ALBERT

Description: The mesozooplankton community, with special emphasis on calanoid copepods, was studied with respect to its species composition, abundance, vertical distribution and developmental structure during the “Ice Station POLarstern” (ISPOL) expedition to the ice-covered western Weddell Sea. Stratified zooplankton tows were carried out nine times between 1 December 2004 and 2 January 2005 with a multiple opening–closing net between 0 and 1000 m depth. Copepods were by far the most abundant taxon, contributing more than 94% of the total mesozooplankton. Numerical dominants were cyclopoid copepods, mostly Oncaea spp. A total of 66 calanoid copepod species were identified, but the calanoid copepod community was characterised by the dominance of only a few species. The most numerous species was Microcalanus pygmaeus, which comprised on average 70% of all calanoids. Calanoides acutus and Metridia gerlachei represented other abundant calanoid species contributing an average of 8% and 7%, respectively. All other species comprised less than 3%. The temporal changes in the abundance and population structure of M. pygmaeus and M. gerlachei were small while a shift in the stage frequency distribution of C. acutus was observed during the study: copepodite stage IV (C IV) dominated the C. acutus population with 48–50% during the first week of December, while C V comprised 48% in late December. C I and C II of C. acutus were absent in the samples, and males occurred only in very low numbers in greater depths. In M. gerlachei, C I was not found, whereas all developmental stages of M. pygmaeus occurred throughout the study. All three species showed migratory behaviour, and they occurred in upper water layers towards the end of the investigation. This vertical ascent was most pronounced in C. acutus and relatively weak in the other two species. In M. pygmaeus and M. gerlachei, copepodids were responsible for the upward migration in late December, while the vertical distribution of adults did not change. In C. acutus, all abundant developmental stages (C IV, C V and females) ascended to upper water layers. Almost exclusively (93%) medium- and semi-ripe females of C. acutus and M. gerlachei were found, and only 3–4% of the ovaries were ripe. The absence of C I and the low number of ripe females indicate that the main reproductive period had not started in C. acutus and M. gerlachei until the end of our study in early January. In contrast, the high portion of C I and C II of M. pygmaeus suggests that reproduction of this species had started in October–November and hence before the onset of the phytoplankton bloom in the water. The community structure did not differ between stations with one exception on 26 December, when the station was strongly influenced by the continental shelf.

Description: In the framework of the “Ice Station POLarstern” (ISPOL) expedition in the western Weddell Sea, two sediment traps were deployed at 10 and 70 m water depth under a drifting ice floe in December 2004. The amount and composition of the vertical particle flux under sea ice were determined during a period of 30 days in order to investigate the influence of biological processes in sea ice and on its underside on the flux. The total mass flux was dominated by diatoms, faecal material, and aggregates, and ranged from 95.28 to 197.67 mg m−2 d−1 at 10 m depth and from 51.54 to 55.34 mg m−2 d−1 at 70 m depth. A strong increase with time of the flux of chlorophyll equivalents, biogenic silica, and faecal material was recorded during the observation period, coincident with the increase in the concentration of chlorophyll a in the bottom ice layer above the trap array. The latter suggests a concomitant increase in the amount of food available for grazers, such as krill, in the bottom ice layer and on the underside of the ice floe, resulting in an increased downward transport of ice-algal material into the water column. The sinking faecal material was dominated by krill faecal strings and contained large amounts of diatom frustule debris, as well as intact diatom frustules, mainly of the species Fragilariopsis curta and F. cylindrus. Single pronounced flux events of Phaeocystis antarctica and aggregates were also observed early in the study period. Low POC/PON and biogenic silica/POC ratios of the sinking particulate matter suggest that the material collected in the traps was relatively fresh.

Description: In the framework of the R.V. Polarstern expedition “Ice station POLarstern” (ISPOL) spatial and temporal trends in composition, abundance and age structure of sea ice inhabiting copepods were investigated in the western Weddell Sea during the transition from the spring to the summer state. For the spatial scale, sea-ice coring was performed at six locations on a transect from the ice edge to the ice-drift station between 14 and 24 November 2004. The temporal changes were investigated in a time series study on a drifting sea-ice floe from 29 November to 30 December 2004. A relatively large number of copepod species (15) were found in the ice with a higher number at the time station (13) than at the transect (9). Drescheriella spp. was by far the most abundant taxon encountered in the sea ice throughout the present study (72–87%). On the transect, Idomene antarctica ranked second in abundance (7%) followed by Stephos longipes (2%) and Ectinosoma sp. (2%). In contrast, Diarthrode cf. lilacinus, which was not found on the transect, was the second most abundant species (11%) at the time station, followed by I. antarctica (9%), Ectinosoma sp. (6%) and S. longipes (1%). Naupliar stages dominated the populations of Drescheriella spp. and S. longipes both on the transect and during the time series. The Ectinosoma sp. population was dominated by nauplii only at the stations of the transect, while copepodite stages made up the largest fraction during the time series. Copepodids always predominated the I. antarctica populations, and it was the only species in which adults occurred in high densities contributing significantly to the abundance. Only Drescheriella spp. and S. longipes occurred throughout the sea-ice cores, while the occurrence of all other species was restricted to the bottom layer of the ice. The distribution of all species was very patchy and varied greatly between the sampling sites.