Plasticity in dormancy behaviour of Calanoides acutus in Antarctic coastal waters

Copepods that enter dormancy, such as Calanoides acutus, are key primary consumers in Southern Ocean food webs where they convert a portion of the seasonal phytoplankton biomass into a longer-term energetic and physiological resource as wax ester (WE) reserves. We studied the seasonal abundance and lipid profiles of pre-adult and adult C. acutus in relation to phytoplankton dynamics on the Western Antarctic Peninsula. Initiation of dormancy occurred when WE unsaturation was relatively high, and chlorophyll a (Chl a) concentrations, predominantly attributable to diatoms, were reducing. Declines in WE unsaturation during the winter may act as a dormancy timing mechanism with increased Chl a concentrations likely to promote sedimentation that results in a teleconnection between the surface and deep water inducing ascent. A late summer diatom bloom was linked to early dormancy termination of females and a second spawning event. The frequency and duration of high biomass phytoplankton blooms may have consequences for the lifespan of the iteroparous C. acutus females (either 1 or 2 years) if limited by a total of two main spawning events. Late summer recruits, generated by a second spawning event, likely benefitted from lower predation and high phytoplankton food availability. The flexibility of copepods to modulate their life-cycle strategy in response to bottom-up and top-down conditions enables individuals to optimize their probability of reproductive success in the very variable environment prevalent in the Southern Ocean.

Epipelagic copepod assemblage in the Gerlache Strait (Antarctica) during the 2015 austral summer

Oblique zooplankton tows were carried out in the Gerlache Strait from 17 to 22 January 2015 as part of the first Colombian oceanographic expedition to the Antarctic continent, to document the composition and structure of the epipelagic copepod assemblage, taking into account variations at a small spatial scale. The epipelagic environment of the Gerlache Strait was divided into two oceanographic regions during sampling: a stratified northern region and a homogeneous southern region. The epipelagic copepod assemblage comprised 17 species belonging to four orders and 13 families. Adult copepods as well as developmental stages ranging from copepodite II (C2) to copepodite V (C5) from eight different species were recorded in the study area. The dominant species by number was Oithona similis, followed by O. frigida, Ctenocalanus citer, Drescheriella glacialis, and Calanoides acutus. There were significant differences between the epipelagic copepod assemblages present in the northern and southern regions of the Gerlache Strait (Anosim, p = 0.01), with a dissimilarity percentage of 52%. Moreover, Aetideus armatus is reported for the first time in the western sector of the Antarctic Peninsula. The oceanographic conditions and the geomorphological characteristics of the study area modulated the surface circulation pattern, as well as the structure and composition of the primary producers during the study period, determining the spatial variation of the abundance and composition of the epipelagic copepod assemblage in the Gerlache Strait during the 2015 Austral summer.

Abundance and grazing impacts of krill, salps and copepods along the 140°E meridian in the Southern Ocean during summer

Abundance and grazing impacts of krill, salps and herbivorous copepods were investigated in Antarctic waters along the 140°E meridian, south of Australia, during the summers of 2002 and 2003. North of the Southern Boundary of the Antarctic Circumpolar Current (SB-ACC), macrozooplankton comprised species of Salpa thompsoni and large herbivorous copepods, while the area south of the SB-ACC was numerically dominated by Euphausia superba or E. crystallorophias. North of the SB-ACC, the estimate of grazing impact revealed that krill, salps and copepods, Calanoides acutus, Calanus propinquus, Rhincalanus gigas and Metridia gerlachei, are able to remove a maximum of 37% of the total phytoplankton standing stock in early to midsummer, but grazing is negligible in late summer. The high grazing impact is attributed to the relatively high zooplankton abundance and low phytoplankton abundance. South of the SB-ACC, overall daily grazing impact of the three zooplankton groups was low and did not exceed 6% of the total phytoplankton standing stock throughout the investigation period. Present results indicate that the contribution of krill, salps and copepods varies seasonally as well as regionally across the SB-ACC. It seems that the carbon transport from surface to deep water by macro- and mesozooplankton in summer in this area is relatively large north of the SB-ACC but small south of the SB-ACC.