatom dominance. Open circles indicate stations were mixing exceeded 90 m at the Barents Sea Polar front and Atlantic water (BS-AW) region in May 1998 (for BS-AW; P/D cells C ratio = 43)

for the delivery of high POC fluxes 10 m above the seafloor. The flux gradient in the Barents Sea is influenced by vertical mixing in frontal zones and less-stratified waters that fertilises primary production and increase sinking speed through active vertical transport of organic carbon and phyto-plankton (Olli et al. 2002; Reigstad et al. 2002). The vertical export of POC can therefore be relatively higher in shallow regions like the North Sea and vertically well-mixed regions such as the central Barents Sea, where retention time and time available for degradation is shortened.

Phaeocystis spp. was present at all stations (Fig. 5), with a Phaeocystis/diatom ratio (cell C/ cell C) of 0.2-5.3. Despite the differences, two aspects were common for the investigated sites. First, PPC never exceeded 45% of POC flux at -100 m depth, with an average of 26% (7-42%). This implies that a large phytoplankton-derived carbon is partly degraded before it reaches depths of 100 m. Second, the contribution from exported Phaeocystis spp. cell C to the POC export was small, ranging from 0.7-7%, with an average of 3% for all stations. The two deep-mixed stations made an exception with 15 and 35% P. pouchetii cell C of POC flux at 90 m depth, increasing the overall average to 6%. Comparatively, the diatom carbon contributed 1-27% to the 100 m measured POC flux, with an average of 11%.

a) 1600

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