Despite occasionally high concentrations and dominance of Phaeocystis spp. in the water column, calculations based on sediment-trap measurements suggest low daily loss rates, implying high retention of Phaeocystis cell carbon in the upper 50-100 m. Unless deep mixing accelerates vertical export, the contribution to the vertical carbon flux at 100 m is on average 3%. The conclusion is that Phaeocystis cell carbon does not contribute significantly to vertical carbon export.

Does mucus contribute significantly to the vertical export of Phaeocystis spp-derived C?

Mucus carbon estimates

The above calculations were provided without taking the contribution from Phaeocystis spp. colony mucus into account. Rousseau et al. (1990) established a relationship for the number of cells per colony of P. globosa, and estimated a carbon content per volume of mucilaginous matter of 335 ng C mm-3. The authors concluded that, when the colony size exceeded 400 im, mucus carbon dominated the biomass, and at colony sizes of 1 mm, mucus contributed 90% of the Phaeocystis colony biomass. Their assumption was a colony filled with mucus structures. Rijssel et al. (1997) challenged this assumption, discovering that the total amounts of sugars and carbon were correlated with the colony surface area for P. globosa, indicating a hollow colony structure with a mucus layer of fixed thickness. The authors argued that the estimates by Rousseau et al. (1990) over- and underestimated the mucus carbon pool for larger and smaller colonies, respectively. Rijssel et al. (1997) measured C content including mucus of 57 pg C cell-1 for field samples and 122 pg C cell-1 for cultures. For P. antarctica, another cell-per-colony ratio and a carbon-per-unit-colony volume of 213 ng C mm-3 was suggested by Mathot et al. (2000) based on investigations from the Ross Sea, Antarctica.

A relationship between cells per colony or colony carbon content has so far not been established for Phaeocystis pouchetii (Schoemann et al. 2005). This species has lobed-formed colonies where the cells are grouped, contrasting the more spherical or elongated colonies characterising P. globosa and P. antarctica. There are also indications that P. pouchetii colonies are more fragile (Wassmann et al. 2005) compared to the tough structure demonstrated for P. globosa (Hamm et al. 1999). It is therefore likely that a difference in the cell per colony ratio as well as carbon per unit of colony exists between P. pouchetii and the other two species. In an attempt to suggest a possible range of contributions from mucus to vertical carbon export based on vertical export rates of P. pouchetii when the mucus sinks as intact colonies with cells we have chosen to apply the relationships for P. globosa and P. antarctica.

First, the range of carbon content in a colony is estimated for the smallest and largest observed colonies in the dataset from the central Barents Sea, and two different cell sizes using the available relationships (Table 3). Estimates of the relative contribution from mucus C to the total Phaeocystis colony carbon based on the relationships given by Rousseau et al. (1990) and Mathot et al. (2000), show that they indicate very similar mucus contributions, adding a maximum of 43-48%, for the largest colonies with small cells (d = 3 im) (Table 3) and 10-12% for similar-sized colonies but larger cells (d = 6 im). As colonies often contain larger cells, the addition of mucus carbon is most likely in the order of 10%, and of minor importance compared to the cell carbon for the total colony carbon content. The relationship suggested by Rijssel et al. (1997) does not allow a separation of cell and mucus carbon, but for the entire colony including cells, the estimated carbon content ranges from the upper levels suggested by the two others, to three times their highest suggested value (Table 3). The reason for this high discrepancy is likely our use of the relatively small colonies (65-115 im) present in the preserved samples examined from the Barents Sea. Rijssel et al. (1997) based their relationship on larger colonies ranging >250-5,000 im in diameter.

Estimated mucus contribution to carbon export

To approximate the potential impact of the estimated mucus contribution to vertical flux, maximum values of colony flux and observed colony size were applied to give an upper range (Table 4). The P. pouchetii fluxes are from the Arctic region of the marginal ice zone in the Barents Sea during spring, 40 m depth and above the vertical mass flux attenuation interval. The estimate of Phaeocystis spp. carbon export based on relationships from Mathot et al. (2000) is 25% lower compared to Rousseau et al. (1990) with 168 mg C m-2 d-1 and 225 mg C m-2 d-1, respectively (Table 4). The mucus contribution to these estimates is however only 10% and 12%, indicating that even with very high vertical fluxes of Phaeocystis spp. colonies, mucus will not add more than about 10% C to estimates based on cell carbon (given the observed colony diameter of 115 im). The total POC export measured at the Barents Sea Arctic water station was about 900 mg C m-2 d-1 (Olli et al. 2002). The Phaeo-cystis spp. carbon export estimates based on Rijssel et al. (1997) exceeded this, and were 2-3 times higher compared to the others, using

Table 3 Estimated carbon content (C) of Phaeocystis pouchetii colonies separated as cells and mucus carbon. Estimates are based on relationships established for living P. globosa (Rijssel et al. 1997; Rousseau et al. 1990) and P. antarctica (Mathot et al. 2000), but using colony size, cell sizes and vertical fluxes measured for P. pouchetii. Maximum and minimum values are examplified including smaller and larger cells and colonies (diameter = d). Cell carbon is calculated according to Menden-Deuer and Lessard (2000)a'b

Cell size (im)

Calculations according to

Colony size (im)

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