Composition and community structure of zooplankton in the sea ice-covered western Weddell Sea in spring 2004—with emphasis on calanoid copepods

Abstract

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.

Introduction

The seasonal fluctuation in sea-ice growth and decay and hence in seasonal light availability is probably the most prominent feature in Polar Seas, and life is strongly affected by this distinct seasonality (e.g., Clarke, 1983). The polar zooplankton is well adapted to this changing environment in particular to the seasonal phytoplankton production, but the species have developed varying capabilities for surviving periods of food scarcity in the pelagial. It has been shown that the calanoid copepod species Calanoides acutus has a life cycle that includes an ontogenetic migration coupled with a diapause at greater depth during winter. However, many Antarctic zooplankton species apparently remain active throughout the year and adjust their feeding behaviour (e.g., Atkinson, 1998; Schnack-Schiel, 2001).

Seasonal and regional studies on zooplankton living in the water beneath sea ice were carried out in various coastal and oceanic parts of the Antarctic Ocean (e.g., Fukuchi and Tanimura, 1981; Foster, 1987; Hopkins and Torres, 1988; Tucker and Burton, 1990; Atkinson and Shreeve, 1995; Schnack-Schiel and Hagen, 1995; Burghart et al., 1999). The western Weddell Sea is one of the few regions of the Antarctic Ocean, which are covered by perennial ice, and to our knowledge zooplankton was studied in that area only during the American-Russian Ice Station Weddell 1 in autumn 1992 (Voronina and Kolosova, 1999; Voronina et al., 2001).

The “Ice Station POLarstern” (ISPOL) expedition provided the opportunity to continue the investigations on zooplankton in the western Weddell Sea but in a different season, in spring 2004. The aims of the present study were the analyses of the zooplankton communities under perennial sea ice cover in the western Weddell Sea during the transition from spring to early summer with emphasis on major differences in abundance, vertical distribution and stage composition of the three dominating calanoid copepods Microcalanus pygmaeus, C. acutus and Metridia gerlachei.