Polar Biol (1993) 13:441-449

�9 Springer-Verlag 1993

Springtime coupling between ice algal and phytoplankton assemblages in southeastern Hudson Bay, Canadian Arctic Christine Michel l, Louis Legendre i, Jean-Claude Therriault 2, Serge Dcmers 2, Thierry Vandevelde i

i D~partement de biologic, Universit6 Laval, Quebec, Quebec, Canada, G1K 7P4 2 Division d'oc6anographie biologique, Institut Maurice-Lamontagne, Minist6re des P~ches et Oc6ans, C. P. 1000, Mont-Joli, Qu6bec,

Canada, G5H 3Z4

Received: 5 January 1993/Accepted: 22 March 1993

Abstract. Microalgal assemblages from the bottom ice, the ice-water interface and the water column were systemat- ically sampled from April to June 1986, in southeastern Hudson Bay (Canadian Arctic). The taxonomic similarity between samples from the three environments was asses- sed using a clustering procedure. There were two groups that comprised samples from both the ice-water interface and the water column, while five other groups were made of samples originating from a single environment. Taxo- nomic compositions of the two mixed groups suggest two types of connexion between the ice-water interface and the water column, i.e. before the phytoplankton bloom, there was seeding of the water column by ice algae and, during ice melt, interfacial algae contributed to the water column communities that were otherwise typically phytoplankton. Overall, the phytoplankton community underwent a succession from pennate to centric diatoms. Sinking rates of algae from the ice-water interface were estimated using settling columns (SETCOL). The sinking rates increased seasonally (0.4-2.7 m d-1), which enhanced accessibility of ice-algal cells to the pelagic grazers. Ice algae contrib- uted to water column production as they became access- ible to the pelagic grazers, and also by seeding the water column before the phytoplankton bloom.

Recent biological studies in ice-covered seas have mainly focussed on the ecology and physiology of ice-algal com- munities which comprise, according to the terminology of Horner et al. (1992), surface (Meguro 1962; Burkholder and Mandelli 1965; Syvertsen 1990), interior (Hoshiai 1977; Ackley et al. 1979; Garrison and Buck 1989) and bottom assemblages (Apollonio 1961, 1965; Meguro et al.

Contribution to the programs of GIROQ (Groupe interuniversitaire de recherches oc~anographiques du Qu6bec) and of the Maurice Lamontagne Institute (Department of Fisheries and Oceans) Correspondence to: C. Michel

1967; Bunt and Lee 1970; Palmisano and Sullivan 1982; Grossi et al. 1987). In some environments (e.g. Baffin Bay, Cross 1982; Hudson Bay, Gosselin et al. 1985; Barlow et al. 1988; Michel et al. 1988; Barents Sea, Johnsen and Hegseth 1991; Hegseth 1992), large concentrations of algal cells are also observed in the sub-ice habitat, either free floating in the upper few centimetres just underneath the ice under- surface or loosely attached to the ice (Syvertsen 1990). The bottom ice and ice-water interracial assemblages are often highly productive (Horner and Schrader 1982; McConville and Wetherbee 1983; Palmisano and Sullivan 1983; Dem- ers et al. 1986), and thus may be important as a food resource for consumers in the water column (Hoshiai et al. 1987; Runge and Ingrain 1988; Runge et al. 1991; Touran- geau and Runge 1991). Moreover, since the bloom of ice algae usually precedes that of phytoplankton in the water column (Alexander 1980; Legendre et al. 1981; Horner and Schrader 1982; Homer 1985), the role of ice algae in initiating the spring phytoplankton bloom (the seeding hypothesis) has received much attention. In both the Antarctic (Smith and Nelson 1985, 1986; Garrison et al. 1987) and the Arctic (Schandelmeir and Alexander 1981), similarities between species from the ice and the water column have sometimes been observed, which would support the seeding hypothesis. This hypothesis is also supported, for some Antarctic sea ice communities, by laboratory experiments (Kuosa et al. 1992). However, other studies in both Arctic and Antarctic have shown that species composition of the ice-algal and water column environments were not related (see Horner 1985).

�9 Questions concerning the fate of ice algae have mainly been adressed using data from sediment traps (Sasaki and Hoshiai 1986; Atkinson and Wacasey 1987; Tremblay et al. 1989) and grazing experiments (Conover et al. 1986; Hoshiai et al. 1987; Stretch et al. 1988; Runge and Ingram 1988, 1991). In the present paper, we investigate the relationships between microalgal assemblages from the bottom ice, the ice-water interface and the water column by comparing seasonal changes in species composition of these three compartments. We also consider changes in sinking rates of ice algae in order to establish possible

442

connexions between the ice and water co lumn assem- blages.

Material and methods

Sampling was conducted in the Canadian Arctic, at a station located on first-year tandfast ice of southeastern Hundson Bay, 22 km off Kuujjuarapik (55 ~ 30.1' N : 77 ~ 44.9' W) (Fig. 1). Data were collected from 3 April to 17 June 1986, thus covering the whole period of the ice-algal and water-column phytoplankton spring blooms. Labora- tory facilities were set up at both the ice station and a shore base in Kuujjuarapik; transportation between the two locations was by airplane or helicopter. Temperature and salinity in the water column were recorded with a Guildline CTD probe.

Every second day (with a few exceptions) from 3 April to 15 May, SCUBA divers collected ice-algae from the bottom ice and from the ice-water interface. Algae in the bottom ice layer were collected from the undersurface of the ice using a submersible ice corer which extracted cores 3 cm long x 6.4 cm diam. The ice cores were immedi- ately processed at the sampling station, after melting at room temperature. In the sampling area, ca. 90% of the ice algal chloro- phyll a is found in the bottom 10 cm of the ca. 1-m ice sheet (Legendre et al. 1991). Free-floating algae at the ice-water interface (i.e. the upper few centimetres of the water column adjacent to the under- surface of the ice) were sampled with a 2.2-L syringe sampler ("slurp gun"), in areas of visually estimated high algal biomass. Samples collected at the ice-water interface (which were free of ice) were immediately divided in two subsamples that were kept in the dark. One of these subsamples was rapidly brought to the shore laboratory in Kuujjuarapik, for biomass and physiological measurements; the other was used at the field station for sinking rate experiments. Physiological measurements, that include photosynthesis, activities of carboxylating enzymes, and nutrient assimilation and internal content, are described in Michel et al. (1988, 1989) and Gosselin et al.

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(1990). From 5 April to 17 June, phytoplankton in the water column were collected with 5-L Niskin bottles, 2.5 and 7.5 m below the ice cover. These samples were immediately processed at the field station.

Duplicate subsamples were filtered on Whatman GF/F glass- fiber filters for the spectrophotometric determination of chlorophyll a and accessory pigments concentrations (equations of Jeffrey and Humphrey 1975) after 24-h extraction (90% acetone at 5~ De- pending on algal biomass, volumes filtered from the ice-water interface ranged from 100 ml at the beginning of the season to 20 ml later. For bottom ice samples, volumes filtered varied between 25 ml and 115 ml and, for the water column samples, the volumes were either 250 or 500 ml. Cell identification and enumeration were carried out under the inverted microscope (Lurid et al. 1958), on algae preserved with acidic Lugol. For each sample, a minimum of 200 cells were counted. Cellular surfaces were estimated for the dominant species, and cellular volumes were calculated using a geometric form corresponding to each species. These data were used as surface/volume ratios.

Flexible linkage clustering (Lance and Williams 1966, 1967; Legendre and Legendre 1983) was used to determine the similarity in species composition among samples from the ice, the ice-water interface and the water column. Cell numbers were first transformed into percent abundances (excluding /~-flagellates, because of their high numbers in the water column and very small biomass). A similarity matrix was then computed among samples (Steinhaus coefficient; Legendre and Legendre 1983), and treated with the flexible linkage clustering algorithm.

Beginning on 9 April, at intervals of 2 or 4 days, sinking rates of algae collected at the ice-water interface were measured, using two settling columns mounted in the field laboratory (SETCOL; Bien- fang 198 la/. Water temperature in the columns was kept constant, by continously circulating under-ice water in the sleeve surrounding each settling column. Homogeneously mixed subsamples (350 ml) were allowed to settle in the columns during 2 h, after which 3 fractions of algae (upper, suspended and settled) were collected and their volumes measured. On each fraction, pigment concentrations were determined and cells enumerated (same methods as described above). Cell sinking rates were computed using the equations of Bienfang (1981a).

Results

The growth season of microalgae at the ice-water interface extended from 3 April to 13 May (Fig. 2a). Dur ing that period, i rradiance at the ice-water interface increased in parallel with the melting of the snow cover (Michel et al. 1988). I r radiance ranged from a m i n i m u m of ca. 4 # E i n s t e i n m - Z s - 1 to a ma x i mum value of ca. 30 gEinstein m - 2 s-1. There was a seasonally increasing trend in chlorophyll a concentra t ions at the interface, from 0.6 mg m - 2 on 3 April to a peak value of 23.6 mg m - 2 on 13 May, just before the algae were released into the water co lumn (Fig. 2a). The end of the season corresponded to rapid ice melt (Michel et al. 1988), which caused the development of a well-defined pycnocline (Fig. 3). At the interface (Fig. 2a), chlorophyll a concentra t ions were well correlated to cell numbers ( r=0.90, P<0.01) . In the bo t tom ice layer (ice-core samples; Fig. 2b), chlorophyll a and cell numbers (r = 0.66, P < 0.01) decreased dur ing the period of increase of algae at the interface. There were no significant l inear correlations between either chlorophyll a concentra t ions or cell numbers from the interface and the bo t tom ice.

In the water column, the highest chlorophyll a and cell concentra t ions were observed after 10 May, at both 2.5

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and 7.5 m (Fig. 4a, b). Increased cell concentrations at 7.5 m were correlated with those at 2.5 m, with a lag of ca. 18 days (r = 0.61, P < 0.01). Concentrations of chlorophyll a at 7.5 m were about half those at 2.5 m and, at the two depths, the high concentrations of chlorophyll a and cells were observed over a period of ca. 27 days.

In order to facilitate a comparison between ice algal and phytoplankton cell abundances, cell numbers in the water column were depth integrated. Until 15 May, cell numbers were integrated down to 2.5 m because no data were available at 7.5 m. Later on, cell concentrations were integrated down to 7.5 m. Areal concentrations of the major diatom species and #-flagellates are shown in Fig. 5 for the bottom ice, the ice-water interface and the water

column. Cell abundances in the water column were in the same range as those observed at the ice-water interface, which were one order of magnitude higher than in bottom ice (Fig. 5a, b,c). Species observed in the three environ- ments were the same but the dominant taxa varied. Assemblages in the bottom ice and at the ice-water interface showed high proportions of Nitzschiafrigida and unidentified species of Nitzschia (Fig., 5a, b). In the water column, Chaetoceros septentrionalis and Navicula pelagica were the dominant diatom species (Fig. 5c). Flagellates were present in the three environments and, in term of cell numbers (not biomass), their contribution to the water column outweighed by far their proportions at the ice- water interface and in the bottom ice (Fig. 5d).

The clustering procedure led to the formation of seven groups (Fig. 6, Table 1). Two of these (groups 4 and 6) comprised samples originating from both the ice-water interface and the water column. Out of the five other groups, one included most of the interracial samples (group 3), two belonged to the bottom ice community (groups 1 and 21 and two were restricted to the water column (groups 5 and 7). Of the latter two, group 5 corresponded to the phytoplankton bloom at 2.5 m and the other to the b loom at 7.5 m (Fig. 4). The relative abundances of species in each group are summarized in Table 1, where abundances are given as percent of total cell numbers (excluding the/~-flagellates).

Figure 7a shows that sinking rates for chlorophyll a and total algal cell numbers were well correlated (r = 0.90, P<0.01) . They increased seasonally from a minimum value of about 0.4 m d - 1, on 9 April, to maximum values

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of about 2.7 m d- 1 on 5 May, before a final decrease to about 0.4 m d- 1 at the end of the sampling season. Except for p-flagellates, sinking rates of the most abundant species varied in accordance with those of the total algal commu- nity (Fig. 7b). Specific sinking rates of the various species

were inversely related to their surface to volume ratios (r=0.96, P<0.01), except for unidentified species of Navicula (Fig. 8, Table 2). N.frigida exhibited the highest sinking rates and smallest surface/volume ratios, followed by another species of Nitzschia, C. septentrionalis, N. pelagica and/~-flagellates (Table 2).

Discussion

The microalgal community can be divided into a 3- compartment biotope. These are: (1) the bottom ice com- partment, where algae bloom early in the season and almost disappear by the beginning of May (Fig. 2b); (2) the ice-water interface compartment (Fig. 2a), where algal growth is favoured until the middle of May by the seasonally increasing under-ice irradiance (Meguro et al. 1967; Horner and Schrader 1982; Palmisano and Sullivan 1982; Gosselin et al. 1985; Michel et al. 1988); and (3) the water column compartment (Fig. 4), where phytoplankton develop during and/or after the ice cover breakup (Grain- ger 1977; Legendre et al. 1981; McConville and Wetherbee 1983; Hsiao 1988). Connexions between the compartments can be influenced by physical mechanisms (e.g., percola- tion of water through the ice, McConville and Wetherbee 1983), trophic interactions (e.g. grazing, Carey 1985; Runge and Ingrain 1988; nutrient regeneration and replen- ishment, Gosselin et al. 1985; Cota et al. 1987), and changes in algal physiology that may affect sinking rates (Steele and Yentsch 1960; Eppley et al. 1967; Smayda 1970; Anderson and Swee'ney 1977, 1978; Bienfang 1981a, b).

Table 1. Average relative abundances of the major species in the 7 groups res- ulting from the flexible linkage clustering (bottom), and numbers of samples from the ice, the ice-water interface and the water column belonging to each group (top). For the clustering of samples, g-flagellates were excluded from the total cell numbers

Groups 1 2 3 4 5 6 7

Number of samples Bottom ice 11 7 Ice-water interface - - 17 3 - 1 - Water column 8 7 5 6

Species Abundances (%) Groups 1 2 3 4 5 6 7

Nitzschiafrigida 18 36 35 15 7 1 0 Nitzschia spp. 13 12 11 31 9 20 2 Navicula pelagica 5 1 11 29 60 21 15 Navicula spp. 11 5 3 9 10 9 1 Entomoneis/Amphora spp. 4 1 1 1 0 0 0 Pleurosygma/Gyrosygma spp. 1 1 1 2 0 0 0 PinnuIaria spp. 1 0 1 3 0 0 0 Chaetoceros septentrionalis 2 6 4 1 9 21 78 Thalassiosira spp. 1 3 1 1 1 3 1

445

The water column exhibits much higher numbers of #-flagellates than the two other compartments but, when #-flagellates are excluded from the total cell numbers, there is more similarity between the interracial and water column assemblages than between those from the ice and the ice-water interface (see groups 4 and 6; Table 1). In the Antarctic, Garrison and Buck (1985) and Garrison et al. (1987) have observed a strong similarity between ice and water column assemblages, which supported the hypo- thesis that planktonic algae were seeded from the ice. In the present study, connexion occurs between the water column and the interfacial layer, but no strong similarity is evidenced between the ice and the interracial or water column layers.

A first connexion between phytoplankton in the water column and algae at the ice-water interface (i.e. group 4; Table 1) occurs before the bloom in the water column (up to 10-15 May; Fig. 6), during a period of low cell and chlorophyll a concentrations in the water column (Fig. 4 a,b). From about 15 April to 15 May, species composition in the water column is intermediate between the interracial and the phytoplankton assemblages (groups 3 and 5, Table 1) with, in addition, a high proport ion of Nitzschia spp. (group 4, Table 1). It thus appears that, before the development of the phytoplankton bloom, ice algae re- leased from the ice-water interface contributed to a mixed water column assemblage with high proportions of species growing chiefly at the interface (Nitzschia spp. and N. frigida; Table 1). The contribution of interracial algae to the water column assemblage during that period can be related to a sedimentation event that occurred from 19 to 22 April and was recorded in sediment traps (Tremblay et al. 1989). As reported by these authors, increased sedi- mentation during that period could be associated with a short warming event, but it could have also resulted from the aggregation of algae in dense patches. Some species of Nitzschia, e.g. Nitzschiafrigida , are particularly suited for aggregation because of their tree-like structure and they can sediment fast compared to other species (Fig. 7b, Table 2).

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Considering the species composition of the water column assemblage during the bloom at 2.5 m (group 5; Fig. 6 and Table 1), it appears that species of Nitzschia do not maintain themselves in the water column (see also Fig. 5c). In contrast, Navicula pelagica dominates the phyto- plankton group observed during that period (group 5, Table 1). This sPecies, also introduced into the water

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Table 2. Mean sinking rates and surface/volume ratios for differ- ent species of algae from the ice-water interface. Standard errors of the means are given (parentheses) as well as numbers of obser- vations (N)

Species Mean sinking rate Surface/volume ratio

(m'd 1) (pm 1)

Nitzschiafrigida 1.54 (0.21) 0.71 (0.01) N = 13 N = 200

Nitzschia spp. 1.33 (0.21) 0.91 (0.03) N = 13 N = 200

Chaetoceros 1.22 (0.18) 0.92 (0.04) septentrionalis N = 13 N = 40 Navicula pelaoica 1.09 (0.29) 0.95 (0.02)

N = 10 N = 100 Navicula spp. 0.74 (0.14) 0.49 (0.04)

N=13 N=10 #-flagellates 0.26 (0.06) 1.20"

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column from the interfacial layer before the phyto- plankton bloom (see groups 3 and 4, Table 1 and Fig. 6), increases in the pelagic environment during the month of May (Fig. 5c). The presence of N. pelagica at the ice-water interface, its quasi-absence from the ice itself, and its considerable increase into the water column after the observed connexion between interracial and water column assemblages would support the hypothesis that this spe- cies from the interface did seed the water column. Actually, cell numbers of N. pelagica in the water column are much higher than at the interface (Fig, 5b, c), which would indicate that it was actively growing in the water column. This first connexion between ice and water column assem- blages is similar to what have been observed by Garrison et al. (1987) along a transect in Antarctic waters. However, the observed connexion did not coincide with ice melt

which became significant only in mid-May (Michel et al. 1988). This is different from what has previously been observed in Arctic waters, where the release of algal cells from the ice has been associated with ice melt (Grainger 1977; Saito and Taniguchi 1978; Legendre et al. 1981).

The second time a connexion was observed between the ice-water interface and the water column was during the period of ice melt, which corresponds to the phyto- plankton bloom at 7.5 m (group 6; Fig. 6). Based on the species composition of group 6 (Table 1), it can be hypothesized that algae from the ice-water interface were contributing to the water column instead of seeding it (i.e. group 4). During the period over which group 6 is present, the composition of the phytoplankton assemblage is inter- mediate between phytoplankton groups 5 and 7, as shown by the proportions of N. pelaoica and C. septentrionalis (Table 1). The contribution from the interracial assemblage consists of Nitzschia spp. (20%) which are abundant in neither of the two phytoplankton groups (groups 5 and 7; Table 1). Group 6 was obviously not seeded from the interface, given its taxonomic composition (Table 1). The presence of Nitzschia spp. in the phytoplankton assem- blage during the connecting period likely reflects a short- term contribution of ice algae to the water column, espe- cially that these species do not persist later on (group 7; Table 1, Fig. 5c). The increase of C. septentrionalis and disappearance of Nitzschia ice algal species in the phyto- plankton during the month of June (group 7; Table 1, Fig. 5c) rules out the seeding hypothesis for this second connexion between the ice-water interface and water column phytoplankton. In Manitounuk Sound (Fig. 1), Legendre et al. (1981) suggested that the release of ice algae at the time of ice melt did not cause the phytoplankton bloom but did superimpose itself on the bloom.

As the phytoplankton bloom developed in the water column, species composition changed in a way similar to the ecological succession described by Bursa (1961), where the spring pennate diatom assemblages were followed in the summer by assemblages of centric taxa. Before the phytoplankton bloom in Hudson Bay (which started in mid-May), the water column community mainly consisted of mixed species from the interface (group 4; Table 1, Fig. 5c). Later on, N. pelagica dominated the phytoplank- ton diatom community, and the month of June showed a rapid increase in the proportion of the centric diatom C. septentrionalis (Fig. 5c). Nitzschia species did not increase in the water column assemblage, unless they were incorp- orated again as did occur for a short period during ice breakup (groups 5 and 6; Table 1, Fig. 5c). In the water column, the pelagic species N. pelagica and C. septentrion- alis took over Nitzschia spp. and N. frigida, which had been released from the ice-water interface, and their num- bers cannot be solely explained by a contribution from the ice interfacial layer (Fig. 5b, c). It thus appears that a phytoplankton bloom did occur and that pelagic species were actively growing in the water column, even under ice cover.

Contrary to the observed connexions between the ice- water interface and the water column (groups 4 and 6; Table 1), there is no mixed group relating the bottom ice and the ice-water interface (Fig. 6). The species composi-

447

tion in these two layers is quite similar, a major difference being that #-flagellates are more abundant in the bottom ice compared to the ice-water interface (Fig. 5d). This suggests that the bottom ice community first develops by incorporating cells form the water column, where the /~-flagellates are largely dominant in numbers. Mech- anisms for cell incorporation in the ice are discussed in a number of studies (e.g. Ackley 1982; Garrison et al. 1983, 1989; Spindler and Dieckmann 1986). In the ice, phyto- plankton species like N. pelagica and C. septentrionalis may be unable to compete with Nitzschia species, which may be better adapted to the ice environment (groups 1 and 2; Table 1, Fig. 5a). This agrees with the view that "once trapped in the ice, natural selection favours species adapted to the ice habitat" (Homer and Schrader 1982).

Sinking rates

The first striking observation concerning sinking experi- ments, conducted on algae from the ice-water interface, is the simultaneous increase (until 5 May) of biomass at the ice-water interface (cell numbers and chlorophyll a con- centrations, Fig. 2a) and sinking rates (Fig. 7a). This could indicate artificial formation of algal aggregates in the settling columns, and therefore be regarded as a methodo- logical artefact. Such aggregates have seldom been ob- served in preserved samples from the ice-water interface (P. Jalbert, pers. comm.), but they may have been broken or destroyed during preservation. However, sedimenta- tion rates of algal cells (sediment trap data; 0.2 2.0 m d - 1), simultaneously measured at the sampling station (Tremb- lay et al. 1989), were in the same range as our own values and also exhibited a seasonal increase. The rough agree- ment between rates computed from these two independent data sets suggests that settling columns did not create a methodological bias, and that the seasonal increase of sinking rates measured in settling experiments also occur- red in the field.

Observed sinking rates (Table 2), and their relation with cell size (Fig. 8), can be compared with results reported by Smayda (1971) for phytoplankton. This au- thor showed that palmelloid stages and senescent phyto- plankton cells have higher sinking rates than actively growing cells. Values from Fig. 8 are in the range of senescent cells or palmelloid stages plotted in Fig. 1 of Smayda (1971). High sinking rates observed for dominant ice algal species (Table 2, Fig. 8) could thus indicate either the formation of aggregates or an alteration of their physiological condition during the season. The formation of aggregates was not documented, but the nutritional status of the cells was investigated (Gosselin et al. 1990). During the month of May, ice algae experienced silicon deficiency. This factor, which is known to enhance phyto- plankton sinking rates (Bienfang 1981b, Bienfang and Harrison 1984), could explain the high sinking rates (Table 2) as well as their seasonal increase (Fig. 7a, b).

It may be assumed that, as the season progresses, part of the algal biomass leaves the ice-water interface with sinking rates as shown in Fig. 7a. The simultaneous increase in both the algal biomass at the ice-water interface

(Fig. 2a) and the sinking rates of these same algae (Fig. 7a) would result in a seasonally increasing flux of cells from the interface towards the underlying water column. This indeed occurred at the sampling station, as shown by Tremblay et al. (1989) using sediment traps.

A seasonally increasing flux of algae from the ice-water interface to the water column could benefit pelagic grazers by increasing the accessibility of ice algae at a time when phytoplankton concentrations are still low in the water column. It is uncertain whether calanoid copepods can graze cells attached to the ice matrix (Conover et al. 1986, Runge and Ingram 1988) but, as observed by Conover et al. (1986), ice algae released from the ice are being grazed easily. Moreover, grazing by the Antarctic krill Euphausia superba is enhanced when ice algae are released from the ice at faster rates (Stretch et al. 1988). In southeastern Hudson Bay, grazing activity of calanoid copepods was observed both at the ice-water interface (Runge and Ingrain 1988, 1991) and in the water column (Runge and Ingrain 1991). Grazing appeared to increase after the start of ice melt and, by mid-May, calanoid females did no longer concentrate at the ice-water interface but were feeding in the water column (Runge and Ingrain 1991). Increased accessibility of ice algae to pelagic calanoid copepods may favour gonad maturation during the ice- algal bloom. According to Tourangeau and Runge (1991), this would influence the timing of egg production, with the result that grazing nauplii would be present in the water column at the onset of the spring phytoplankton bloom. During the spring of 1986 in Hudson Bay, Tremblay et al. (1989) estimated that only about 10% of the ice algal production sank to depth, most of the production being incorporated into the pelagic system. The present study shows that ice algae could contribute directly to water column primary production by possible seeding of pelagic algal species such as Navicula pelagica. Another contribu- tion would result from an increased flux of ice algae from the ice-water interface as the season progresses, which would make them more accessible to pelagic grazers. Through these pathways, ice-algal production appears to be closely linked to the pelagic food web in polar waters, where the development of phytoplankton in the water column occurs after the peak of ice algal production.

Acknowledgements. This research was funded by the Natural Sci- ences and Engineering Research Council of Canada (strategic and individual grants to Li.), by the Maurice Lamontagne Institute (Department of Fisheries and Oceans), and by grants to GIROQ from the Fonds FCAR of Qu6bec, NSERC and the Canadian Donner Foundation. C. M. received a post-graduate scholarship from the Fonds FCAR, and financial support was received from the Department of Indian and Northern Affairs for field work. Heli- copter time was provided by Fisheries and Oceans. Housing was at the Kuujjuarapik field station of the Centre d'6tudes nordiques, Universit6 Laval, where we benefited from the invaluable assistance of the superintendent C. C6t6. We thank E. Bonneau, M. Dub6, A. Gagn6, and P. Joly for assistance in the field, and the SCUBA divers for under-ice sampling. We thank P. Jalbert and L. B6rard- Therriault for cell identification and enumeration. Physical data were kindly provided by R. G. Ingram and K. Shirasawa. We also thank three anonymous reviewers for their comments and sugges- tions.

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