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Microalgal samples collected at the ice-water interface (Hudson Bay, Canadian Arctic) were size fractionated in order to determine i f small algae, corresponding to the slze of picoplankton in open waters, were present m the seaIce environment. The <5 pm fraction was found to account for up to 4 % of total chlorophyll a and 20 % of total chlorophyll c. Photosynthetically active algae were present in the < l km fraction. Their photosynthetic efficiency was lower than that of the large ice diatoms, which indicated better adaptation of these larger cells to low under-ice light intensities. This is contrary to what has generally been observed for picoplankton in open waters. The ecological significance of ice picoalgae remains to be assessed. Phytoplankton in oceanic waters are dominated by small cells less than a few pm in diameter designated as picoplankton (0.2 to 2.0 pm: Sieburth et al. 1978) or ultraplankton (<5 to 10 vm: Sverdrup et al. 1942). Picoplankton consist of phycoerythrin-rich cyanobacteria and minute eucaryotic algae (see the reviews by Carr & Wyman 1986 and Thomsen 1986). In oceanic waters, they typically account for more than 50 % of the chlorophyll a (e.g. Berman 1975, Herbland & Le Bouteiller 1981, Platt et al. 1983, Takahashi & Bienfang 1983, Takahashi & Hori 1984, Takahashi et al. 1985, Putt & Prezelin 1985, Glover et al. 1986a). Given their high photosynthetic biomass, picoplankton also account for a large proportion of the primary production. They are typically responsible for about 50 to 60 % of total primary production in oceanic oligotrophic waters (Paerl 1977, Herbland & Le Bouteiller 1981, Li et al. 1983, Platt et al. 1983), and proportions higher than 75 % have been reported in shelf waters by Berman (1975), Takahash & Bienfang (1983), Putt & Prezelin (1985), Glover et al. (1986a) and Prezehn et al. (1986). Lower proportions (20 to 30 %) have been found in the Celtic Sea by Joint & Pomroy (1983) and Joint et al. (1986). In addition, picoplankton have higher photosynthetic efficiency (photosynthesis per unit photon fluence rate) at low irradiance than larger cells, which could enhance their contribution to primary production, at least at depth. This was shown for both laboratory cultures and natural populations (e.g. Glover & Moms 1981, Moms & Glover 1981, Platt et al. 1983, Barlow & Alberte 1985, Glover et al. 1985, 1986a, b, Putt & Prezelin 1985, Joint & Pomroy 1986). In the chlorophyll maximum for example, photosynthetic efficiency per unit chlorophyll a (aB) was higher for picoplankton than for larger cells by a factor of 1.5 in California coastal waters (Putt & Prezelin 1985) and by a factor of 2 in the North Atlantic Ocean (Platt et al. 1983). In the surface mixed layer of the Celtic Sea, Joint & Pomroy (1986) reported orB values with a factor of = 3 between the < 1 pm and the > 5 pm fractions. In the North Atlantic Ocean, Murphy & Haugen (1985) found a latitudinal gradient in the taxonomic composition of picoplankton; from 36.51" to 61.35ON, cyanobacteria decreased by a factor of 10 to 100 while eucaryotic cells increased 5-fold. T h s gradient correlated, at least in part, with decreasing temperature, but other factors such as light and nutrients may have been involved as well. In the Antarctic Ocean, Probyn & Painting (1985) observed high proportions of particulate organic carbon (up to 94 %) and nitrogen (up to 60 %) in the < 1 pm fraction near the ice edge. It is therefore expected that very small algal cells should be present in polar waters. In polar seas, ice microalgae constitute an important component of the primary biomass and production (e.g. ' Contribution to the programs of GIROQ (Groupe Subba Rao & Platt 1984, Horner 1985a, Demers et al. interuniversitaire de recherches oceanoqraphiques du 1986). In addition, bacteria are both present and active Quebec) and of the Maurice-Lamontagne Institute, Canada in the sea ice (see the review of Sullivan 1985). Given O Inter-Research/Printed ~n F. R. Germany 0171-8630/8?/0040/0199/$ 03.00 200 Mar. Ecol. Prog. Ser 40: 199-203, 1987 the possible presence of picoplanktonic cells in polar waters and the actual occurrence of bacteria in the sea ice, it was hypothesized that very small organisms should be found among the ice microalgae. In order to test this hypothesis, ice algae were size-fractionated for measurements of chlorophyll and photosynthetic carbon uptake. Sampling was conducted during April and May 1985 at a field station located on first-year ice 27 km off Kuujjuarapik (55"30.1 N, 77O44.5 W), Hudson Bay, Canadian Arctic. Microalgae were collected at the icewater interface by SCUBA divers using a 2.0 l syringe sampler (slurp gun). Details of sampling procedures are given in Barlow et al. (in press). On 3 different occasions, 6 subsamples (125 m1 on 29 Apr; 60 m1 on 3 and 7 May) were inoculated with 10 pCi H14C03 and put in a Hawaiian-type incubator (Doty & Oguri 1959), in which under-ice water was continuously circulated in order to keep the temperature close to natural conditions. Incubations were under 43, 17, 11, 8, 1 and 0 pEinst m-' S-' of blue-green light that approximated the colour of the under-ice illumination (Rochet et al. 1986); bags of black tulle were used to obtain the various light intensities, and the dark bottles were wrapped in aluminum foil. After 4 h incubations, samples were sequentially filtered onto 5.0, 1.0 and 0.8 pm Nuclepore filters. Filters were put into scintillation vials with 0.2 m1 HC1 0.5 N (Lean & Burnison 1979), and 10 m1 Aquasol added before counting the samples by liquid scintillation (Pugh 1973). Simultaneously, and also on 23 Apr, 50 m1 subsamples were size-fractionated for chlorophyll analyses. Duplicate subsamples were sequentially filtered onto Nuclepore 5.0 pm and Whatman GF/F filters, and also onto Nuclepore 1.0 pm and GF/F filters; this resulted in 4 fractions (>5.0, c 5 . 0 , > 1.0 and < 1.0 pm). Four subsamples were also directly filtered onto GF/F, the filtrate of 2 of them being filtered onto 0.2 pm Nuclepore to determine if chlorophyll-containing particles passed through the GF/F filters. Pigments were extracted in 90 % acetone during = 24 h in the dark, and absorbances measured with a spectrophotometer according to Strickland & Parsons (1972). Concentrations of the various chlorophylls were calculated using the equations of Jeffrey & Humphrey (1975). The 3 size fractions for photosynthetlc activities (>5.0, 5.0 to 1.0 and < 1.0 pm) did not directly correspond to the 4 chlorophyll fractions. In order to make the 2 sets of measurement correspond, size-fractionated activities were combined as follows: > 5 I.tm; <5.0 pm = (5.0 to 1.0 + <1.0 um); > 1.0 pm = (>5.0 + 5.0 to 1.0 pm); and < 1.0 pm. Combining the fractions could also reduce the effects of possible inaccuracies resulting from sequential filtering. Rochet et al. (1986) found that I, of ice algae incubated under blue-green light was >40 liEinst m-* S-' during the whole sampling season, which indicates that maximum photon fluence rate in the incubator (43 ~ ~ E i n s t m-2 S-') was not high enough to achieve saturation. Consequently, the initial slope (photosynthetic efficiency; cuB) was the only parameter estimated for the photosynthesis versus irradiance curves. This was done by linear regression of photosynthetic activities, normalized per unit chlorophyll (B: biomass), on photon fluence rates. Potential errors resulting from size fractionation have been reviewed by Li (1986). He concluded that there is no general rule for cell retention on filters, since it depends not only on pore size and vacuum pressure but also on differences in species, clones and physiology. For example, high pressure may force the passage of cells through the upper bound filter, but it may also reduce the number of cells in the filtrate as a consequence of particle impaction on the filter and cell rupture. It cannot therefore be excluded that the true importance of the smaller cells be underestimated in the <5.0 and < 1.0 pm fractions. On the other hand, fragments of eukaryotic cells may be caught on the small pore filter during 14C experiments, and incorrectly attributed to carbon fixed by picoplankton (Waterbury et al. 1986). However, given the low specific activities in the small-sized fractions (Table 2), it is unlikely that this occurred in the present study. The proportion of pigments passing through Whatman GF/F filters (and retained on 0.2 pm Nuclepore filters) was on the average less than 5 %, which indicates good efficiency of the GF/F filters. Chlorophyll analyses provided 8 independent estimates of pigments retained on GF/F filters; these are the duplicate fractionations (>5.0 + <5.0 pm) and (> 1.0 + < 1.0 pm), and the 4 GF/F filtrations. Chlorophyll b was almost always below detection, and values for chlorophylls a and c were close enough to provide good estimates of the means (Fig.1). Ratios of chlorophylla to chlorophyll c (5.1 to 8.0) were lower than those (15.0 to 24.4) reported by Barlow et al. (in press) for corresponding subsamples analysed by high performance liquid chromatography using the method of Mantoura & Llewellyn (1983). This is because the standard spectrophotometric technique yielded higher estimates of chlorophyll c than HPLC analysis. Table 1 gives the relative proportions in both chlorophyll a and chlorophyll c in the < 5 and < 1 pm size fractions. These indicate that very small cells were indeed present in the sea-ice microflora, and that they were richer in chlorophyll c relative to chlorophyll a than the larger cells. Such microalgae cannot be called picoplankton, since ice algae are not planktonic but rather grow i.n close association with sea ice. Unlike picoplankton in oceanic waters, picoalgae at the icewater interface accounted for only a small proportion of Legendre et a1 Chlorophyll and photosynthesis in slze-fractionated sea-ice microalgae 201 Fig. 1. Mean values for chlorophyll a and c in samp

Chlorophyll and photosynthetic efficiency of size-fractionated sea-ice microalgae (Hudson Bay, Canadian Arctic)