Vertical migrations of Subantactric and Antarctic zooplankton were previously studied in the Drake Passage and adjacent Southern Ocean both in diel and seasonal aspects16,19,20,21,28,29. One of the common ways to describe the magnitude of diel migrations is a night/day ratio of mesozooplankton characteristics20,29,30. As an example, the mean night/day ratios of the mesoplankton abundance and biomass were reported to be 1.06 and 1.17, respectively, in the upper 100 m of the Amundsen Sea29. In waters around South Georgia the night/day rations varied from 0.61 to 1.69 and from 0.73 to 1.45 for abundance and biomass, respectively20. The ratios were obtained from the station pools not divided by any of the hydrological fronts20,29. However, the fronts may drastically influence mesoplankton distribution19,20,21,22, so we hypothesized that the observed variations are linked to the position of main hydrological fronts. Indeed, in our dataset, the ratio depended on the position respective to the PF, varying in wide range from 0.47 north of the PF to 3.45 south of the PF (Table 2). Additionally to the mesoplankton abundance and biomass, the similar night/day differences were also demonstrated for the diversity values, which was never reported before. The ES-100 changes indicate that the vertical migration involves many mesoplankton species in a similar way (including the rare ones), as this index includes both number of species and the evenness of their abundances31,32.
The diel vertical migrations are thought to be triggered by the light intensity changes during the day9,16,20,29,30. The upward migrations were previously recorded around 7:00–9:00 p.m., with the following morning descent at 5:00–6:00 a.m.16. The difference in time is explained by changes in dusk and dawn time linked to different latitude or date15,16,33. In our samples, according to the distribution of integral parameters along the day, the nighttime extremum (maximum or minimum, depending on the PF position and the depth range) fell within the 11:00 p.m. and 3:00 a.m. north from the PF and within 1:00 a.m. and 4:00 a.m. south from the PF. The exact time of mesoplankton ascent and descent is hard to assess in our dataset.
The range and direction of diel vertical migrations in our samples depended not only on the position respective the PF (north or south of that) but also on the season (spring or summer). Seasonal changes in vertical migrations of zooplankton were regularly observed in temperate and polar seas16,21,28,30,33,34,35. The seasonal variation is often attributed to the overall decrease of diel light/dark cycles in summer30,34,35. In particular, the midnight sun period can stop any vertical movement of certain copepod species, as it was shown for Calanus species35,36. In winter, during the polar night, the mesoplankton migrates dielly despite the lack of obvious light/dark cycles37. In the Southern Ocean, an overall downward migration of zooplankton was observed in winter, as it was shown by summer/winter comparison16,21,28. Cisewski et al.16 distinguished two periods of migration patterns: from February to October (late summer to early spring), when the migrations are driven by the day/night rhythm; and from October to January, when the most of the zooplankton rises to the uppermost waters (< 50 m) for feeding and reproduction. Our samplings, taken in October–November (spring) and January (summer), therefore, fall into the second migration period described by16.
Differences between the spring and summer samples were significant and concerned integral community parameters and distribution of certain species. The seasonal shifts in mesoplankton vertical migrations depended on the position respective to the PF. Spring/summer vertical distribution of the abundance and biomass demonstrated the overall summer ascent north of the PF and summer descent south of the PF (Figs. 5, 6). In particular, north of the PF the mesoplankton was mainly concentrated in the upper layers by the summertime. South of the PF, the nighttime maxima of the abundance and biomass descended to the depth of ~ 150–300 m by summer (possibly even deeper, to the depths unsampled in this study) (Figs. 5, 6). Seasonal zooplankton migrations are linked to the food source distribution (i.e. primary production rate, often expressed in Chl a values) which concentrates in the upper mixed layer during spring and summer16. In the Drake Passage, the maximum surface chlorophyll values were observed around November–December north of the PF and in December-January south of the PF38,39. Concentration of the mesoplankton in the UL north of the PF in spring and summer is thus explainable (Figs. 5, 6). However, the summer mesoplankton descent from the most productive UL to the deeper layers south of the PF is unexpected. A possible explanation may be in vertical distribution of the phytoplankton: e.g., in the Eastern Atlantic Sector of the Southern Ocean Chl a was concentrated in the UL north of the PF, but evenly distributed within the upper 100 m south of the PF40. In the Drake Passage south from the PF, the Chl a vertical distribution demonstrated deep maxima below the UL (pers. comm. by Dr. A. Demidov, unpublished data). The dynamics of individual species distribution expectably demonstrated similar migration patterns as the integral community parameters. Most recognized taxa migrated normally with the nighttime surface ascent, confirming previous studies2,5,16,21,22,28. However, several taxa demonstrated the reverse migration pattern with the night descent to the deeper waters. Previously the negative pattern was shown for species of Oithona similis and Oncaea curvata copepods near the Antarctic coast (south off ACC)13.
Seasonal shifts (reflected mainly in summer ascent) were previously reported for several species of copepods in the Southern Ocean, including Eucalanus longiceps, R. gigas, Neocalanus tonsus, Calanoides acutus, Calanus simillimus and Calanus propinquus21. Our data confirmed the spring/summer shifts for most of the species with possible exception of C. acutus. E. longiceps and N. tonsus were not statistically represented in our samples. According to our data, R. gigas, O. plumifera, Oncaea sp. and Aetideus spp. copepods changed direction of their diel migrations from normal to inversed depending on season and position respective to the PF. Inversed migrations are generally explained by a possible avoidance of nocturnal predators with normal migration patterns5,15,30, which is in accordance with our dataset: Aetideus sp. and O. plumifera show certain negative correlations with predators, including cnidarians and Euchaetidae copepods. These predatory mesoplankton organisms are listed as consumers of copepods, possibly including Oithona species41. We suggest that the predators influence migratory behavior of the taxa and may switch direction of migrations on both sides of the Polar Front. The observed details of the vertical migrations show fine adaptive adjustments of taxa to local factors depending on season (spring or summer), hydrological setting (north or south of the PF), and predators.
The PF has a great impact on seasonal and vertical migrations of mesoplankton. Vertical dynamics differ on both sides the PF in seasonal and diurnal aspects. North of the PF the mesoplankton concentrates in the UL both in spring and in summer, while south of the PF the mesoplankton concentrates in the UL in spring and descends in deeper layer in summer. In spring north of the PF, most of the taxa dielly migrate within the upper 300 m, ascending from the DL to the UL at night, while south of the PF vertical migrations encompass deeper layers from below 300 m (unsampled in this study) to the DL. In summer and north of the PF, migrations of mesoplankton are concentrated in the UL and large-scale diel migrations are insignificant, which mirrors feeding and reproduction in phytoplankton rich strata. Conversely, south of the PF the mesoplankton is concentrated below the UL: a possible result of an even vertical distribution of phytoplankton with no prominent surface maximum. The described trends of the seasonal and diel migrations are shown in a simplified scheme representing a balance-like swing of mesoplankton maxima on northern and southern sides of the PF (Fig. 8). Individual taxa such as Aetideus sp. and O. plumifera showed both common (nighttime ascend) and inverted (nighttime descend) vertical migrations depending on season and position related to the PF. According to42, the SF and the SAF act as more important boundaries for zooplankton communities than the PF. Although, we do not have a representative data set to compare the areas south from the SF and north from the SAF, we suggest that the differences in migration patterns in those areas may differ significantly from the described in this study.