Keywords

1 Introduction

The Atlantic sector of Antarctica is the area of the greatest abundance of Antarctic krill (Euphausia superba Dana, 1851) (Mackintosh 1972; Murphy et al. 2007). Antarctic krill, as well as other zooplankton species, feeds on phytoplankton. Euphausia superba is the keystone species in the Southern Ocean ecosystem, and plays a crucial role in the Antarctic food web providing an important source of food for enormous populations of seabirds, penguins, and marine mammals (Loeb et al. 1997; Ross and Quetin 1995; Schofield et al. 2017; Smith et al. 2008; Steinberg et al. 2015). The ability to reproduce at a fast rate allows E. superba to create an immense biomass varying from 61 million tons to 600 million tons (Atkinson et al. 2019; McQuillan 1962; Nemoto 1972; Samyshev 2002). Furthermore, krill contribute to the biological pump by production of faecal pellets (Mauchline and Fisher 1969). The highest concentrations of krill were found in the northwestern part of the Antarctic Peninsula as well as in the Scotia and Weddell seas where the eastward-flowing Antarctic Circumpolar Current (ACC) and waters from the Weddell–Scotia Confluence resulted in a strong advective flow, intense eddy activity, and mixing (Hewitt et al. 2004; Murphy et al. 2007). The Weddell Sea is a marginal sea of the Southern Ocean that is situated along the coast of Antarctica south of the Atlantic Ocean. The continental shelf along the eastern coast of the Antarctic Peninsula and the Antarctic continent covers about a quarter of the total Weddell Sea area. The ecosystem of the Weddell Sea deserves special study due to many aspects. The Weddell Sea is characterized by an extremely cold environment, the biota lives here most of the year in the presence of ice, and this sea is very important for biodiversity conservation in the Southern Ocean (Teschke et al. 2020). The Weddell Sea is a region of high phytoplankton productivity due to the mixing of micronutrients in the surface waters. The upper layer of cyclonic outflow of the Weddell Sea gyre over the shelf regions becomes enriched in sediment-derived nutrients. The northerly outflow of the Weddell Sea shelf water affects the deep-water Powell Basin located in the northeastern part of the Weddell Sea. The Powell Basin is isolated from the Weddell Sea by seamounts with the associated Weddell Front and characterized by cyclonic circulation (Heywood et al. 2004). A high abundance of E. superba was reported in the Powell Basin (Kasatkina et al. 2004; Siegel et al. 2004; Siegel and Watkins 2016); therefore, it is likely that krill juveniles from the western part of the Weddell Sea accumulate here.

In addition to the regular seasonal and interannual fluctuations in krill abundance associated with hydrological features in various years, there has been a decrease in krill catches up to 80% in recent decades in various regions of Antarctica, observed together with a significant increase of salp populations (Salpa thompsoni Foxton, 1961) and (Ihlea racovitzai, Van Beneden and De Selys Longchamp, 1913). The main cause of the decline in krill stock and the increase in salp abundance was the increase in the water temperature as a result of global warming and the retreat of Antarctic ice from the tip of the Antarctic Peninsula, because ice algae are important for krill recruitment.

It should be noted that the summer water temperature near the Antarctic Peninsula increased by 1.3 °C during a period of 50 years (Meredith and King 2005). Even the deep-water layers of the Weddell Sea warmed up by 0.032 °C over a decade (Whitehouse et al. 2008). Thus, considering the commercial and trophic importance of krill, it is important to study the abundance and biomass of krill, salps, and mesozooplankton in the Powell Basin that allows us to verify the hypothesis of the transport of juvenile krill from the Weddell Gyre to the Powell Basin.

2 Materials and Methods

In the Powell Basin area, the samples were collected in January–February 2020 during cruise 79 of the R/V “Akademik Mstislav Keldysh” (AMK) (Fig. 27.1). In a network of stations located in the Powell Basin (Fig. 27.1), the abundance, species composition, and biomass of macro- and mesozooplankton were analyzed using a double Bongo net (mouth area 0.6 m2, mesh size 0.300 mm) hauled vertically between a depth of 200 m (or less in shallower stations) and the surface. The volume of water filtered was measured with sensing device placed at the mouth of the net. The samples were mixed and homogenized in a container, and then concentrated and fixed in 4% formalin in sea water (final concentration) for abundance, taxonomic, and biomass studies.

Fig. 27.1
figure 1

Map of the study region during the AMK 79 Antarctic cruise and locations of stations (6597–6627) in the Powell Basin

Full size image

Biological analysis of krill was carried out after fixing the samples with formalin and reaching a constant weight of crustaceans based on the formalin concentration. Biological analysis of krill specimens included measurement of their individual length, mass, sex determination, and maturity stage (the degree of development of external and internal genitals). The length of krill crustaceans was measured with an accuracy of 1 mm from the outer edge of the orbit to the end of the telson. The individual mass of krill was determined by weighing the crustaceans dried with filter paper, on a torsion balance (WT-1000) (SC-CCAMLR 2010). The maturity stages of krill were determined according to the scale proposed by Makarov and Denys (1980).

Each individual salp was measured and weighted. Zooplankton samples were fixed with 4% formaldehyde and then identified under binocular MBS-9 (LOMO, Saint Petersburg, Russia) using a Bogorov chamber.

The dominant species were identified by the index of coenotic significance (ICS) according to formula (27.1):

$$ \mathrm{ICS}=p\surd B, $$
(27.1)

where р is species occurrence (%), and В is species average biomass (g/m3), respectively. This index indicates the significance of a given species in the community (Mordukhay–Boltovskoy 1975).

Data processing and statistical analyses were performed using the MS Excel 2010 program and STATISTICA 12.0 (StatSoft) software package. The PCA analysis, multiple regression analyses, and correlation coefficients between abundance and biomass and hydrological and hydrochemical parameters were calculated with the standard MS Excel 2010 program. Cluster analysis was conducted in the STATISTICA 12.0 (StatSoft) software package. This analysis was applied to visualize similarities in the physicochemical parameters as well as macro- and mesozooplankton species between the stations.

3 Results

In the Powell Basin, zooplankton taxa consisted of 7 phylums, 10 classes, 14 orders, and 34 species (Table 27.1).

Table 27.1 List of collected species in the Powell Basin
Full size table

In the order Euphausiacea, there were five species, but only three of them (E. superba, E. frigida, E. crystallorophias) belong to the designation of «krill». Among the phyla, the greatest number of species belonged to Arthropoda (17 species), then Annelida (four species), the phylums Mollusca, Chordata, Chaetognatha, Cnidaria included 3 species each and the phylum Ctenophora one species. The distribution of zooplankton biomass was not uniform.

The average biomass varied from 0 mg/m3 at station 6620 to 2777.6 mg/m3 at station 6599; the average value was 760.36 ± 777.1 mg/m3 (Fig. 27.2).

Fig. 27.2
figure 2

Biomass of salp (S. thompsoni, I. racovitzae), krill (E. superba, E. frigida), and other groups of zooplankton (mesozooplankton) in the Powell Basin.

Full size image

Note. 6596 – 6627 are the designations of the stations of observations.

The biomass of salps was the maximal compared to krill as well as other zooplankton groups, but it varied to the greatest extent from 0–2761.2 mg/m3, making on average 737.3 ± 767.5 mg/m3. Between the stations, the percentage of Salpa thompsoni in the zooplankton community varied from 0% to 98.47 %. The biomass of krill varied from 0 to 496.3 mg/m3, on average 40.6 ± 110.1 mg/m3, while the biomass of mesozooplankton varied from 0 to 158.4 mg/m3, on average 21.2 ± 36.9 mg/m3. The maximum biomass was observed in the central deep-water region (stations 6597–6604 and 6618–6619), as well as at stations 6625–6627, where salps were absolutely dominant, amounting to 97.4% in biomass.

The distribution of zooplankton abundance varied greatly among the study sites (Fig. 27.3).

Fig. 27.3
figure 3

Abundance of salp (S. thompsoni, I. racovitzae), krill (E. superba, E. frigida), and other groups of zooplankton (mesozooplankton) in the Powell Basin.

Full size image

Note. 6596–6627 are the designations of the stations of observations.

At other stations, krill or mesozooplankton prevailed, with maximal biomass up to 496.3 mg/m3 in the western part of the study site where juvenile krill developed significantly. The share of krill in the total zooplankton abundance was only 3.1%, whereas the proportion of salps was 39.5% and the proportion of mesozooplankton was 57.4%. According to ICS, the species S. thompsoni dominates and its ICS exceeds 2.35 times that of E. superba (Fig. 27.4).

Fig. 27.4
figure 4

Structure of zooplanktocenosis in the Powell Basin according to the ICS index

Full size image

Due to the considerable dominance of salps, the ICS distribution of the species was very uneven.

The size of species Euphausia frigida, E. crystallorophias, and Thysanoessa macrura, which are Antarctic circumpolar neritic species, varied from 10 to 20 mm. The species Thysanoessa vicina was encountered occasionally and was represented by crustaceans ranging in size from 12 to 15 mm. Of the copepods, the Calanoides acutus species had the greatest weight according to the ICS; species Calanus propinguus, Rhincalanus gigas, and Metridia gerlachei were also found. In the Powell Basin, the biomass of copepods was low: from 0.38 to 2.24 mg/m3 and composed from 0.03% to 9.47% of zooplankton biomass. Siphonophores were represented by three species, of which Dimophyes antarctica and Diphyes chamissonis ranked in the fourth and fifth places, respectively, according to the ICS. Chaetognaths, which are predators, mainly on copepods or euphausiids larvae, were represented by three species: Sagitta maxima, Sagitta gazellae, and Eukrochia hamata. The species Sagitta maxima ranked tenth according to the ICS, but the biomass of sagitta was relatively low, from 0.08 to 1.23 mg/m3, and their proportion ranged from 0.01% to 5.19%. The biomass of hyperiids being another group of predators, was low, from 0 to 0.38 mg/m3. Hyperiids were represented by four species: Themisto gaudichaudii, which was the most abundant, as well as Primno macropa, Cyllopus magellanicus, and Scina antarctica. It is likely that low occurrence and abundance of the species was associated with the low abundance of copepods and euphausiids.

The size of species Salpa thompsoni varied from 5 to 120 mm, with a prevalence of specimens from 30 to 40 mm (25.4%) and from 40 to 50 mm (26.0%) (Fig. 27.5).

Fig. 27.5
figure 5

Size structure of S. thompsoni in the Powell Basin (6596–6627 are station numbers)

Full size image

Many salps were confined to deep-water stations due to their biotopic allocation in a 1500-m thick layer of water according to literature data (Voronina 1998). Another salp species I. racovitrae ranked third in the ICS index and its size varied from 2 to 6 mm. I. racovitrae is a salp species confined to the surface (0–50 m deep) layer of the water column. The population of E. superba in the Powell Basin was represented by crustaceans from 10 to 53 mm in size, which were found throughout the study site. The maximum abundance of E. superba was at station 6596, where juveniles of 20–30 mm predominated; they were transported here, probably by waters from the Weddell–Scotia Confluence. The size of species E. superba varied from 10 to 53 mm, with prevalence of specimens from 40 to 50 mm (35.2%); those from 30 to 40 mm (33.5%) predominated in the size structure of krill of the Powell Basin (Fig. 27.6).

Fig. 27.6
figure 6

Size structure of krill (Euphausia superba) in the Powell Basin.

Full size image

The correlation between mesozooplankton abundance and turbidity was high (Table 27.2).

Table 27.2 Correlation coefficient matrix for the main oceanographic parameters and quantitative indicators of krill, salps, and mesozooplankton in the Powell Basin (significant values are in bold)
Full size table

Krill biomass positively correlated significantly with oxygen content and negatively with temperature (Table 27.2). The abundance of zooplankton, primarily copepods, increases with a decrease in the number of salps and krill, which feed not only on phytoplankton, but also on small zooplankton (Loeb et al. 1997). Salps consume phytoplankton competing with euphausiids. This explains the low inverse correlation between the abundance of salps and krill, which was r = −0.23, but, however, was not significant. Multiple regression analysis of the krill, salps, and mesozooplankton indices and oceanographic parameters revealed a significant cumulative effect of temperature, oxygen, salinity, and turbidity on krill abundance (p-value ˂ 0.001) and biomass (p-value ˂ 0.001). The oxygen level exerted a significant stimulatory effect (p-value ≤ 0.05) on both abundance and biomass of krill and salps. Thus, temperature decreased the combined effect of oxygen, turbidity, and salinity, which could be attributed to the delayed effect of temperature on the quantitative indicators of krill, salps, and mesozooplankton of the Powell Basin compared with the influence of oxygen, which determines the level of metabolism. Thus, oxygen content plays a leading role in the determination of krill development, especially in the conditions of the Powell Basin.

The PCA was used to assess the most important factors explaining the variation in the environmental data set (Fig. 27.7). The PCA is an indirect ordination technique for obtaining a low-dimensional representation of multivariate data, such that the data can be presented visually in a two-dimensional PCA correlation biplot. The PCA of the physicochemical variables and the abundance and biomass of krill, salps, and mesozooplankton through the first two components explained 93.4% of the total variance.

Fig. 27.7
figure 7

Correlation biplot diagram based on the principal component analysis (PCA) of the physicochemical variables and the abundance and biomass of krill, salps, and mesozooplankton in the Powell Basin

Full size image

The PCA confirmed the stimulatory role of oxygen and salinity and the inhibitory effect of temperature and turbidity on both abundance and biomass of krill and salps. In addition, salinity was the main parameter (with a loading of 85.4%) in the second principal component and explained 24.4% of the total variance in the data. Nevertheless, turbidity had a positive effect on mesozooplankton abundance, which was confirmed by the correlation analysis. Thus, PCA showed that, of the physicochemical parameters, only temperature was negatively correlated with abundance and biomass of krill, salps, and mesozooplankton in the Powell Basin.

Cluster analysis taking into account all physicochemical and biotic parameters did not show complete coincidence of physicochemical parameters on the one hand and quantitative indicators of zooplankton on the other. (Fig. 27.8). Cluster analysis conducted on the basis of the parameters of hydrobionts showed similarity among two groups of stations. The first cluster included stations 6607, 6608, 6609, 6614, 6620, and 6596, whereas the second cluster included stations 6597, 6618, 6601, 6602, 6598, 6604, 6600, 6603, and 6599. The stations of the first cluster had lower total biomass but higher quantitative characteristics of krill and mesozooplankton compared to the second cluster due to high salp development at the stations of the latter.

Fig. 27.8
figure 8

Cluster analysis of physicochemical variables (a) as well as abundance and biomass of krill, salps, and mesozooplankton (b) in the Powell Basin in January–February 2020

Full size image

4 Conclusions

Two subareas differing in the ratio of salps, krill, and mesozooplankton exist in the study site based on the cluster analysis. The maximum biomass of zooplankton (1285.96 mg/m3) was recorded in the central deep-water area and in the Antarctic Sound due to salps. Results of the correlation and principal component analysis showed the key role of oxygen and temperature in the development of zooplankton in the Powell Basin. To fully understand the “salps–krill” relationships, wider and deeper studies are needed.