Sea-surface temperature (SST) decreased by about 10°C from the south (latitude 20°N) to the north (55°N). The main gradient of SST was found between 40°N and 47°N (Figure 2A). We found the North Atlantic Central Water (NACW) between 20 and 30°N and 40-48°N from the surface to 700 m depth (8°<T<20°C). An increasing of temperature and salinity in the mesopelagic zone was observed around 35°N coinciding with the Mediterranean Water (MW; 2.6°<T<11°C). Antarctic Intermediate Water (AIW) was also detected between 20 and 30°N at 800–1200 m depth (4°<T<8°C). Finally, North Atlantic Deep Water (NADW) was found below 1200 m depth for most stations but approaching the surface between 50 and 55°N (4°<T<1.5°C; see details in Sarmiento-Lezcano et al., 2022). Highest salinity values at the surface were found in Sts. 2 and 3 (37.1), sharply declining northward with a minimum value of 35.1 in Station 10 (Figure 2B). We also observed the Mediterranean Water (MW) at around 1000 m depth centered at 35°N, showing a Meddy because of the high salinity observed. Dissolved oxygen (DOx) increased from low to high latitudes, increasing towards the north (Figure 2C), and an oxygen minimum zone (OMZ) was found in the lowest latitude of the transect (20-25°N). This OMZ was found in Sts. 1 and 2 located between 200 and 1000 m. Chl a was highly variable (Figure 2D) and ranged from very low values at 25°N (St. 2) to the highest values observed at 47°N (St. 7). A frontal zone was observed between stations 6 and 7 in the limit between the stratified zone (surface water >14°C in the upper 50 m layer) and the mixed water column to the north. The southern oligotrophic zone was characterized by a deep chlorophyll maximum (DCM), and further north (St. 10) values were similar in the upper 50 m layer (Figure 2). Chl a was observed to reach down to 200 m depth between 40 and 47°N at the frontal zone.

Across the study area, although 15 zooplankotn groups were found, 11 zooplankton groups predominated (>0.2%), with copepods being the most dominant taxa (Table 1). Copepod nauplii and copepods in the range of 100-200 µm displayed the highest abundance at the frontal zone (Sts. 6 and 7; Figure 3A). where the contribution of copepods increased to 99%. Lowest abundance values corresponded to the southern stations (south of 40°N) where the contribution of copepods was lower (86%). In the northern stations (north of 50°N), the abundance of copepods also decreased as well as the copepod contribution to the mesozooplankton community (63%, Table 1). In these northern stations, the abundance of cladocera (22%), doliolids (7%), small jellyfishes (2%), and siphonophores (2%) increased sharply (Table 1). Zooplankton biomass (Figure 3B) was low in the upper 200 m depth southwards of 45°N (microzooplankton <5 mg DW·m^-3 and mesozooplankton <10 mg DW·m^-3). Both size classes increased northward declining in the northernmost station. The larger community (>200 µm) showed the highest biomass in St. 9 (>40 mgDW·m^-3).

Using the hierarchical clustering analysis of the total abundance of zooplankton and their dominant groups, we observed three different station assemblages (Figure 4). The first five stations in the south (similarity of 80%) were segregated from the group of Sts. 6 and 7 (similarity of 72%), coinciding with the highest gradient of temperature and salinity (82% of similarity at 45-47°N). Finally the northern stations (north of 50°N) formed by Sts. 8, 9, and 10, also showed a high similarity among them.

In the upper 200 m layer, copepods were the dominant group among all zooplankton groups. We also found a gradient in richness from the southern stations (83 copepod species) to the northern stations (42 copepod species). The southern stations had less than 300 ind·m^-3 while at the frontal zone (Sts. 6 and 7) the total abundance was 5 times higher. In the northern stations, the abundance decreased at least half from the frontal area. Besides the different copepods mentioned later, the cladoceran Evadne nordmanni (23%) appeared only north of 50°N.

We identified 41 genera and 95 copepod species in this layer. The dominant copepods in the whole area were Paracalanus spp. (32%), Oithona spp. (25%), and Clausocalanus spp. (10%), followed by large Calanoids. (5%), Calocalanus spp. (4%), Oncaea spp. (3%) and Metridia spp. (2%). Paracalanus spp. dominated in the frontal (Sts. 6-7) area (51%) and Oithona spp. were more relevant in the northern stations (Table 1). Paracalanus parvus was the dominant species in the frontal region (15%) although it was also present in the southern (7%) and in the northern regions (5%). However, Oithona spp. increased in the northern area (40%), being quite diversified (O. atlantica, O. similis, O. tenuis, and O. setigera). Calanus finmarchicus dominated the northern station (St. 10; 58°N) and C. helgolandicus was found further south disappearing at St. 10. Juveniles of the genus Metridia were also abundant in the northern station in the epipelagic layer as well as M. lucens. A higher number of copepod species (83) were identified in the southern stations decreasing by half in the frontal zone and further north. Hierarchical clustering of the dominant taxa (>1%) including cladocera, showed three main different assemblages in relation to the frontal region (Figure 5A). Group a (37% of similarity) was formed by species more abundant in the northern stations (E. nordmanni, C. finmarchicus, Eucalanus hyalinus, Rhincalanus nasutus, and juveniles of Metridia spp.). Group b (>50% of similarity) represented the bulk of the dominant taxa. In this group we found different species of Paracalanus, Clausocalanus, Calocalanus, Acartia, Candacia and all Corycaeidae, as well as Mesocalanus tenuicornis, Lucicutia gemina, Mecynocera clausi, Pleuromamma gracilis, and C. helgolandicus. Group c (40% of similarity) was formed by those species only found in the southern stations: Evadne spinifera, Nannocalanus minor, Oncaea venusta, Euchaeta acuta, Temora stylifera but also deeper species such as Pareucalanus sewelli or Subeucalanus subtenuis. The abundance of the dominant genera (Figure 5B) clearly showed the dominance of Paracalanus spp. in the frontal zone, while Oithona spp. dominated north of the frontal zone.

The vertical distribution of the dominant groups during day- and nighttime (copepods, chaetognaths, siphonophores, ostracods, and pteropods) showed a sharply decrease of abundance and biomass with depth (Figure 6). The contribution of copepods at each layer (Table 2) was usually higher than 90% with the exception at the 400–700 m layer, decreasing during daytime to 85% due to the presence of other zooplankton groups at same layer. All groups had their highest abundances in the epipelagic zone and besides copepods, very few organisms were found below 700 m depth. Ostracods were more abundant in the upper 200 m layer observing their increasing in the upper 100 m layer during night, as well as the pteropods. Siphonophores and chaetognaths showed similar abundance in the upper 100 m depth during day, although at night both seemed to be dispersed in the upper 200 m layer (Figure 6).

Zooplankton abundance and biomass declined sharply with depth showing a rather similar vertical copepod profile of abundance among the different stations (Figure 7A). The highest abundances were found in the upper 100 m layer either by day or by night, gradually declining to 400–700 m layer. Zooplankton biomass also showed similar profiles except the increase observed at St. 10 in the 400–700 m layer (Figure 7B). Below 700 m depth we observed a strong decline in both abundance and biomass. About 90% of the abundance and 50% of the biomass was found in the upper 100 m layer (Table 2). Zooplankton abundance and biomass contributed to the mesopelagic layer (200–700 m depth) just 3% and 27%, respectively. Below 700 m depth, abundance was only <1% of the total abundance (0–1900 m layer) and 11% of the total biomass. During the day, 89% of the abundance was found in the upper 100 m layer, while at night it was 91%. In this layer, the biomass accounted for 46% during the day and 53% at night, while the nearest layer below (100–200 m depth) changed from 9% by day to 16% by night due to the decrease of the layer below (200–700 m) from 33 by day to 21% by night. Below 700 m depth, we did not find significant differences. We observed a low number of organisms (503 ind·m^-3) and a high biomass (10 mg DW·m^-3) at St. 10 (58°N), showing the overall different mean size of the dominant organisms. Here, we found a high abundance of C. finmarchicus during the day in the mesopelagic layer (400–700 m depth). Part of this population moved at night to the upper layers increasing twice their abundances (980 ind·m^-3).

Using the hierarchical clustering of the dominant zooplankton groups at the different layers, we found that the upper 700 m was segregated (55% of similarity) from the layers below (Figure 8). Three main groups were observed: (1) the upper epipelagic (0–100 m depth with 78% of similarity), the 100 to 700 m depth layers (70% of similarity), and below them to 1900 m depth (68%). The NMDS analysis also showed the three groups in which the epipelagic and the mesopelagic layers to 700 m depth were segregated from the other layer (Figure 8). The similarity was also high (72%) from 700 m to 1600 m depth, either by day- and nighttime. Nevertheless, slight differences were observed in the deepest strata (1600-1900m) between day- and nighttime.

We observed differences in the composition of copepods between the southern and northern latitudes (see groups in Figure 5A) and according to Sundby (2000) it could be related to temperature and stratification as two of the most important factors determining the structure of the copepod community. However, although in the epipelagic layer (0–200 m) the copepod community showed different structure among regions, most copepods were, as in our study, cosmopolitan (Bucklin et al., 2010). Among the Calanoids, Clausocalanus spp., and Paracalanus spp. were rather abundant, being P. parvus and their juveniles the dominant copepods in the frontal zone. Among the non-calanoids, Oithona spp. was by far the most abundant copepod, an ubiquitous organism in the world ocean whose information about its distribution pattern is still limited (Gallienne and Robins, 2001). In our study, O. setigera was found in the whole transect while O. tenuis and O. plumifera were found in the southern area. Other species such as O. similis and O. atlantica were only found in the northern area with a southern limit at 47°N.

Some copepods such as C. styliremis or M. tenuicornis were found in the whole area, while C. arcuicornis, N. minor, M. clausi as well as Sapphirina spp., F. rostrata and other Corycaeidae spp. were abundant in the southern stations. They are characteristic species of warm latitudes (Vives and Shmeleva, 2007, 2011; Schnack-Schiel et al., 2010; Wootton and Castellani, 2016). By contrast, in the northern region the latter copepods were absent observing a high abundance of Metridia spp. and C. finmarchicus in agreement with authors working in the area (Gaard et al., 2008; Gislason et al., 2008). C. finmarchicus dominated the northern latitudes replacing C. helgolandicus observed toward the south as expected (Fleminger and Hulsemann, 1977; Conover, 1988; Planque et al., 1997; Beaugrand et al., 2001).

A high abundance of P. parvus was also noticed in other areas of the North Atlantic Ocean also showing a surface distribution (Frost and Fleminger, 1968; Vinogradov et al., 1970; Deevey and Brooks, 1977) which could be related to the warm waters found at the east of the MAR (Pollard et al., 2004). The northeastward trajectory of the North Atlantic currents enables species of southern warmer areas to appear east of the MAR and north the frontal system. As a result, the northern limit of warm water species could be observed further north in the eastern side than the western side of the ridge. Accordingly, the copepod distribution pattern showed a non-symmetrical horizontal distribution, underlining the complexity of the biogeographic area (Beaugrand et al., 2001; Gallienne et al., 2001; Hays et al., 2001; Stemmann et al., 2008; Barnard et al., 2011). In any case, Paracalanus spp. are a widespread cosmopolitan epipelagic species, living in high chlorophyll conditions (Siokou-Frangou et al., 2010), which could be found forming intense patches and being well adapted to many ecosystems and food resources (Siokou et al., 2019).

To our knowledge, the number of genera found in the present study was slightly higher than previous studies carried out in the North Atlantic (Gaard et al., 2008). This might be probably due to our extensive sampled area from low latitudes (20°N) and to the important depth range of our study (Roe, 1984; Woodd-Walker et al., 2002). In similar latitudes, Grice and Hulsemann (1965) found 66 calanoid genera between 0 and 5000 m depth, most of them in the upper 2000 m depth, close to our record of 68 calanoid genera.

In relation to the rather large area covered from subtropical to temperate latitudes, we found only one third of the copepod species and one half of the genera in the northern stations. Copepod diversity as the number of copepod genera and species declined northward as previously observed in different Atlantic areas (Angel, 1993; Pierrot-Bults, 1997; Woodd-Walker et al., 2002; Bode et al., 2018). Variable diversity and less clear latitudinal trend were observed north of 40°N (Woodd-Walker et al., 2002). This could be explained by the complex distribution of currents, the warmer water supplied from southern areas (Pollard et al., 2004), and the intensity of the frontal system (Beaugrand and Ibañez, 2002). The west-east asymmetry also confirmed the different diversity of calanoids on either side of the MAR forming different species association (Beaugrand et al., 2001, 2002).

The highest diversity observed from 100 to 700 m depth during daytime and at the 100–200 m layer at night is also in agreement with other authors working in the subtropical and temperate oceans (Gaard et al., 2008; Bode et al., 2018). The concentration of available food but also environmental gradients decreasing with depth, could be behind the decline of the number of ecological niches and consequently in species diversity (Longhurst, 1985). It was interesting to observe a high diversity in the deepest layers during the night (3, Table 2) which can be due to the low abundance and the inhabiting species of copepods probably reaching up from below depths.

We found a large number of copepods living in the upper 100 m layer. Group a was formed by species quite abundant during the day in the epipelagic layer (35%). This abundant group was mainly composed by different species of Paracalanus, Clausocalanus, Ctenocalanus and Calocalanus. Another group seemed to display a reverse migration since they were rather abundant during the day in the epipelagic layer and at night were dispersed by the different adjacent layers in quite low abundances. This was the case of Group c composed by different species of Oithona, Oncaea and Microcalanus. Because of this, it was not easy to define their vertical movements. By opposite, some species were rather abundant in the epipelagic layer particularly during the night but in low abundance during the day (Group b). Most copepods living in the upper layers were associated with the highest food abundance as it was the case of Paracalanus spp. and Clausocalanus spp. (omnivores-herbivores) and Oithona spp. and Oncaea spp. (detritivores), reflecting their trophic regime disparity (Kiørboe, 2011; Benedetti et al., 2016). Fernández de Puelles et al. (2023) found, in lower latitudes, many of these copepods moving daily around the upper 100 m depth, and increasing their abundances during the night in the upper 50 m. However, we were not able to observe this migration due to our wider sampling strategy in this layer.

We also found strong interzonal migrants which were a group of species moving from meso- or bathypelagic layers toward the epipelagic layer at night (Group d, 13%). We also showed that each stratum was inhabited by different copepod assemblages, indicating differences among day- and nighttime. We found two groups of strong migrants moving from deep layers to the surface, one with bimodal vertical profiles (Group d: different species of Euchirella, Gaetanus, Paraeuchaeta, Scaphocalanus and H. norvegicus) and the other with also vertical migration during the night (Group b: R. cornutus, M. lucens, M. brevicauda and P. robusta or P. piseki). All of these were migratory copepods too, which were found in high abundances in the epipelagic waters at night (15%), exhibiting similar vertical migrations as in tropical and subtropical latitudes (Deevey and Brooks, 1977; Roe, 1984; Roe et al., 1984; Longhurst et al., 1990; Fernández de Puelles et al., 2023).

We also observed the group of species characteristic in the twilight layers (Group e: 21%) showing closer abundances during day- and nighttime (such as species of Conaea, Tortanus, Nullosetigera, Subeucalanus, P. sewelli or M. princeps). They could be present slightly shallower at night but never reaching the upper 100 m layer, exhibiting a similar vertical distribution to the ones found at lower latitudes (Roe, 1984; Roe et al., 1984; Fernández de Puelles et al., 2023).

Although in low abundances, other copepods were always living in bathypelagic layers such as Monacilla. Some deep copepods such as P. sewelli in our data exhibited different vertical distribution than in lower latitudes (Fernández de Puelles et al., 2023). In this sense, it is important to mention that some deep copepods such as Euchirella spp., Heterorhabdus spp., or Gaetanus spp. showed differences in their vertical distribution with latitude, usually becoming deeper in southern regions (Gride, 1963; Bucklin et al., 2010). We also found that deep copepods collected in southern stations did not appear in the northern ones (e.g., T. mayumbaensis, P. sewelli, different species of Scottocalanus or Nullosetigera), in agreement with earlier studies in close areas (Gaard et al., 2008). A similar pattern was observed for decapods and ostracods in different regions of the North Atlantic Ocean (Foxton, 1972; Fasham and Angel, 1975).

Particular mention is given to the well-studied C. finmarchicus (Marshall and Orr, 1972) which was highly abundant in northern latitudes (Gaard et al., 2008). In our study, C. finmarchicus showed a bimodal vertical profile, more abundant at night in the upper layer (Figure 10). This copepod is known to display a large variability in the vertical distribution of their developmental stages, including diel variation (Kaartvedt, 1996; Mauchline, 1998). The apparent absence of regular DVM of some species (e. g. C. helgolandicus in our data), does not necessarily mean that a population does not migrate vertically since they could change their vertical displacements with latitude (Roe et al., 1984). There are many difficulties in interpreting vertical distribution patterns since it is possible that apparent non-migrants move acyclically or migrate every two days. The vertical distribution patterns of copepods could vary in time and space exhibiting a complex behavior and sometimes showing irregular and chaotic movements (Sameoto, 1976; Roe and Badcock, 1984).

In summary, this study provides the vertical distribution of the dominant copepods in the North Atlantic Ocean across an extensive region and spanning down to bathypelagic layers. We found that almost half of the total copepods were living in the epipelagic layer while others were abundant in deeper layers during the day moving or not during the night towards the surface. This pattern was close to the distribution observed in tropical and subtropical latitudes at the basin scale (Fernández de Puelles et al., 2023). In this study, we observed a latitudinal decreasing gradient northward in the number of genera and species, and an increasing gradient in their abundances. Small copepod nauplii and Paracalanus parvus predominated in the frontal zone found between 45 and 47°N, while in the northern region, Calanus finmarchicus and several Oithona dominated. South of this latitude, warmer species such as C. helgolandicus, N. minor, and M. clausi were more abundant. Deep copepods were also of importance in the vertical distribution and structure of the community with the larger species living in meso- and bathy-pelagic layers, moving upward at night. Accordingly, we found at least five groups in their vertical distribution: (1) the non-migrant epipelagic copepods, (2) those migrant copepods observed in high abundances in the epipelagic layer only at night and in deeper layers during the day, (3) the copepods abundant in the epipelagic layer by day performing reverse migrations, (4) the strong migrants moving from the meso- and bathypelagic layers to the epipelagic zone at night and finally (5) those just moving into the twilight zone. Here, we disentangled the structure of these communities in the above related five groups shedding light about their role in the ocean as epipelagic non-migrant consumers and those performing strong diel vertical migrations, being also key copepods promoting active flux (Turner, 2015). Accordingly, all of them are essential contributors to the ocean biogeochemistry and carbon flux through the water column. Finally, we underline the need for taxonomic studies in more extensive areas and seasons for further understanding of the role of these organisms in the biological carbon pump.