* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Mesozooplankton in the Arctic Ocean in summer
Indian Ocean wikipedia , lookup
Marine debris wikipedia , lookup
Physical oceanography wikipedia , lookup
Deep sea fish wikipedia , lookup
The Marine Mammal Center wikipedia , lookup
Marine microorganism wikipedia , lookup
Marine life wikipedia , lookup
Ocean acidification wikipedia , lookup
Blue carbon wikipedia , lookup
Effects of global warming on oceans wikipedia , lookup
Marine habitats wikipedia , lookup
Marine pollution wikipedia , lookup
Critical Depth wikipedia , lookup
Marine biology wikipedia , lookup
History of research ships wikipedia , lookup
Climate change in the Arctic wikipedia , lookup
Arctic Ocean wikipedia , lookup
Ecosystem of the North Pacific Subtropical Gyre wikipedia , lookup
Deep-Sea Research I 46 (1999) 1391}1415 Mesozooplankton in the Arctic Ocean in summer Delphine Thibault , Erica J.H. Head, Patricia A. Wheeler* INRS-Oce& anologie, 310 Alle& e des Ursulines, Rimouski, Que& bec, Canada G5L 3A1 Biological Oceanography Division, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada B2Y 4A2 College of Oceanic and Atmospheric Oceanography, Oregon State University, Corvallis, OR 97331, USA Received 29 December 1997; accepted 13 November 1998 Abstract The biomass, species and chemical composition of the mesozooplankton and their impact on lower food levels were estimated along a transect across the Arctic Ocean. Mesozooplankton biomass in the upper 200 m of the water column was signi"cantly higher (19}42 mg DW m\) than has previously been reported for the Arctic Ocean, and it reached a maximum at ca. 873N in the Amundsen Basin. The lowest values were recorded in the Chukchi Sea and Nansen Basin, where ice cover was lower (50}80%) than in the central Arctic Ocean. In the deeper strata (200}500 m) of the Canadian and Eurasian Basins, the biomass was always much lower (4.35}16.44 mg DW m\). The C/N (g/g) ratio for the mesozooplankton population was high (6.5}8.5) but within the documented range. These high values (when compared to 4.5 at lower latitudes) may be explained by the high lipid content. Mesozooplankton accounted for approximately 40% of the total particulate organic carbon in the upper 100 m of the water column. Mesozooplankton species composition was homogeneous along the transect, consisting mainly of copepods (70}90% of the total number). It was dominated by four large copepod species (Calanus hyperboreus, C. glacialis, C. ,nmarchicus and Metridia longa), which together accounted for more than 80% of the total biomass. According to measurements of gut pigment and gut turnover rates, the mesozooplankton on average ingested between 6 and 30% of their body carbon per day as phytoplankton. Microzooplankton may have provided an additional source of energy for the mesozooplankton community. These data emphasize the importance of mesozooplankton in the arctic food web and reinforce the idea that the Arctic Ocean should no longer be considered to be a &&biological desert''. 1999 Elsevier Science Ltd. All rights reserved. * Corresponding author. Fax: 001 541 737-2064; e-mail: [email protected] 0967-0637/99/$ } see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 9 8 ) 0 0 0 0 9 - 6 1392 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1. Introduction Information on the mesozooplankton community in the Arctic Ocean is relatively scarce. The earliest data were collected during the drift of the Fram (expedition of Fridtjof Nansen, 1893}1896). Subsequently, data were collected from Soviet ice breakers (Sadko in 1935, Sedov in 1937}1939), ice islands (Johnson, 1963; Hopkins, 1969a, 1969b, Brodskiy and Pavshtiks, 1976; Dawson, 1978; Kosobokova, 1982; Rudyakov, 1983) and submarines (Grice, 1962). Except for the drifting stations and submarines, previous data were mainly collected in the eastern section of the Arctic Basin (e.g., Hirche and Mumm, 1992). The western part of the Arctic Ocean has been especially poorly sampled due to more extensive ice cover. In previous studies, four large species represented the main bulk of zooplankton, with two species of Calanus, Calanus hyperboreus and Calanus glacialis, being characterized as arctic species (Lee, 1974; Dawson, 1978; Rudyakov, 1983; Conover and Huntley, 1991; Hirche, 1991; Hirche et al., 1994; Hirche and Nieho!, 1996). In the Arctic, C. glacialis overwinters as stage V copepodite (Conover and Huntley, 1991) completing its life cycle in 2 years. C. hyperboreus shows a 3 year life cycle overwintering as stage III}V copepodite (Hirche, 1991; Hirche and Nieho!, 1996). The other major species, Metridia longa, was classi"ed by Grice (1962), Gronvik and Hopkins (1984) and Ba mstedt et al. (1985) as an arctic deep water species that overwinters as stage V copepodite and adults. The fourth main species, Calanus ,nmarchicus was "rst characterized as a boreal species by Dawson (1978), but it is now generally regarded as a North Atlantic species (Smith and Vidal, 1986; Smith, 1988; Smith and SchnackSchiel, 1990). The life cycle duration for this species is still debated, but C. ,nmarchicus is known to overwinter in diapause in deep-water (Ba mstedt et al., 1990). This species is imported into the Arctic Ocean by the main in#ow of Atlantic water running through the Fram Strait (Ba mstedt, 1986; Hirche, 1991; Gislason and Astthorsson, 1995), so that its distribution in the Arctic Ocean is associated with the cyclonic circulation of Atlantic waters (Jaschnov, 1970). Thus patterns of its dispersal in the Arctic basins should be associated with patterns of #ow of Atlantic water into the Arctic Ocean. The Arctic Ocean was, until the Canada/USA joint expedition (AOS'94) in 1994, characterized as an area of very low primary production (Apollonio, 1959; English, 1961) with an annual phytoplankton production rate of 1 g C m\ yr\. Preliminary data reported by Wheeler et al. (1996) and Gosselin et al. (1997), however, have shown that the central Arctic Ocean is at least 10 times more productive than was previously reported. The di!erence lies in part in the contribution of ice algal production and the assessment of the contribution of DOC release by pelagic and ice algae. Concentrations of chlorophyll a and primary production rates in the Arctic Ocean during the summer of 1994 were similar to values reported for other oligotrophic areas (Gosselin et al., 1997). Thus, a complex pelagic food web should be expected, which may have a classical food chain as well as microbial food web elements. In this study, in addition to examining the biomass and the species composition of mesozooplankton, their role in the Arctic food web was investigated during the AOS'94 cruise using the so-called &&gut #uorescence'' method. D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1393 2. Materials and methods 2.1. Cruise track and sampling From 26 July to August 1994, the USCGC Polar Sea made a transect across the Arctic Ocean running from Nome, Alaska, to Reykjavik, Iceland. The transect started on the continental shelf of the Chukchi Sea, crossed the Chukchi Abyssal Plain, the Arlis Plateau, the Mendeleyev Ridge, the Makarov Basin, the Lomonosov Ridge, the Amundsen Basin, and ended in the deep Nansen Basin (Figs. 1 and 2a). Eighteen stations were sampled for zooplankton along this transect (Fig. 1, Table 1). At each station, water samples were collected using 10 L Niskin bottles mounted on a rosette sampler equipped with a CTD probe (Sea-Bird Electronics). Sampling and determination of chlorophyll a and primary production in the water column are reported elsewhere (Gosselin et al., 1997). Mesozooplankton samples were collected at each station by vertical net hauls using a 200 lm mesh size WP2 net with a 0.75 m diameter mouth. No #ow meter was available, but as the hydrodynamic properties of the WP2 (200 lm mesh size) net (UNESCO, 1968) were carefully designed, this net can be considered quantitatively Fig. 1. Cruise track, stations position and main topographic features. 1394 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 Fig. 2. Spatial variation of bottom depth (a), surface temperature (b) and salinity (c) across the Arctic Ocean from CTD measurements. accurate in the absence of actual volume measurements. The hauling speed was ca. 1.5 m s\. Where possible, three di!erent depth strata were sampled (0}100, 100} 200 m and 200}500 m) using an opening}closing device. Samples were collected at di!erent times of the day, but the entire expedition occurred during the period of 24}h daylight conditions. 2.2. Biomass Immediately after capture, mesozooplankton samples from each depth were split using a Motoda splitter. The "rst-half of each net tow was split again for dry weight measurements and chemical composition analysis. These two sub-samples were "ltered separately onto preweighed GF/A "lters (47 mm), quickly rinsed with distilled water and dried (603C) for 3 d onboard. The "lters were reweighed upon return to the laboratory using a microscale electrobalance. Total dry weight (DW) represented the di!erence in the weight of the "lter before and after "ltration. Zooplankton biomass (DW) was expressed per m and m, assuming that the net sampled a vertical cylinder (0.75 m in diameter) of water. One of the "lters was ground with distilled water and homogenized, and 3 subsamples were taken for analysis of the chemical composition. Each sub-sample was D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1395 Table 1 Coordinates of sampling sites for mesozooplankton study in the Arctic Ocean Station no. Sampling date Sampling time (h min) Latitude Longitude 2 3 4 5 7 8 13 17 19 21 23 24 25 26 30 31 32 33 07/26 07/27 07/28 07/30 08/01 08/03 08/06 08/09 08/11 08/13 08/15 08/16 08/18 08/19 08/20 08/22 08/25 08/26 15.30 19.40 21.00 2.00 6.00 8.00 4.45 5.45 12.10 11.30 7.10 4.00 10.00 7.00 11.00 4.30 10.00 13.30 70300N 72300N 74301N 75325N 76339N 78308N 80309N 81315N 82326N 84306N 85354N 87309N 88304N 88348N 89301N 90300N 85343N 84316N 168345W 168351W 168350W 170344W 173320W 176348W 173317W 179300E 175350E 174359E 166350E 160332E 147347E 143329E 137341E 037350E 034337E Geographical region Chukchi Sea Canada Basin Makarov Basin Amundsen Basin Nansen Basin Ice cover (%) Ice thickness (m) 55 70 80 100 100 90 100 100 95 90 95 100 95 95 90 95 90 80 1.08 2.55 1.6 1.75 2.68 2.32 2 2.49 2.05 1.63 1.94 1.94 1.73 2.4 2.28 1.92 3.14 placed in a tin cup and dried at 603C for 3 d. The chemical composition (i.e. particulate organic carbon and nitrogen) of the samples was measured using a Perkin-Elmer CHN 2400 Analyzer. 2.3. Abundance and species composition The second-half of the net tow was preserved in a 4% bu!ered seawater : formalin solution for later species determination, staging and sizing of the whole mesozooplankton community. Once back on shore, these sub-samples were sieved onto a 350 lm mesh. The total fraction larger than 350 lm was counted, whereas only 5}10% of the fraction smaller than 350 lm was counted for the dominant zooplankton species. Regarding the copepod groups, the di!erent copepodite stages were counted and measured under a dissecting microscope (equipped with a calibrated eyepiece reticule). Prosome lengths and morphological characteristics were used to distinguish between the three species of Calanus (C. hyperboreus, C. glacialis and C. ,nmarchicus) as described by Grainger (1961) and Jaschnov (1970). It was di$cult to distinguish between the "rst three copepodite stages (CI}CIII), so that these individuals were classi"ed only as Calanus spp. Species determinations were conducted using the descriptions of Rose (1933). Individual dry weights of the dominant copepod species were determined on preserved samples. A 20% loss in dry weight due to formaldehyde preservation was assumed (Hopkins, 1968; Omori, 1978). For the di!erent species and stages, 5}15 1396 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 individuals, depending on size, were pooled, rinsed with distilled water, placed on a preweighed GF/A "lter and dried at 603C for 3 d. Triplicate measurements were made whenever enough animals were collected. 2.4. Gut pigment contents and gut evacuation rates An additional net haul from 0 to 100 m also was taken at each station to assess mesozooplankton grazing using the gut #uorescence technique. The time of sampling depended on the time of arrival at the station; thus sampling often occurred at di!erent times of day. As soon as possible after collection ((5 min), the mesozooplankton samples were sieved through 1000, 500 and 200 lm mesh sizes. Each subsample was collected on a 200 lm mesh size nylon disk, quickly rinsed with distilled water and immediately frozen. Within 10 d of collection, samples were quickly sorted to species and stages with a dissecting microscope under dim light using cold "ltered sea water (at approximately 03C) and treated as described by Mackas and Bohrer (1976). Aliquots of 15 individuals were sorted out for stage IV of Calanus species and Metridia; for larger individuals or older stages (stage V and adults), groups of "ve individuals were picked and analyzed for gut pigment contents. Gut pigment analysis was done on board with a Turner Design #uorometer. Pigments were extracted in 10 ml of 90% acetone at !203C overnight. Chlorophyll a and pheopigment concentration were measured according to the method of Yentsch and Menzel (1963) modi"ed by Holm-Hansen et al. (1965). The gut pigment contents (ng Chl a eq ind\) were calculated according to Wang and Conover (1986), where ng chlorophyll a equivalents (ng Chl a eq)"Chl a#pheopigment. Instantaneous gut evacuation rate (GER) was measured at each station. Freshly harvested animals were screened over a 2000 lm mesh netting to exclude large animals and concentrated on a 350 lm mesh, quickly rinsed with fresh "ltered sea water and transferred to 8 L plastic carboys containing "ltered sea-water. The carboys were kept in a controlled temperature incubator in the dark. Zooplankton usually were subsampled at 0, 5, 10, 15, 30, 45, 60, 90, 120, 180 min and frozen immediately in liquid nitrogen. Pigment measurements were conducted as for the gut contents. Gut evacuation rate constant (k) was estimated by "tting values to a negative exponential curve following the equation: C "C e\IR, R where C is the gut pigment content after time t (in min) and C is the initial gut R content. Gut pigment contents were transformed into in situ ingestion rates (I) assuming the animals were feeding at equilibrium, i.e. defecation rate equaled ingestion rate, following this equation: I"C k. If we assume ingestion rate is constant over 24 h and if we know the abundance of the species, we can obtain its total daily ingestion rate (lg Chl a eq d\). The grazing impact of each species then can be estimated, i.e. its daily removal of phytoplankton. D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1397 In order to express gut pigment contents in term of carbon and thus to assess the rate of phytoplankton ingestion in terms of copepod daily ration, the phytoplankton carbon content must be determined. The PC/Chl a ratio of phytoplankton was derived from the slope of the regression line between PC and Chl a. The ordinate at the origin gives an estimate of the detrital part of the seston. The ingestion of phytoplankton carbon by copepods can then be calculated in terms of their daily ration (% copepod body carbon per day), or of their grazing impact on the phytoplankton (% primary production of C per day, or % standing stock of phytoplankton C per day). This ingested phytoplankton carbon can then be used by the copepods to fuel their physiological needs (i.e. respiration, reproduction and growth). Linear correlation (Pearson Product Moment) and linear regression for assessing relationships among variables were determined with the Sigma-Stat and Statgraphics statistics package. 3. Results 3.1. Characterization of the study area Sea surface temperatures decreased slightly along the transect from !1.273C in the Chukchi Sea to !1.653C in Nansen Basin with a minimum of !1.733C over the Lomonosov ridge (Fig. 2b). The 0}100 m layer had the coldest temperatures, which varied little along the transect (Swift et al., 1997). The temperature increased evenly between 100 and 200 m. The layer of Atlantic water characterized by the 03C isotherm (potential temperature) was located between 200 and 800 m (see Fig. 2a in Swift et al., 1997). In the eastern sector the 03C isotherm shoaled towards the surface reaching ca. 150 m depth. Warm cores within the Atlantic layer were observed along the transect, where the temperatures reached 13C in the western sector and '1.53C in Nansen Basin. Salinity increased from 23.5 in the Chukchi Sea to 33.0 in Nansen Basin (Fig. 2c). Ice cover was lowest (55}70%) at Stations 2 and 3 the Chukchi Sea (Table 1). Thick ice covered most of the transect at this time of year, but thin ice was observed in the Chukchi Sea and over the Lomonosov Ridge (Table 1). In Table 2 we report latitudinal variations of algal biomass and primary production. More details can be found in Gosselin et al. (1997). Total phytoplankton biomass was very high ('100 mg Chl a m\) at Stations 2 and 3, but was much lower between Stations 4 and 32 (1.18}26.82 mg Chl a m\). There was an increase in algal biomass at the last station in Nansen Basin (Table 2). The biomass of large cells ('5 lm) showed a pattern of variation similar to that of total biomass. At Station 21 the primary production rate measured for the large cells was'60% of the total primary production rate (Table 2), but at most stations with '80% ice cover the large cells accounted for (50% of the total primary production. 3.2. Zooplankton biomass The integrated mesozooplankton standing stock (g DW m\, 0}500 m) ranged from a minimum of 0.4 in the Chukchi Sea, to a level ca. 7 in Nansen Basin, with 2 3 4 5 6 7 8 11 13 16 17 19 21 23 24 25 26 30 31 32 33 Station 0.01 0.10 3.29 0.21 0.55 0.38 27.58 1.57 273.42 3.50 49.49 1.94 1.25 0.45 3.74 0.62 0.83 0.74 27.92 2.91 309.52 3.50 51.76 4.04 1.63 Large cells production mgC m\ d\ 0.61 56.09 Total production mg C m\ d\ Ice algae 0.96 4.20 0.64 0.37 14.25 0.07 0.03 0.1 0.14 1.88 0.17 0.02 0.39 0.07 2.90 Total biomass mg Chl a m\ 15.01 23.35 520.65 27.36 10.22 9.39 73.17 56.52 26.48 12.37 55.16 2569.06 750.00 87.52 24.40 Total production mg C m\ d\ Phytoplankton 2.6 4.5 287.3 4.2 1.2 0.1 44.7 6.0 9.4 2.0 10.1 2282.9 600.0 0.6 4.5 Large cells production mg C m\ d\ 4.51 8.89 62.03 445.04 162.43 8.08 22.41 26.82 12.21 11.43 9.67 11.21 9.04 4.34 1.18 22.32 9.45 5.61 4.85 10.55 Total biomass mg Chl a m\ 1.22 2.16 37.72 0.98 1.01 0.27 15.49 1.6 1.34 3.31 1.91 437.23 101.48 1.97 4.28 Large cells biomass mg Chl a m\ Table 2 Latitudinal variations of ice algae biomass and primary production and phytoplankton biomass and primary production. Data from Figs. 5 and 6 in Gosselin et al. (1997) 1398 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1399 Table 3 Mesozooplankton biomass for the three layers (0}100, 100}200 and 200}500 m) and integrated over the upper 500 m. PC (g C m\) for the upper 100 m is taken from Wheeler et al. (1997). Percent zooplankton carbon (C-Zoo) is calculated as [C-Zoo/(PC#C-Zoo)]a;100 Biomass (mg DW m\) Station no. 0}100 m 2 3 4 5 7 8 13 17 19 21 23 24 25 26 30 31 32 33 18.79 7.12 19.07 24.9 37.42 28.56 34.68 38.36 36.48 31.37 37.75 37.6 42.23 41.29 42.51 39.01 25.18 26.24 100}200 m n.s n.s 7.19 n.s 23.82 19.54 22.68 40.37 40.11 20.69 27.32 21.29 32.54 22.5 24.01 31.94 10.4 19.18 200}500 m Integrated biomass (g DW m\) PC (g C m\) C-Zoo Abundance % (ind m\) n.s n.s n.s n.s 4.35 9.66 5.11 16.44 13.23 8.86 10.53 11.75 4.9 7.84 8.62 n.d 5.81 8.43 0.66 0.36 4.33 2.49 7.43 7.71 7.27 12.80 11.63 7.86 9.67 9.14 8.95 8.79 9.24 n.d 5.30 7.07 n.d n.d 5.02 n.d 4.81 5.03 4.99 3.55 3.68 4.48 4.09 n.d 3.93 n.d n.d 4.09 3.73 4.21 n.d n.d 25.66 n.d 38.19 38.00 36.82 59.06 55.83 41.25 48.60 n.d 47.66 n.d n.d n.d 36.24 40.16 79.95 35.85 33.77 53.83 42.96 40.61 35.99 74.70 41.74 55.28 60.62 54.55 37.85 30.74 80.38 The sampling depth was 0 to 35 m. Biomass estimated from preserved sample. n.s: not sampled. n.d: not determined (1) from Wheeler et al. (1997). a maximum of 12.8 in Makarov Basin and otherwise levels of about 9 in Amundsen Basin (Table 3). Highest mesozooplankton biomass concentrations (19} 42 mg DW m\) were observed in the upper layer (0}100 m) and lowest concentrations (4}16 mg DW m\) in the deepest layer (200}500 m) (Table 3). The biomass in the intermediate layer (100}200 m) ranged between 20 and 40 mg DW m\ and was on average 30% lower than in the upper 0}100 m layer except at Stations 17 and 19 in Amundsen Basin, where it exceeded that in the upper layer (Table 3). This di!erence in observed vertical distribution was probably not associated with daily migration. Vertical migrations are known to be very limited or absent under conditions of constant light (BeH et al., 1971; Hirche and Mumm, 1992; Mumm, 1993; Nielsen and Hansen, 1995; Falkenhaug et al., 1997). Between 82 and 93% of the total biomass was located in the top 200 m at this time of the year. Zooplankton biomass in the upper 100 m showed signi"cant (p(0.05) negative correlation with total phytoplankton biomass (r"!0.758), biomass of large cells of phytoplankton (r"!0.740) and with primary production (r"!0.721) (Table 4). On the other hand, zooplankton biomass was signi"cantly correlated with ice cover 1 1 2 !0.553 * 1 1 ns ns 3 4 1. 1 * ns ns 5 0.992 *** 0.993 *** 1 !0.553 * ns 6 ns 1 ns !0.559 * 0.913 *** ns 7 ns 0.927 *** 1 ns !0.549 * 0.994 *** ns 1 } Latitude. 2 } Phytoplankton total primary production (mg C m\ d\). 3 } Ice Algae total primary production (mg C m\ d\). 4 } Ice algae primary production (mg C m\ d\) by large cells. 5 } Ice algae total biomass (mg Chl a m\). 6 } Large cells phytoplankton primary production (mg C m\ d\). 7 } Phytoplankton total biomass (mg Chl a m\). 8 } Phytoplankton large cells biomass (mg Chl a m\). 9 } Water depth (m). 10 } Ice cover (%). 11 } Ice thickness (m). 12 } Zooplankton abundance (ind m\) for the 0}100 m layer. 13 } Zooplankton biomass (mg DW m\) for the 0}100 m layer. *P'0.01, **P'0.0001, ***P(0.0001. 11 12 13 9 10 8 7 5 6 4 3 2 1 1 ns 0.934 *** 0.992 *** ns !0.548 * 0.992 *** ns 8 1 ns ns ns ns ns ns 0.820 *** ns 9 Table 4 Correlation table (correlation coe$cient in bold, P value) between biological and physical variables 10 !0.866 *** !0.839 *** ns 1 ns !0.806 ** ns 0.483 * !0.879 *** ns 11 1 ns ns ns ns !0.568 * ns ns ns ns ns 12 ns !0.508 * ns 1 ns ns ns ns ns 0.589 * ns ns 13 0.740 ** ns 0.765 ** ns ns 1 !0.758 ** ns ns ns 0.749 ** !0.72 ** ns 1400 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1401 Fig. 3. Spatial variations of C/N ratio (a) and %C (b) for the mesozooplankton population in the Arctic Ocean for di!erent depth intervals. (䊏) 0}50 m; (*): 0}100 m, (䊉) 100}200 m, (䉭): 200}500 m. (r"0.765), and also with temperature (r"0.710, not shown). Zooplankton biomass showed a positive correlation with latitude (r"0.749), but the latitude per se might not be the driving force since it re#ects a combination of di!erent physical features of the environment (including ice cover and temperature) that vary with latitude as well. Ice cover and latitude showed a low but signi"cant correlation (r"0.483). The abundance of large zooplankton ('350 lm) was not signi"cantly (p'0.1) correlated with total biomass, but the abundance of the largest copepod species, Calanus hyperboreus, was strongly correlated with total biomass (r"0.910, not shown). 3.3. Carbon and nitrogen contents With the exception of the two shallow stations in the Chukchi Sea (Stations 2 and 3, sampling depth 0}35 and 0}50 m, respectively), the C/N ratio for mesozooplankton (Fig. 4a) varied between 6 and 8.5, and these values are in the range previously observed for zooplankton at high latitudes (between 6.3 and 12.5, see Ba mstedt, 1986). The mesozooplankton carbon content was relatively constant over the whole survey area (around 40% of the dry weight), although it was lower at stations at the ice edge (Stations 3 and 4, Fig. 3b). No variations with depth or latitude were observed (Fig. 3a and b). At this time of the year, the mesozooplankton accounted for approximately 40% of the total particulate carbon in the upper 100 m of the water column (Table 3). 1402 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 3.4. Zooplankton species composition Only the results from the 0}100 m layer are presented here. Community composition was consistent along the transect, with 21% of the taxa present at every station and 70% present at more than "ve stations. The population was clearly dominated by copepods, which represented 80}93% of the total number of individual zooplankters. Twenty-three di!erent species of copepod were identi"ed. Only a few genera were found in the 200}350 lm size class. These included Oithona, Oncaea and Pseudocalanus, which, because of their small size (average individual weight around 3 lg during this study), were not an important component of the total biomass. Since the smaller species represent a small proportion of the total biomass, the focus of this paper will be on the larger species. Four species of copepods dominated the size class over 350 lm: Calanus hyperboreus, C. glacialis, C. ,nmarchicus and Metridia longa. Those four species represented, in term of numbers, over 70% of the population larger than 350 lm (Fig. 4a). Non-copepod genera included chaetognaths, pteropods, amphipods, ostracods and gelatinous forms (Fig. 4b). Those genera showed highest values at Station 13 in Canada Basin (main components: amphipods and pteropods) and at Station 26 in Amundsen Basin (main components: chaetognaths and ostracods). The copepod community showed three maxima in terms of abundance (Fig. 5a). Fig. 4. Spatial distribution of abundance of copepods (a) and non-copepods (b). D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1403 Fig. 5. Spatial variations of abundance of the 4 main species along the transect: Calanus hyperboreus (a), Calanus glacialis (b), Calanus ,nmarchicus (c) and Metridia longa (d). (䊏) Copepodite C4, (䊉) Copepodite C5, (*) female. The "rst was at Station 4 in the Chukchi Sea, at which stage IV C. hyperboreus was dominant. At the other end of the transect, at Station 32 in Nansen Basin, another maximum was observed, but in this case young stages and adults of both C. glacialis and C. ,nmarchicus were dominant. The third maximum, located at Station 23 in Makarov Basin, was dominate by adults and stage V C. hyperboreus, which accounted for over 50% of the copepods (Fig. 5a). The highest concentration of C. hyperboreus was found in the central part of Makarov Basin, and there was a minimum in their abundance in Nansen Basin (Fig. 5a). In the Chukchi Sea, at Station 4, young stages (copepodite II and III) of C. hyperboreus were very abundant (13.67 and 49.44 ind m\, respectively). Calanus glacialis densities were of the same order of magnitude as C. hyperboreus, and both showed a maximum in Makarov Basin (Fig. 5b). Very low concentrations also occurred in Nansen Basin. The numerical abundance of Metridia longa showed two maxima (81.153N in Canada Basin and 893N in Amundsen Basin) and a very low level in Makarov Basin (Fig. 5d). Young stages of M. longa were not 1404 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 abundant, maximal values of 2}3 ind m\ (copepodite II and III), while older stages (copepodite IV and V) reached abundance of 14}16 ind m\. Calanus ,nmarchicus was also present but in lower numbers (range from 0}7 ind m\). Although considered to be a North Atlantic species, it was found even in the western part of Canada Basin (Fig. 5c), but highest concentrations were found at the last two stations, the closest to the Fram Strait, which carries the main in#ow of Atlantic water into the Arctic ocean (Jaschnov, 1970; Aagaard et al., 1991; Hirche and Mumm, 1992; McLaughlin et al., 1996). The total abundance of copepods was negatively correlated with the ice cover (r"!0.508, Table 4) due to the high number of copepods that occurred at Station 2 (i.e. high number of copepodite C III and IV of C. hyperboreus) and the last stations (i.e. high number of C. ,nmarchicus/glacialis copepodite and female), where the ice cover was the lowest (Table 1). However, integrated biomass and individuals are lowest over the shelf (see Table 3). The total abundance was positively correlated with total phytoplankton primary production (r"0.589, Table 4). 3.5. Phytoplankton ingestion The C/Chl a ratio was estimated from the slope of the regression line between PC and Chlorophyll a (R"0.85, p(0.001: Spearman Rank Order). Over the whole survey area the PC/Chl a ratio was estimated to be 29.8. Three stations (2, 3 and 33 shallow station) had the greatest PC and Chl a concentrations (Fig. 6), distinguishing them from all other stations. These three stations were at or close to the ice edge. If these three stations are removed from the regression, the PC/Chl a ratio becomes 41, which is similar to the value found using microscopic measurements (Booth and Horner, 1997). Gut pigment values of Calanus hyperboreus females exhibited a wide range of values from 0.5 to 34 ng Chl a eq. ind\. The highest values were found in Nansen Basin and Fig. 6. Comparison of Chlorophyll a concentration (Gosselin et al., 1997) and PC (Wheeler et al., 1997) across the Arctic Ocean. (**) Linear regression line (PC"63.5#29.8 * Chl a, R"0.85, p(0.001). D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1405 Fig. 7. Spatial variations of gut pigment contents for Calanus hyperboreus female, Calanus glacialis and Metridia longa and phytoplankton biomass '5 lm along the transect. also at 843N in Makarov Basin (Fig. 7). The variation in gut contents may be explained in part by the variation in concentration of phytoplankton bigger than 5 lm (R"0.42). Gut pigment concentrations (except for Station 21) increased with concentrations of phytoplankton larger than 5 lm. Younger stages had lower gut contents (0.44}17.61 ng Chl a eq ind\). The other common Calanus species, C. glacialis, showed values about 2}3 times lower than those of C. hyperboreus, and highest values were also observed at the two last stations (Fig. 7). Gut pigment levels of C. glacialis females did not show any correlation with either total or '5 lm chlorophyll a concentration. The few data available for younger stages (copepodite IV and V) were similar to those of the females. Metridia longa female gut contents (Fig. 7) showed a scattered variation with a tendency to increase towards the east. Overall M. longa showed the lowest values near the Lomonosov Ridge (87}883N), in the eastern part of the Makarov Basin and in the Amundsen Basin (except at the North Pole station). Higher gut contents observed for all three species at the eastern end of the eastern section were related to a higher proportion of centric and pennate diatoms (see Table 3 in Gosselin et al., 1997). Gut evacuation rates were calculated only at eight stations and estimated for female and copepodite VI and V of the large species pooled together (Table 5). Values of k (gut evacuation rate constant) were variable, lower values being observed at Station 21 in Makarov Basin (0.28 h\), and at Station 30 in Amundsen Basin (0.57 h\). For the other stations k was relatively constant at about 0.8}1 h\. The gut evacuation rate constants were in the same range as those (0.66}1.04 h\) given by Head (1988). Canada Basin 13 13 17 17 19 19 21 21 23 23 25 25 30 30 31 31 Total phytoplankton. Phytoplankton '5 lm. Amundsen Basin Makarov Basin Geographical region Station no. 1.00 0.570 0.870 0.858 0.283 0.761 1.187 0.751 k (h\) 562 33 187 55 65 14 541 354 291 50 114 29 272 65 187 67 Chl a lg m\ 24.22 8.68 11.19 11.18 47.05 30.48 66.91 26.65 ng pig ind\ h\ 1.13 0.59 1.02 1.18 3.53 2.47 4.46 1.57 lg pig lg C\ d\ Ingestion 6.39 9.67 16.51 21.05 32.82 29.38 50.52 22.037 lg pig d\ 7.84 '100.00 54.04 '100.00 90.40 '100.00 12.14 18.54 14.46 84.18 28.96 '100 7.18 30.06 7.84 19.08 Grazing impact on phytoplankton % 16.88 41.48 '100.00 '100.00 '100.00 '100.00 25.84 41.08 18.94 '100.00 25.30 '100.00 25.84 '100.00 6.78 31.60 Impact on primary production % 7.54 3.97 6.78 7.84 23.52 16.48 29.74 10.46 Daily ration % Table 5 Gut evacuation rate constant (k), Ingestion, impact of herbivorous grazing on phytoplankton biomass and on primary production and daily body ration along the transect for the 0}100 m layer. Pig"Chl a eq 1406 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1407 When copepods feed on phytoplankton some of the chlorophyll a they ingest is not converted to a-type pheopigment, but rather to a non-#uorescent colorless product. Since this process happens early during ingestion, a correction factor must be applied in order to get a more realistic estimate of ingestion. Degrees of chlorophyll a destruction reported in the literature are variable (Kiorboe and Tiselius, 1987; Redden, 1994; Head and Harris, 1996). Head (1992) and Head and Harris (1996) showed that the degree of pigment destruction varies with ingestion rate and hence is related to ambient chlorophyll a concentration. In the case of low in situ chlorophyll concentrations, the correction factor will be high (Redden, 1994; Head and Harris, 1996). They found degrees of destruction of between 20 and 100% for chlorophyll a concentration range of 19}0.2 lg L\. Head and Harris (1996) carried out grazing incubation experiments with mixtures of Calanus spp. feeding on natural mixtures of phytoplankton. In their experiments, when the phytoplankton mixtures were dominated by diatoms and concentrations were (2 lg Chl a L\. (see Fig. 1, Expt. A1), the degree of destruction was ca. 90%. By contrast, when the mixtures contained prymnesiophytes and total phytoplankton were (2 lg Chl a L\ (see Fig. 1, Expt. A2), degrees of destruction were even higher. During the present study, chlorophyll concentrations were low (0.1}0.5 lg L\). Thus, a correction factor of 10 is considered to be reasonably conservative. Overall the ingestion of phytoplankton by the large species ranged from 8.68 to 66.91 lg Chl a eq ind\ d\ (Table 5). The lowest values were observed in Makarov and Amundsen Basins and the highest value at Station 17 on the east side of Makarov Basin. If we use a correction factor of 10, the daily impact of grazing on the total phytoplankton biomass ('0.7 lm) by copepods would have varied between 7 and 90% (Table 5), while for chlorophyll a'5 lm, it would have reached over 100%. At Station 21 the community ingestion rate was high, but the impact on phytoplankton and primary production was very low, so that at this station the phytoplankton was apparently poorly used by the zooplankton population. The impact of grazing on primary production was higher on the cells larger than 5 lm than on the total phytoplankton. The daily body ration in terms of carbon ranged from 4 to 30% (Table 5). The lowest values occurred in Makarov and Amundsen Basins. Mesozooplankton grazing measured by gut #uorescence does not assess for ingestion of heterotroph #agellates/ciliates, and mesozooplankton species can sometimes feed extensively on microzooplankton. During this study, biomass of heterotrophic protists in the size class 6}20 lm (Sherr et al., 1997) and mesozooplankton biomass were relatively well correlated (r"!0.696, P(0.05) suggesting that, even during the productive season of the year, both phytoplankton and protists may contribute to the mesozooplankton diet. 4. Discussion 4.1. Zooplankton biomass The Arctic Ocean was for a long-time considered as a biological desert but recent studies have shown that there is signi"cant biological activity at various trophic levels 1408 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 (Wheeler et al., 1996; Gosselin et al., 1997; Sherr et al., 1997). Zooplankton also show relatively high biomass. In our study, the integrated mesozooplankton biomass in the upper 500 m was much greater than previously reported in the literature for the same region. Conover and Huntley (1991), in a review of mesozooplankton biomass throughout the Arctic Ocean, reported values between 0.12 g DW m\ in the central Arctic Basin and 4.52 g DW m\ in Fram Strait. On the other hand, recent studies in the Eurasian Basin (Hirche and Mumm, 1992; Mumm, 1993; Nielsen and Hansen, 1995), showed mesozooplankton biomass in the same range as the data presented here. Di!erences or similarities observed between this survey and previous ones may have occurred because of di!erences in the size of the mesh of the net that was used for sampling. Mesh size in previous studies ranged from 64 to 500 lm (Conover and Huntley, 1991). Small mesh nets (64 lm) will sample the small class fraction e!ectively but will lead to a higher avoidance rate by larger organisms, and thus may underestimate the mesozooplankton biomass (Hopkins, 1969a, b), which is, in this area, dominated by large copepods. Larger meshes (333 and 500 lm) will, on the other hand, undersample small organisms but are more e!ective for sampling the larger ones (Hirche and Mumm, 1992; Mumm, 1993; Nielsen and Hansen, 1995). The standard net (WP2 with a mesh size of 200 lm) used in this study gives a more representative sampling of both small and large species (Thibault et al., 1994). Another explanation of the di!erence may lie in the preservation techniques. Most of the previous studies measured biomass from preserved (formaldehyde) or frozen samples, and a loss of weight is known to occur under these circumstances (Hopkins, 1968; Omori, 1978). In this study, biomass was measured on fresh samples. The high biomass was not due to increase in the depth or thickness of the Atlantic layer. During this survey, the mesozooplankton biomass was concentrated in the upper 200 m, above the Atlantic layer. Deep layers (around and below 1000 m) were not sampled, so that the biomass of individuals occupying the deep waters (e.g. diapausing Calanus species) may have been missed. Kosobokova (1982), however, found in summer the maximum zooplankton biomass was in the upper 300 m in the central Arctic Ocean, even though sampling was to depths of 1000}2000 m. Also, the data presented here is only for one period of the year, and it may represent a maximum annual level, since samples were taken at the end of the productive season. In fact, Kosobokova (1982) reported an increase of 6}8 times zooplankton biomass between June and August in the central Arctic Basin. 4.2. Contribution of zooplankton to PC C/N (g/g) ratio for zooplankton was high (6.5}8.5), and values were within the range of reported data for high latitudes (6.3}12.5; Ba mstedt, 1986). This ratio is higher than commonly observed at lower latitudes (4.5 in temperate waters: Ba mstedt, 1986). The C/N ratio is indicative of the proportion of fat and protein in the zooplankton body. Nitrogen is present in protein and carbon in both protein and fat, hence the high C : N ratios observed were probably indicative of the animals' accumulation of lipid reserves, which would be expected to be maximal at the end of the productive season (Hopkins et al., 1984, 1985; Conover and Cota, 1985; Arashkevich and Kosobokova, D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1409 1988). Lower C/N ratios were observed at the two "rst stations on the continental shelf of the Chukchi Sea, where a higher proportion of young stages of C. hyperboreus were observed. Percentage of body lipid is known to be smaller for young stages (Kattner and Krause, 1987). The percentage of carbon (around 40%) at these two locations was in the same range as has been reported for other latitudes (Ba mstedt, 1986; Ikeda and Skjoldal, 1989). In terms of carbon, the overall zooplankton biomass represented, over the course of this survey at the end of the productive season, a large portion of the carbon standing stock, up to 48% of the total particulate carbon in the Arctic Ocean. During the JGOFS North Atlantic Bloom Experiment, mesozooplankton represented in late spring only a small fraction of the POC (1.6}16.7% in Lochte et al. (1993), calculated from their Figs. 3 and 10). 4.3. Structure of the zooplankton assemblages The species composition of the zooplankton observed during the survey was similar to that reported previously (Grice, 1962; Jaschnov, 1970; Brodskiy and Pavshtiks, 1976; Dawson, 1978), with a high predominance of copepods. Copepods have always been identi"ed to be the major component of the mesozooplankton community in the Arctic Ocean (Jaschnov, 1970; Dawson, 1978; Longhurst et al., 1984, 1989; Longhurst, 1985; Hirche et al., 1994; Nielsen and Hansen, 1995). The copepod community clearly was dominated by calanoid copepods, and in terms of biomass, speci"cally by C. hyperboreus, C. glacialis and M. longa. The two Calanus species are commonly found in the Arctic Basin (Lee, 1974; Dawson, 1978; Rudyakov, 1983; Hirche et al., 1994). The fourth main species, C. ,nmarchicus was usually limited to Fram Strait and part of Nansen Basin (Grainger, 1961), but during this study it spread over the central Arctic Basin. As this species was always associated with Atlantic water entering the Arctic Ocean through Fram Strait as the North Atlantic current (Ba mstedt, 1986; Hirche, 1991; Gislason and Astthorsson, 1995), an alteration in the characteristics or in#ow of the Atlantic water could modify the dispersion of the species. Actually during this cruise we observed an Atlantic layer with internal core 13C warmer than previously reported (Swift, 1996; Swift et al., 1997), which could have allowed C. ,nmarchicus individuals to survive longer as they were advected further north. Jaschnov (1966) noted also that in years of a strongly pronounced warm Atlantic current, this species was found in northern areas of the Laptev Sea. Also, during this cruise Wheeler et al. (1997) measured nutrients concentrations and found that the front separating the two main in#ows of saline waters, the Atlantic waters (i.e. high N : P ratio, and low silicate concentrations) and the Paci"c waters (i.e. low N : P ratio and high silicate concentrations), had shifted west from over the Lomonosov Ridge to over the Mendeleyev Ridge. This shift seems to have taken place in the last decade (McLaughlin et al., 1996; Swift et al., 1997; Wheeler et al., 1997). In an area near the North Pole, Brodskiy and Pavshtiks (1976) found a maximum of 15 ind m\ in August for C. hyperboreus, which is in the same range as our data. Hirche (1991) observed lower numbers of individuals (females, CVs, CIVs) in the Atlantic water and the Greenland Sea gyre in November (18.7}128 ind 10 m\). An abundance of 102 ind 1000 m\ was observed for C. hyperboreus by Hirche et al. 1410 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 (1994) in the northern part of Nansen Basin, which is close to the abundance found at Station 33 (Fig. 3a). C. glacialis was present in the Atlantic waters and the Greenland Sea gyre in November in lower numbers (1.8}29.3 ind 10 m\: Hirche, 1991) than we observed during this study. The number of female M. longa was much lower here than that found in September in Balsfjorden, northern Norway (Bas mstedt et al., 1985). For the Atlantic waters o! the coast of Greenland, Hirche (1991) found only 6.8}37.3 ind m\ of M. longa in November. Stage CV copepodite and adults dominated the population larger than 350 m at this time of the year, representing up to 80% of the total population. Hirche (1991) and Gislason and Astthorsson (1995) observed only late stages (CIVs and CVs) and adult C. hyperboreus in the Greenland Sea in late fall. Females were concentrated in the surface waters only between April and August where they were feeding and forming large lipid reserves (this study; Hirche and Nieho!, 1996). On the other hand, Longhurst et al. (1984) found in the Canadian Archipelago that during August female C. hyperboreus were already leaving the upper 400 m to concentrate at depths of 500}1500 m. In summary, the mesozooplankton population observed in this study followed the main characteristics already noted for the Arctic Ocean: small numbers of species, large body size resulting from slow growth rates and generation times of 1}3 years. 4.4. Herbivory and omnivory The copepod community was dominated by omnivorous and herbivorous species (e.g., Calanus sp., Metridia sp.), and only a few predatory forms were found (e.g., Euchaeta sp., Appendicularia, Parathemisto sp.). Longhurst (1985) and Longhurst et al. (1989) also found that predatory mesozooplankton represented only 23% in the Arctic compared with 38% in temperate oceans and 47% in tropical oceans. The importance of the classical food web (i.e. ingestion of phytoplankton by zooplankton) was assessed through the measurements of gut pigment contents. Gut pigment contents for C. hyperboreus and C. glacialis were similar to previously reported values. For example, in Ba$n Sound in September Head et al. (1985) found gut pigment contents ranging from 5.7 ng Chl a eq ind\ during the day to 27.1 ng Chl a eq ind\ at night for CV C. hyperboreus and from 0.8 to 7 ng Chl a eq. ind\ for female C. glacialis. In the present study, no attempt was made to study the daily variations of gut contents because of time and personnel constraints, but vertical migrations are known to be limited under conditions of 24 h daylight (Bogorov, 1946), and it is possible that variations in ingestion rates were not following a diel pattern. In this study, M. longa females showed gut pigment contents that were much higher than those previously reported (0}3.2 ng Chl a eq ind\) by Ba mstedt et al. (1990). During this survey copepods were apparently consuming a wide proportion of the total daily primary production (3}55%). However, the proportion of large cells in the total phytoplankton community was generally low (Table 3) with cells '5 lm representing only from 5}30% of the total biomass at most station. Copepods generally consume large phytoplankton cells more e$ciently than small ones (Poulet, 1973, 1978). If this is true for the copepods in the Arctic Ocean, then their rates of D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1411 consumption would often have exceeded the primary production rates for the large phytoplankton. On the other hand, grazing by phagotrophic protists could have accounted for much of the primary production of the small phytoplankton cells (Sherr et al., 1997). Daily rations for C. hyperboreus similar to these have been observed previously (Ba mstedt et al., 1990) and it has been suggested that a daily ration around 20% body C d\ implies that food was not limiting (Herman, 1983), so that during this transect, the mesozooplankton may have been able to obtain su$cient energy from ingested phytoplankton to sustain their metabolism and to form lipid reserves. During this study, a great deal of e!ort was put into studying ice algae (Wheeler et al., 1996; Gosselin et al., 1997). Most of the time ice algae represented a lower biomass than phytoplankton, but at Stations 25 and 31, they represented a larger pool of carbon. Ice algae can be consumed by copepods (Grainger, 1991), but in this study we did not evaluate its role as a food source for copepods. When the phytoplankton population is dominated by small cells, a signi"cant source of carbon for copepods may be microzooplankton (Ba mstedt and Ervik, 1984; Ba mstedt et al., 1985; Hopkins et al., 1985; Gi!ord, 1993; Landry et al., 1994, 1995; Nielsen and Hansen, 1995). Fessenden and Cowles (1994) estimated that non-phytoplanktonic prey in copepod diets can represent 16}100% of the carbon ingested. Non-phytoplanktonic prey may have provided an extra source of carbon for the omnivorous species. Both Metridia longa and Calanus hyperboreus/glacialis have been shown to utilize microzooplankton (Haq, 1967; Huntley et al., 1987). In this study mesozooplankton biomass was relatively well correlated (p(0.05) with the biomass of heterotrophic protists in the size class 6}20 lm (r"!0.696), suggesting that, even during the productive period of the year, protists may contribute to the mesozooplankton diet. Copepods in the Arctic Ocean were for a long time considered to be essentially herbivorous (Hopkins et al., 1984), but our results suggest that there may be an omnivorous component. 5. Conclusions This "rst transect across the Arctic Ocean showed that zooplankton biomass was higher than has previously been reported and that at the end of the productive season mesozooplankton carbon accounted for about 50% of the PC. The distributions of the di!erent species were in#uenced by currents and environmental conditions. At this time of the year zooplankton were not food limited and their diet included phytoplankton and probably microzooplankton. During summer the pelagic food web in the Arctic Ocean may be as complex as those at lower latitudes, including elements of the classic food chain together with those of a microbial food web. In this study biomass and production rates of various trophic levels (e.g. phytoplankton, microzooplankton and mesozooplankton) were higher than reported in past studies. Clearly, signi"cant biological production and consumption of carbon take place in the central Arctic. More complete data sets on the temporal variations over the annual cycle are needed to determine whether autotrophic and heterotrophic processes are balanced, or if the Arctic is a net source or sink for organic carbon. 1412 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 Acknowledgements We are grateful to the crew of the USCGC Polar Sea for technical assistance during sampling, and the Bedford Institute of Oceanography for supplies and for providing excellent facilities. We thank M. Gosselin for providing the algal biomass and production data. We appreciate critical comments by two anonymous reviewers on an early draft. This work was supported in part by an NSF grant (OPP-9400256) awarded to P.A. Wheeler). References Aagaard, K., Fahrbach, E., Meincke, J., Swift, J.H., 1991. Saline out#ow from the Arctic Ocean: Its contribution to the deep waters of the Greenland, Norwegian, and Iceland Seas. Journal of Geophysical Research 96(C11), 20433}20441. Apollonio, S., 1959. Hydrobiological measurements on IGY drifting station Bravo. Transactions, American Geophysical Union 40, 316}319. Arashkevich, E.G., Kosobokova, K.N., 1988. Life strategy of plant-eating copepods: physiology and biochemistry of overwintering Calanus glacialis under starvation conditions. Oceanology 28, 513}517. Ba mstedt U., 1986. Chemical composition and energy content. In: Corner, E.D.S., O'Hara, S.C.M. (Eds.), The Biological Chemistry of Marine Copepods. Clarendon Press, Oxford, pp. 1}58. Ba mstedt, U., Ervik, A., 1984. Local variations in size and activity among Calanus ,nmarchicus and Metridia longa (Copepoda, Calanoida) overwintering on the west coast of Norway. Journal of Plankton Research 6(5), 843}857. Ba mstedt, U., Ha kanson, J.L., Brenner-Larsen, J., BjoK rnsen, P.K., Geertz-Hansen, O., Tiselius, P., 1990. Copepod nutritional condition and pelagic production during autumn in Kosterjjorden, western Sweden. Marine Biology 104, 197}208. Ba mstedt, U., Tande, K.S., Nicolajsen, H., 1985. Ecological investigations on the zooplankton community of Balsfjorden, northern Norway: physiological adaptations in Metridia longa (Copepoda) to the overwintering period. In: Gray, J.S., Christiansen, M.E., (Eds.), Marine Biology of Polar Regions and E!ects of Stress on Marine Organisms, Wiley, New York, pp. 313}327. BeH , A.W.H., Forns, J.M., Roels, A.A., 1971. Plankton abundance in the North Atlantic Ocean. In: Costlow, J.D. (Ed.), Fertility of the Sea, pp. 17}50. Gordon Breach, New York. Bogorov, B.G., 1946. Peculiarities of diurnal vertical migrations of zooplankton in polar seas. Journal of Marine Research 6(1), 25}32. Booth, B.C., Horner, R.A., 1997. Microalgae on the Arctic Ocean Section, 1994: species abundance and biomass. Deep-Sea Research II 44(8), 1607}1622. Brodskiy, K.A., Pavshtiks, Y.A., 1976. Plankton of the central part of the Arctic Basin (based on collections of the North Pole Drifting stations). Polar Geography 143}161. Conover, R.J., Cota, G.F., 1985. Balance experiments with arctic zooplankton. In: Gray, J.S., Christiansen, M.E., (Eds.), Marine Biology of Polar Regions and E!ects of Stress on Marine Organisms Wiley, New York, pp. 217}236. Conover, R.J., Huntley, M., 1991. Copepods in ice-covered seas } distribution, adaptations to seasonnally limited food, metabolism, growth patterns and life cycle strategies in polar seas. Journal of Marine Systems 2, 1}41. Dawson, J.K., 1978. Vertical distribution of Calanus hyperboreus in the central Arctic Ocean. Limnology and Oceanography 23(5), 950}957. English, T.S., 1961. Some biological observations in the central North Polar Sea. Drift Sta. Alpha. 1957}1958. Arctic Institute of North America, Research Paper 13, 8}80. Fessenden, L., Cowles, T.J., 1994. Copepod predation on phagotrophic ciliates in Oregon coastal waters. Marine Ecology Progress Series 107, 103}111. D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1413 Falkenhaug, T., Tande, K.S., Semenova, T., 1997. Diel, seasonal and ontogenetic variations in the vertical distributions of four marine copepods. Marine Ecology Progress Series 149, 105}119. Gi!ord, D.J., 1993. Protozoa in the diets of Neocalanus spp. in the oceanic subarctic Paci"c Ocean. Progress in Oceanography 32, 223}237. Gislason, A., Astthorsson, O.S., 1995. Seasonal cycle of zooplankton southwest of Iceland. Journal of Plankton Research 17(10), 1959}1976. Gosselin, M., Levasseur, M., Wheeler, P.A., Horner, R.A., Booth, B.C., 1997. New Measurements of phytoplankton and ice algal production in the Arctic Ocean. Deep-Sea Research II 44(8), 1623}1644. Grainger, E.H., 1961. The copepods Calanus glacialis Jaschnov and Calanus ,nmarchicus (Gunnerus) in canadian arctic-subarctic waters. Journal of Fisheries Research Board Canada 18(5), 663}678. Grainger, E.H., 1991. Exploitation of arctic sea ice by epibenthic copepods. Marine Ecology Progress Series 77, 119}124. Grice, G.D., 1962. Copepods collected by the nuclear submarine Seadragon on a cruise to and from the North Pole, with remarks on their geographic distribution. Journal of Marine Research 20(1), 97}109. Gronvik, S., Hopkins, C.C.E., 1984. Ecological investigations of the zooplankton community of Balsfjorden, northern Norway: generation cycle, seasonal vertical distribution, and seasonal variations in body weight and carbon and nitrogen content of the copepod Metridia longa (Lubbock). Journal of Experimental Marine Biology and Ecology 80, 93}107. Haq, S.M., 1967. Nutritional physiology of Metridia lucens and M. longa from the Gulf of Maine. Limnology and Oceanography 12, 40}51. Head, E.J.H., 1988. Copepod feeding behavior and the measurement of grazing rates in vivo and in vitro. Hydrobiologia 167/168, 31}41. Head, E.J.H., 1992. Gut pigment accumulation and destruction by arctic copepods in vitro and in situ. Marine Biology 112, 583}592. Head, E.J.H., Harris, L.R., 1996. Chlorophyll destruction by Calanus spp. grazing on phytoplankton; kinetics, e!ects of ingestion rate and feeding history, and a mechanistic interpretation. Marine Ecology Progress Series 135, 223}235. Head, E.J.H., Harris, L.R., Abou Debs, C., 1985. E!ect of daylength and food concentration on in situ diurnal feeding rhythms in Arctic copepods. Marine Ecology Progress Series 24, 281}288. Herman, A.W., 1983. Vertical distribution patterns of copepods, chlorophyll, and production in northeastern Ba$n Bay. Limnology and Oceanography 28(4), 709}719. Hirche H.-J. (1991) Distribution of dominant calanoid copepod species in the Greenland Sea during late fall. Polar Biology 11, 351}362. Hirche, H.-J., Hagen, W., Mumm, N., Richter, C., 1994. The Northeast Water Polynia, Greenland Sea. III. Meso- and macrozooplankton distribution and production of dominant herbivorous copepods during spring. Polar Biology 14, 491}503. Hirche, H.-J., Mumm, N., 1992. Distribution of dominant copepods in the Nansen Basin, Arctic Ocean, in summer. Deep-Sea Research 39(Suppl. 2), S485}S505. Hirche H.-J., Nieho!, B., 1996. Reproduction of the arctic copepod Calanus hyperboreus in the Greenland Sea-"eld and laboratory observations. Polar Biology 16, 209}219. Holm-Hansen, O., Lorenzen, C.J., Holmes, R.W., Strickland, J.D., 1965. Fluorometric determination of chlorophyll. Journal du Conseil permanent international de l'Exploration de la Mer 30(1), 3}15. Hopkins, C.C.E., Tande, K.S., Gronvik, S., 1984. Ecological investigations of the zooplankton community of Balsfjorden, northern Norway: an analysis of growth and overwintering tactics in relation to niche and environment in Metridia longa (Lubbock), Calanus ,nmarchicus (Gunnerus), ¹hysanoessa inermis (Kroyer) and ¹. Rishii (M. Sars). Journal of Experimental Marine Biology and Ecology 82, 77}99. Hopkins C.C.E., Tande, K.S., Gronvik, S., Sargent, J.R., Schweder, T., 1985. Ecological investigations on the zooplankton community of Balsfjorden, northern Norway: growth, and quanti"cation of condition, in relation to overwintering and food supply in Metridia longa, Calanus ,nmarchicus, ¹hysanoessa inermis and ¹hysanoessa raschi. In: Gray, J.S., Christiansen, M.E. (Eds.), Marine, Biology of Polar Regions and E!ects of Stress on Marine Organisms, pp. 83}101. John Wiley and sons Ltd. 1414 D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 Hopkins, T.L., 1968. Carbon and nitrogen content of fresh and preserved Nematoscelis di$cilis, a euphausiid crustacean. Journal du Conseil permanent international de l'Exploration de la Mer 31, 300}304. Hopkins, T.L., 1969a. Zooplankton biomass related to hydrography along the drift track of Arlis II in the Arctic Basin and the East Greenland current. Journal of Fisheries Research Board Canada 26(2), 305}310. Hopkins, T.L., 1969b. Zooplankton standing crop in the Arctic Basin. Limnology and Oceanography 14, 80}85. Huntley, M., Tande, K., Eilertsen, H.C., 1987. On the trophic fate of Phaeocystis pouchetii (Hariot). II. Grazing rates of Calanus hyperboreus (Kroyer) on diatoms and di!erent size categories of Phaeocystis pouchetii. Journal of Experimental Marine Biology and Ecology 110, 197}212. Ikeda, T., Skjoldal, H.R., 1989. Metabolism and elemental composition of zooplankton from the Barents Sea during early Arctic summer. Marine Biology 100, 173}183. Jaschnov, W.A., 1966. Water masses and plankton. 4. Calanus ,nmarchicus and Dinophyes artica as indicators of atlantic waters in Polar basin. Oceanology 6(3), 12}20. Jaschnov, W.A., 1970. Distribution of Calanus species in the seas of the northern hemisphere. International Revue der gesamten Hydrobiologia 55(2), 197}212. Johnson, M.W., 1963. Zooplankton collections from the high polar basin with special reference to the copepoda. Limnology and Oceanography 8(1), 89}102. Kattner, G., Krause, M., 1987. Changes in lipids during the development of Calanus ,nmarchicus s. I. from copepodid I to adult. Marine Biology 96, 511}518. Kiorboe, T., Tiselius, P.T., 1987. Gut clearance and pigment destruction in a herbivorous copepod, Acartia tonsa, and the determination of in situ grazing rates. Journal of Plankton Research 9(3), 525}534. Kosobokova, K.N., 1982. Composition and distribution of the biomass of zooplankton in the Central Arctic Basin. Oceanology 22(6), 744}750. Landry, M.R., Lorenzen, C.J., Peterson, W.K., 1994. Mesozooplankton grazing in the Southern California bight. II. Grazing impact and particulate #ux. Marine Ecology Progress Series 115, 73}85. Landry, M.R., Peterson, W.K., Lorenzen, C.J., 1995. Zooplankton grazing, phytoplankton growth, and export #ux: inferences from chlorophyll tracer methods. ICES Journal of Marine Science 52, 337}345. Lee, R.F., 1974. Lipid composition of the copepod Calanus hyperboreus from the Arctic Ocean. Changes with depth and season. Marine Biology 26, 313}318. Lochte, K., Ducklow, H.W., Fasham, M.J.R., Stienen, C., 1993. Plankton succession and carbon cycling at 473N 203W during the JGOFS North Atlantic Bloom Experiments. Deep-Sea Research II 40(1/2), 91}114. Longhurst, A., Sameoto, D., Herman, A., 1984. Vertical distribution of Arctic zooplankton in summer: eastern Canadian archipelago. Journal of Plankton Research 6(1), 137}168. Longhurst, A.R., 1985. The structure and evolution of plankton communities. Progress in Oceanography 15, 1}35. Longhurst, A.R., Platt, T., Harrison, W.G., Head, E.J.H., Herman, A.W., Horne, E., Conover, R.J., Li, W.K.W., Rao, D.V.S., Sameoto, D., Smith, J.C., Smith, R.E.H., 1989. Biological oceanography in the canadian high arctic. Rapport Proces-verbal Re& union du Conseil international d'Exploration de la Mer 188, 80}89. Mackas, D., Bohrer, R., 1976. Fluorescence analysis of zooplankton gut contents and an investigation of diel feeding patterns. Journal of Experimental Marine Biology and Ecology 25, 77}85. McLaughlin, F.A., Carmack, E.C., Macdonald, R.W., Bishop, J.K.B., 1996. Physical and geochemical properties across the Atlantic/Paci"c water mass front in the southern Canadian Basin. Journal of Geophysical Research 101(C1), 1183}1197. Mumm, N., 1993. Composition and distribution of mesozooplankton in the Nansen Basin, Arctic Ocean, during summer. Polar Biology 13, 451}461. Nielsen, T.G., Hansen, B., 1995. Plankton community structure and carbon cycling on the western coast of Greenland during and after the sedimentation of a diatom bloom. Marine Ecology Progress Series 125, 23}9}257. D. Thibault et al. / Deep-Sea Research I 46 (1999) 1391}1415 1415 Omori, M., 1978. Some factors a!ecting dry weight, organic weight and concentration of carbon and nitrogen in freshly prepared and in preserved zooplankton. International Revue der gesamten Hydrobiologia 63(2), 261}269. Poulet, S.A., 1973. Grazing of Pseudocalanus minutus on naturally occurring particulate matter. Limnology nd Oceanography 18(4), 564}573. Poulet, S.A., 1978. Comparison between "ve coexisting species of marine copepods feeding on naturally occurring particulate matter. Limnology and Oceanography 23(6), 1126}1143. Redden, A.M., 1994. Grazer-mediated chloropigment degradation and the vertical #ux of spring bloom production in Conception Bay, Newfoundland. Ph.D. Thesis, Memorial University of Newfoundland, 254 pp. Rose, M. 1933. Faune de France, O$ce central de Faunistique, Paris, 26, 374 pp. Rudyakov, Y.A., 1983. Vertical distribution of Calanus hyperboreus (Copepoda) in the Central Arctic Basin. Oceanology 23(2), 249}254. Sherr, E.B., Sherr, B.F., Fessenden, L., 1997. Heterotrophic protists in the central Arctic Ocean. Deep-Sea Research II 44(8), 1665}1682. Smith, S.L., 1988. Copepods in Fram Strait in summer: Distribution, feeding and metabolism. Journal of Marine Research 46, 145}181. Smith S.L., Schnack-Schiel, S.B., 1990. Polar zooplankton. In Polar Oceanography, Part B: Chemistry, Biology and Geology, Academic Press, San Diego, pp. 527}598. Smith, S.L., Vidal, J., 1986. Variations in the distribution, abundance, and development of copepods in the southeastern Bering Sea in 1980 and 1981. Continental Shelf Research 5(1/2), 215}239. Swift, J., 1996. A CTD/Hydrographic section across the Arctic Ocean. In: Tucker, W., Cate, D. (Eds.), The 1994 Arctic Ocean Section. The First Major Scienti"c Crossing of the Arctic Ocean, U.S. Army Cold Regions Research and Engineering Laboratory, pp. 17}19. Swift, J.H., Jones, E.P., Aagaard, K., Carmack, E.C., Hingston, M., Macdonald, R., McLaughlin, F., Perkin, R., 1997. Waters of the Makarov Basin. Deep-Sea Research II 44(8), 1503}1529. Thibault D., Gaudy, R., Le Fèvre, J., 1994. Zooplankton biomass, feeding and metabolism in a geostrophic frontal area (Almeria-Oran front, western Mediterranean). Signi"cance to pelagic food webs. Journal of Marine Systems 5, 297}311. UNESCO, 1968. Zooplankton Sampling. UNESCO Monographs on Oceanographic Methodology, 2. Wang, R., Conover, R.J., 1986. Dynamics of gut pigment in the copepod ¹emora longicornis and the determination of in situ grazing rates. Limnology and Oceanography 31(4), 867}877. Wheeler, P.A., Gosselin, M., Sherr, E., Thibault, D., Kirchman, D.L., Benner, R., Whitledge, T.E., 1996. Active cycling of organic carbon in the Central Arctic Ocean. Nature 380, 697}699. Wheeler, P.A., Watkins, J.M., Hansing, R.L., 1997. Nutrients, organic carbon and organic nitrogen in the upper water column of the Arctic Ocean: implications for the sources of dissolved organic carbon. Deep-Sea Research II 44(8), 1571}1592. Yentsch, C.S., Menzel, D.W., 1963. A method for the determination of phytoplankton chlorophyll and phaeophytin by #uorescence. Deep-Sea Research 10, 221}231.