Age Structure and Spatial Distribution of Euphausia superba Larvae off the Antarctic Peninsula, Southern Ocean

Abstract

The Antarctic krill, Euphausia superba Dana, 1850, is a species forming high biomass and, therefore, playing a major role in the Antarctic marine food web. The age structure and patterns of spatial distribution of E. superba larvae in the waters of the Bransfield Strait (Antarctic Sound, Powell Basin), and off the South Orkney Islands, were studied based on data collected through a research survey in January and February 2022. Eggs and larvae (naupliar, calyptopis, and furcilia stages) of E. superba were found in these regions. Eggs and nauplii were concentrated in the southern, deep-sea part of the Antarctic Sound and over the northeastern and southwestern slopes of the Powell Basin, while calyptopis and furcilia larvae were concentrated north of the South Orkney Islands. The larvae abundance increased in an easterly direction. Four groups of communities comprising krill larvae at different development stages were identified. These groups were located in two subregions with the border between them running off the South Orkney Islands. The distribution and abundance of E. superba larvae showed a clear relationship with environmental conditions, in particular with a combination of such factors as sea surface temperature and chlorophyll a concentration.

Recent Changes in Composition and Distribution Patterns of Summer Mesozooplankton off the Western Antarctic Peninsula

“…The highest euphausiid egg abundances were observed at the deep-sea stations which were associated with the highest oxygen concentrations (>10 mg/L), very cold water (−1 • C), and high, bloom-like phytoplankton concentrations. Mackintosh [91] found that euphausiid eggs and larvae at early stages tend to certain zones as the thermocline disappears; in addition, an abundance of krill larvae at early developmental stages may be associated with temperature and Chl a concentration [27,29,92,93]. Further studies are expected to be carried out to elucidate the relative contributions of the factors driving the abundance of copepods that dominate the mesozooplankton of the Southern Ocean and form the major food supply in the food chain between primary producers and predators [94,95].…”

“…Previously, data on the mesozooplankton together with the other plankton categories (phytoplankton, macroplankton, and ichthyoplankton) were obtained as preliminary results, e.g., [20,29]; in the present study, we provide more details and carried out additional analyses.…”

“…The detailed dynamics of some environmental variables and the pattern of main currents in the study region were described earlier elsewhere [20,26,29,[57][58][59][60]. During the summer of 2022, the average values of the environmental variables at the sampling stations were as follows: water temperature, −0.02 ± 0.70 • C; water salinity, 34.56 ± 0.15 psu; oxygen concentration, 8.84 ± 1.46 mg/L; and chlorophyll a concentration, 0.30 ± 0.62 µg/L.…”

“…In the West Antarctic sector of the Southern Ocean, climate change has caused the following consequences over the past few decades: an increase in sea surface temperature (SST) of up to 0.2 • C per decade, with projections indicating further extensive increase at a rate of 0.27-1.08 • C per decade that may occur by the late 21st century [5][6][7], a reduction in sea-ice cover near the Antarctic Peninsula [8,9], and significant interannual variations in chlorophyll a (Chl a) concentrations [10]. Some significant changes in the plankton communities were reported previously, including replacement of dominant phytoplankton species by others [11][12][13], variations in the dominance and abundance of taxonomic groups of zooplankton, in particular copepods [14][15][16][17][18][19][20], variations in the abundance, biomass, and spatial distribution of the pelagic tunicate Salpa thompsoni [21,22], krill Euphausia superba [23][24][25][26], and krill larvae [18,[27][28][29]. Such changes might be either part of a natural ecosystem mechanism [30,31] or a negative trend that caused communities to adapt to new habitat conditions [15,32,33].…”