Materials for this study were collected on 9–13 July 2008 during a cruise in the framework of the project Evolution of a Glacial Arctic Continental Margin: the Southern Svalbard Ice Stream-dominated Sedimentary System. An SBE 21 thermosalinograph (TSG; Sea-bird Electronics, Bellevue, WA, USA) was used to record surface-water temperature and conductivity. Salinity was calculated from the values measured by the SBE 21 sensor. By means of the pump connected to the TSG surface seawater samples were collected every degree or half a degree along the south-to-north transect from 65 to 78°N (Fig. 1).

Water samples (5 l) were concentrated with a 10-µm mesh, reduced to 250 ml and immediately fixed with buffered formaldehyde (4% final concentration). We are aware that naked ciliates have been underestimated because of the fixative chosen (Karayanni et al. 2004); otherwise, the amount of water sampled allowed us to feel confident that even rare organisms could have been detected in our samples.

Subsamples (100 cc) were examined in a settling chamber using a Leitz (now Leica Microsystems, Wetzlar, Germany) Labovert inverted microscope, equipped with phase contrast and bright-field illumination (magnitude:× 200), according to the Utermöhl method (1958). The entire surface of the chamber was examined.

Among the microzooplankton community, four main groups were considered: ciliates (naked and tintinnids), heterotrophic dinoflagellates, other protozoa and micrometazoans.

Empty loricae of tintinnids were not differentiated from the filled ones because tintinnid protoplasts are attached to the lorica by a fragile strand, which detaches with ease during collection and fixation of the samples. The phototrophic ciliate Mesodinium rubrum is an obligate autotroph (Lindholm 1985), but it was considered to be in the list as it was only a minor part of the ciliates biomass. Dinoflagellates were considered heterotrophic based on the previous studies (Lessard 1991). Only species that belong to Gyrodinium genus could not be identified to species level and the contribution of autotrophy or mixotrophy could not be evaluated by this method but their contribution to total biomass was less than 1%.

For each taxon, biomasses were estimated by measuring the linear dimension of each organism with an eyepiece scale and equating shapes to standard geometric figures. Cell volumes were converted into carbon values using appropriate formulae or conversion factors, as follows—tintinnids: pg C cell⁻¹=µm³×0.053 + 444.5 (Verity & Langdon 1984); naked ciliates: pg C cell⁻¹=µm³×0.14 (Putt & Stoecker 1989); athecate dinoflagellates: pg C cell⁻¹=µm³×0.11 (Edler 1979); thecate dinoflagellates: pg C cell⁻¹=µm³×0.13 (Edler 1979); others: pg C cell⁻¹=µm³×0.08 (Beers & Stewart 1970); foraminifera: pg C cell⁻¹=µm³×0.089 (Gifford & Caron 2000).

To test whether microzooplankton changed among stations, the cluster analyses and similarity percentage analyses (SIMPER) were performed using PRIMER software version 5. For the cluster analysis, square root transformation of the data matrix and Bray–Curtis similarity with a complete linkage were applied (Clarke & Warwick 2001). To detect which microzooplanktonic groups were mainly responsible for the discrimination of subclusters, a SIMPER analysis was carried out. A percentage dissimilarity cut-off of 90% was applied.

To elucidate the relationship between biotic (total microzooplankton, naked ciliates, tintinnids, heterotrophic dinoflagellates, Foraminifera and micrometazoans) and abiotic variables (salinity and temperature), Spearman correlation analysis (R) was performed using STATISTICA (P<0.05*; P<0.01**; P<0.001***).

Temperatures ranged from 2.7°C (station 16) to 13.3°C (station 1) and salinity between 33.3 (station 16) and 35.2 (station 6). Temperature showed a clear south-to-north gradient; with the exception of station 16, with similar salinity values (Fig. 2).

The microzooplankton community was composed of heterotrophic dinoflagellates, foraminifers, ciliates (naked ciliates and tintinnids) and micrometazoans. In total, 27 taxa were counted (Table 1).

Total microzooplankton abundance ranged from 17 to 438 ind l⁻¹. Tintinnids and heterotrophic dinoflagellates were the most abundant organisms, ranging from 1.5 to 292.5 ind l⁻¹ and from 0 to 232 ind l⁻¹, respectively (Fig. 3). An increasing gradient in both abundance (Fig. 3) and number of microzooplankton taxa (Fig. 4) was observed from south to north, with temperature decreasing from 13 to 2°C.

Micrometazoans, mainly copepod nauplii and gastropoda larvae, reached the maximum of 45.5 ind l⁻¹ at station 7. Larval stages of the pteropod Limacina helicina were detected with a maximum of 5.5 ind l⁻¹ (Table 1). Aloricate ciliates’ abundance was always less than 50 ind l⁻¹; no clear pattern with respect to the transect was discerned. Other protozoa, in this study constituted only by Foraminifera, were very scarce at all stations, with a maximum value of 1.4 ind l⁻¹ (Table 1).

Acanthostomella norvegica (Daday), Parafavella denticulata (Ehrenberg), Parafavella gigantea (Brandt) and Ptychocylis obtusa Brandt were the most common species among tintinnids (Fig. 5). P. denticulata and A. norvegica were the most abundant species, reaching their maxima at station 11 (227.5 ind l⁻¹) and station 15 (146 ind l⁻¹), respectively (Table 1). A. norvegica and P. gigantea were present all along the transect, whereas P. denticulata, P. obtusa and Coxiella cf. pseudoannulata were detected in the northern part of the transect. Leprotintinnus pellucidus (Cleve) (Fig. 5) was recorded only at station 17, close to Svalbard. The community of naked ciliates was dominated by Strombidiidae and Holotrichia. Mesodinium rubrum was always very scarce with a maximum value of 0.7 ind l⁻¹. Protoperidinium was the most representative genus among heterotrophic dinoflagellates. P. bipes (Paulsen) Balech, with a maximum of 116 ind l⁻¹ at station 15, and Dinophysis rotundata Claparède and Lachmann were detected in the northern part of the transect, P. cf. curtipes in the southern part (Fig. 5).

Cellular carbon content ranged from 0.87 to 5.18 µg C l⁻¹ (Fig. 6). Tintinnids and micrometazoa made up the largest part of the microzooplankton biomass with higher values at stations 10, 11 and 17 (Fig. 6). The high biomass of the tintinnids was due principally to genus Parafavella, consisting of large species such as P. gigantea and P. denticulata. Along the entire transect, micrometazoans showed quite constant carbon biomass values, due to copepoda nauplii and gastropoda larvae. The biomass of naked ciliates was dominated by Strombidiidae and Holotrichia that could not be identified to species mainly due to the fixative used. Dinoflagellates never formed the major part of carbon values, as they were mainly constituted by small species from the genus Protoperidinium, such as P. bipes.

Cluster analysis divided the stations into two groups of stations with different microzooplanktonic communities (Fig. 7). The first group (south), which comprised stations 1–8, was characterized by low microzooplankton abundance. The second group (north) was characterized by stations with intermediate (stations 9 and 16) and high abundance (stations 10, 11, 14, 15 and 17). The SIMPER outputs showed an average dissimilarity between north and south groups of 76.51%. The main differences were due to tintinnids (45.20%) and dinoflagellates (22.68%), which were more abundant in the northern stations.

Highly significant negative correlations were observed between total microzooplankton (R=−0.88***), heterotrophic dinoflagellates (R=−0.87***), tintinnids (R=−0.86***) and temperature. No significant correlations were observed between microzooplankton abundances and salinity.

Temperature and salinity values measured during this study resembled what were found in the same area and season by other authors (Boltovskoy et al. 1991; Seuthe et al. 2011). Particularly, low temperature (2.7°C) and salinity (33.3) values recorded at station 16 could be related to the presence of colder fresh surface Arctic Water from the east (Cottier & Venables 2007). It was possible to identify three different types of density patterns. Stations 1–5, close to the Norwegian coast, presented temperatures between 10 and 13.3°C. For stations 6–17, between Norway and Svalbard, two subgroups were evident: stations 6–8 with temperature > 8°C and stations 10, 11, 14, 15 and 17 with temperature < 8°C. Stations 9 and 16, as suggested by cluster analysis, seemed to present particular characteristics. The former, with intermediate values of temperature and salinity, could be considered as the shifting from one situation (south) to another (north), while station 16 differed from all the rest for the minima of both temperature and salinity.

Tintinnids showed a south-to-north gradient, with highest abundance when the temperature was between 4 and 8°C (stations 10, 11, 14, 15 and 17).

The distribution of the most representative species showed a different pattern along the transect. P. denticulata appeared at station 7 and increased at stations 10–15. Even P. obtusa was present only in the northern stations with temperature < 6°C. Boltovskoy et al. (1991) found P. obtusa and P. denticulata 1.4–2 times more abundant in waters of Atlantic origin, in comparison to colder Arctic waters. Burkovskii (1976) reported that optimum temperature ranges for P. denticulata and P. obtusa were 8–16 and 6–11°C, respectively. In our samples, P. denticulata (Fig. 5) showed dimensions similar to the one found in Greenland samples by Boltovskoy et al. (1995). A comparison of the morphometric data for P. denticulata in the Greenland Sea vs. the Barents Sea showed a larger dimension of the oral diameter and bowl length in the Greenland Sea. The oral diameter of the specimens of P. denticulata in the Barents Sea was between 45 and 55 µm, while in the Greenland Sea it measured between 60 and 65 µm. Boltovskoy et al. (1995) argued that the difference between the mean size of P. denticulata in the two areas was due to dissimilarities between the trophic status and thermic regimes of the two areas.

In this study, we found P. gigantea all along the transect, with a maximum of 14.5 ind l⁻¹. In previous studies in nearby areas (Boltovskoy et al. 1991, 1995; Cordeiro & Sassi 1997; Cordeiro et al. 1997; Seuthe et al. 2011) P. gigantea was never recorded, not even at three stations close to the area considered in this study (stations 3, 11 and 12 in Boltovskoy et al. 1991). It was present in the central Barents Sea (Hansen & Jensen 2000) and off the west coast of Sweden (Hedin 1974, 1975). Hedin (1975) described P. gigantea as a summer species, most frequent in surface waters with two peaks during the warmer months (April–June and October). Hedin (1974) reported dimensions varying from 211 to 265 µm, while in Marshall (1969) length ranged from 200 to 750 µm, which is closer to our findings (400–500 µm; Fig. 5c). This species is common in all northern seas, including the Arctic (Marshall 1969), and it is considered a boreal species by Pierce & Turner (1993). Recently, Hinder et al. (2012) observed a general increase of abundance of this species in the eastern North Atlantic, with more northern displacement towards Iceland and Greenland in less warm waters. These findings can explain the differences between our results and those of Boltovskoy et al. (1991), who did not report any P. gigantea in their samples.

We also found A. norvegica all along the transect, even if the abundance increased in the northern stations with a temperature below 8°C. Boltovskoy et al. (1991) found high values of this species at the stations dominated by Arctic waters, and Taniguchi (1983) reported peaks in colder waters along a transect across the Oyashio front. Other authors (Boltovskoy 1995; Cordeiro et al. 1997; Seuthe et al. 2011) reported high levels of abundance of this species in nearby areas and the absence in the samples counted by the continuous plankton recorder survey in the north-east Atlantic and North Sea was probably due to the counting method (Hinder et al. 2012). A. norvegica is considered cosmopolitan (Pierce & Turner 1993) and it was the only one present in both subpolar areas, as in the South Atlantic (Alder 1999), the Strait of Magellan (Fonda Umani et al. 2011), the Barents Sea and the Greenland Sea (Boltovskoy et al. 1991, 1995).

L. pellucidus was found only in the northernmost station sampled (station 17). Hedin (1975) considered this as a cold species most common in surface waters all through the year with the exception of summer and Boltovskoy et al. (1991) associated this species with waters of Atlantic origin.

The distribution patterns of tintinnids and dinoflagellates species found during the cruise showed a certain relationship with the circulation of waters within the Norwegian Atlantic Current and the West Spitsbergen Current.

It is not clear to us whether the biogeographic dissimilarities we report here are responses to differences in associated biological parameters or if they point to specific distributional preferences with respect to different water masses.

In previous studies in the Barents Sea (Boltovskoy et al. 1991), tintinnids ranged between <1 and 314 ind l⁻¹, with an average of 46 ind l⁻¹ for the 0–50 m layer. In the Greenland Sea, tintinnid abundance was lower, not exceeding 135 ind l⁻¹ (Boltovskoy et al. 1995).

A. norvegica, P. denticulata and P. obtusa accounted for almost 90% of the individuals in the Barents and Greenland seas (Boltovskoy et al. 1991, 1995). The Greenland Sea tintinnid assemblages were generally less diverse and had a lower equitability than those from the Barents Sea. Boltovskoy et al. suggested a direct relationship between lower temperature in the Greenland Sea (around 0–1°C) and lower diversity/higher dominance. In the study reported here, temperatures from 4 to 8°C were associated with higher abundance and diversity; at station 16, where the temperature fell to 2.5°C, abundance reduced.

Boltovskoy and coworkers found lower values for dinoflagellates in comparison to what we found in this study, but they considered only large and thecate forms (Boltovskoy et al. 1991, 1995). In general, maximum concentrations for microzooplankton that occurred in the area were detected at or above 25 m (Boltovskoy et al. 1991, 1995). Recently, Seuthe et al. (2011) found dinoflagellate rather than aloricate ciliates dominating at one station located in a fjord on the west coast of Svalbard. In our study, aloricate ciliates never reached high abundance values and micrometazoans were higher than previously reported (Melle & Skjoldal 1989; Boltovskoy et al. 1991).

In terms of biomass, Boltovskoy et al. (1991) found for tintinnids a carbon average for the 0–50 m layer of 0.33 µg C l⁻¹, with a maximum of 2.1 µg C l⁻¹. In the central Barents Sea, Jensen & Hansen (2000) analysed ciliates and heterotrophic dinoflagellates in the Marginal Ice Zone and they described a maximum biomass of tintinnids of 1 µg C l⁻¹. The higher biomass in our results is principally due to the presence of P. gigantea and its larger dimensions, as the contribution to the total carbon amount of other frequent species, such as A. norvegica and P. obtusa, was never significant because of their small dimensions.

In conclusion, the microzooplankton community in the area affected by the North Atlantic Current seemed to be constituted principally by tintinnids and heterotrophic dinoflagellates. The abundance and the number of taxa changed along the south-to-north transect, with higher values at lower temperatures.

The forecasted warming of the Arctic could affect the geographical displacement of tintinnids cold species, which were significantly more abundant in the northernmost part of our transect, pushing them even further north and causing a reduction of both abundance and diversity in the southern part of the study area.