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being advected into the fjord basin (Wassmann et al. 2000). Increased chlorophyll a concentrations, fresh cells and DMS at the bottom of the fjord (180 m depth) did not result from vertical export, but rather from horizontal advection combined with mixing processes in bloom-containing surface water (Wassmann et al. 2000; Belviso et al. 2006). In a similar manner, fresh biogenic material and nutrient-depleted water were present at >100 m depth in the Polar Front and Atlantic water region of the Barents Sea, resulting from extensive mixing processes of non-stratified surface waters (Reigstad et al. 2002). This process was effective in bringing P. pouchetii cells well below the euphotic zone in the deeper-mixed Barents Sea stations (Olli et al. 2002; Ratkova and Wassmann 2002; Reigstad et al. 2002). Locally such processes might be of importance for vertical export of Phaeocystis spp. especially as it has been suggested that Phaeo-cystis spp. is favoured in regions with deeper and well-mixed waters (60-80 m) (Goffart et al. 2000; Lancelot et al. 1998; Sakshaug 2004).

Concluding remarks

The main body of evidence based upon direct observations suggests that Phaeocystis spp. blooms are to a large extent recycled in the upper layers. Contradictory evidence derives from indirect evidence, suggesting that sediment traps may underestimate vertical export of Phaeocystis spp. An alternative to the deployment of sediment traps, the 234Th technique, has recently been applied in concert with sediment traps in the Barents Sea during the presence of P. pouchetii (Lalande 2006). Sediment-trap POC fluxes were much lower than large-volume POC fluxes at almost every station in summer 2005 (Lalande 2006). This may reflect either an under-collection by the drifting sediment traps or an over-collection by the large-volume 234Th sampling. The offset between the two methods may be attributed to the prominent presence of P. pouchetii in summer 2005, which is potentially causing the large variation observed in POC/234Th ratios (Lalande 2006). Due to the large proportion of carbon released by P. pouchetii and because

P. pouchetii cell C does not contribute significantly to the vertical export of biogenic matter, the utilization of large-volume sampling of 234Th may yield relatively high, and possibly incorrect, POC/234Th ratios and hence POC fluxes in regions where P. pouchetii is dominant. Large-volume 234Th sampling appears to be an inadequate alternative for vertical flux estimates in Phaeocystis spp.-dominated ecosystems. The use of sediment traps may be more reliable for the accurately measurement of the vertical export of biogenic matter in the Barents Sea (Lalande 2006).

The fate of Phaeocystis spp. blooms, and the possible contribution to carbon export have been investigated and discussed for several decades (Wassmann 1994; Lancelot et al. 1998; Schoe-mann et al. 2005; Whipple et al. 2005). Clearly, there is extraordinary flexibility and plasticity in the ecology, life cycle and fate of this genus, related to species, morphotypes and environmental conditions (Veldhuis and Wassmann 2005). Beside the many mechanisms and processes promoting retention and recycling of Phaeocystis spp.-derived carbon, there are also several pathways for vertical carbon export. An attempt to summarize the different pathways is presented in a conceptual diagram (Fig. 7). The evidence presented in this publication attempts to quantify or at least examplify these export mechanisms, and illustrate their regional and episodic significance. Considering the biomass and dominance of Phaeocystis in pelagic systems, the vertical export of cell carbon is of minor importance. This is based on the following findings:

• A strong decline in the vertical export of P. pouchetii cells between 40 m and 100 m has been shown in the Barents Sea and North Norwegian fjords.

• Despite potential Phaeocystis spp. cell C dominance in the water column, vertical export of diatom cell C was always larger than that Phaeocystis spp. cell C at 100 m depth, unless deep mixing short-circuits retention.

• Phaeocystis spp. cell carbon contributed on average only 3% to the vertical POC flux at 100 m depth.

Fig. 7 Conceptual diagram indicating possible pathways for P. pouchetii-derived carbon

Vertical flux of TEP, bacteria and flagellates

Faecal pellet export (especially from krill)

Vertical flux of TEP, bacteria and flagellates

Faecal pellet export (especially from krill)

The estimated contribution from mucus to the colony sizes observed in the north-eastern North Atlantic suggest that mucus may contribute an additional 40% (maximum) to the estimated cell C export. In concert, Phaeocys-tis spp. cell and colony C may thus contribute <5% of the vertical POC export. Possible grazing from krill inducing FP export, downwelling of Phaeocystis cell/colonies/remains through deep physical mixing processes or nutrient-related flocculation and export of senescent Phaeocystis spp. mucus may induce episodic Phaeocystis flux events. TEP measurements provides a method to estimate vertical export of Phaeocystis-de-rived mucus-remains that otherwise are easy to overlook based on their semiparticulate and transparent nature.

Vertical carbon export

Euphotic zone

Euphotic zone

> y-asymptote (export production)

Phaeocystis spp. cell carbon

Diatom carbon x-asymptote (input into benthic boundary layer)

Fig. 8 Attenuation of vertical flux below the euphotic zone is stronger for Phaeocystis spp. cells compared to diatoms despite similar biomass in the upper mixed layer (modified from Wassmann et al. 2003). Export of transparent exopolymer particles (TEP) can reduce the attenuation of Phaeocystis-derived carbon

> y-asymptote (export production)

Phaeocystis spp. cell carbon

Diatom carbon x-asymptote (input into benthic boundary layer)

Fig. 8 Attenuation of vertical flux below the euphotic zone is stronger for Phaeocystis spp. cells compared to diatoms despite similar biomass in the upper mixed layer (modified from Wassmann et al. 2003). Export of transparent exopolymer particles (TEP) can reduce the attenuation of Phaeocystis-derived carbon

The increased vertical flux attenuation efficiency in Phaeocystis spp.-dominated regions has thus to be taken into account and the channelling of the Phaeocystis spp. biomass through the pelagic has to be carefully considered (Fig. 8). We suggest that there must be significant differences between the vertical export of Phaeocystis spp. and diatoms. The potential loss of TEP and mucilaginous matter on filters prior to analysis may result in an underestimation of the Phaeocystis spp.-associated C flux, and Fig. 8 indicates that the vertical flux attenuation efficiency during Phaeocystis spp. blooms may be smaller then the vertical export of cell carbon may suggest.

Acknowledgement The tedious microscopic effort by Tatjana Ratkova made this contribution possible, and is highly acknowledged. Contributions from colleagues through discussions and fieldwork over many years during the ESCAPE (EU-MAS3-CT96-0050) and BIOHAB (EU-EVK3-CT99-00015) projects, the fjord projects within North Norwegian Coastal ecology (NFR), ALV (NFR 121521/720), CABANERA (NFR 155936/ 700), MACESIZ (NFR 155945/700) and the Arctic Ocean Expedition-2001, are highly acknowledged as well as the economic support received through these projects. This publication is a contribution to all those projects, and to the SC0R#120 working group. We appreciate constructive comments from two anonymous referees, and thank the organisers of the symposium ''Phaeocystis, major link in the biogeochemical cycling of climate-relevant elements'' for the opportunity to publish these data.

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Biogeochemistry (2007) 83:235-244 DOI 10.1007/s10533-007-9094-2

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