Quantitative studies Water column

The major gear employed for the quantitative sampling of mesozooplankton was the multiple opening and closing net equipped with five nets of 100 and /or 55 µm each.

Stratified vertical hauls were carried out in order to study differences in the vertical and regional distribution of copepod species in relation to hydrographic regimes. On the shelf and slope stations the hauls covered the entire water column between the surface and maximal 1,000 m, while at oceanic stations, the net was deployed down to 2,000 m. The depth ranges were defined according to the temperature profiles at the respective station. In addition, a ”maxi type“ multiple opening-closing net with an opening of 0.5 m² equipped with nine nets of 100 µm mesh size and with a digital flowmeter was deployed twice. Nine successive depth layers were sampled between near the sea floor and the surface. The net samples were preserved in 4 % buffered formalin and at one station in 100 % ethanol for molecular genetic purposes.

For each of the plankton stations, water from a CTD was filtered for subsequent analyses of chlorophyll a, POC and PON concentrations.

Species composition, abundance, biomass, population structure and vertical distribution, maturity of gonads and gut contents will be analysed from these samples.

In general, we encountered a winter situation at the beginning of the cruise in the Scotia Sea and in the Scotia Weddell Confluence. The zooplankton abundance at greater depths were high and the communities were dominated by the large calanoid copepods Calanoides acutus and by numerous small cyclopoid and calanoid copepod species. In contrast, the abundances were low in the upper water layers. Six weeks later, we visited again our first station in the Scotia Sea on our route back to Cape Town. At this time, a spring bloom had developed and the phytoplankton biomass had increased approximately twelve-fold. The zooplankton had left their winter quarters at greater depth and had returned to the highly productive surface layers.

6.2 Life cycle strategy of pelagic and sympagic zooplankton

The net samples on the Larsen shelf reveiled a very different picture. The water column seemed to be completely mixed and sediment particles were found up to the surface layers. The pteropod Limacina sp. and small cyclopoid copepods clearly dominated the zooplankton communities and the overall abundance of all other species was very low. hole. Discrete water samples for analysis of algal pigments were collected from 0 and 5 m depth below the ice with a polyethylene tube with a valve at one end. The unequipped end of the tube was lowered into the water through a core hole with the valve closed. At the sampling depth, the valve was opened and closed again and the tube with the enclosed water sample was hoisted to the surface. For determination of Chl a concentration, samples were filtered through Whatman GF/F filters, extracted in 90 % acetone, homogenized and analyzed fluorometrically.

Organisms from the under-ice water (0 and 5 m depth below the ice) were quantitatively sampled with an under-ice pumping system equipped with a standardized water meter (accuracy 0.1 l) and inserts of plankton gauze (mesh size 50 µm) to concentrate the organisms. Samples were fixed in borax buffered formalin in seawater (4 %). Enumeration of species and stages from the sub-ice layer will be done in the home laboratory.

Sea ice

At most stations seven ice cores were drilled with a motor-powered KOVACS ice corer (internal diameter: 9 cm) in areas of non-deformed ice. At stations with an ice thickness of more than 2 m, only three ice cores were drilled. Generally, the uppermost parts of one core were cut into 10-cm segments, only the lowermost part was cut into a 5-cm segment. All segments were melted in the dark at 4 °C in a surplus of 0.2-µm-filtered seawater to avoid osmotic stress. Once melted, the samples were concentrated on a 20-µm mesh sized gauze and fixed in borax buffered formalin (4 % final concentration). In order to analyze the composition and abundances of sympagic meiofaunal organisms the specimens will be sorted into taxa under a stereo-microscope in the home laboratory.

From the other six (two) cores only the lowermost 5 cm were taken. Three (one) of these cores were melted in a surplus of 0.2-µm-filtered seawater, concentrated over a 20-µm gauze and than analysed unfixed. The other three (one) bottom sections were placed into a box with 50 ml filtered seawater immediately after coring, and shaken gently for about one minute. Then the water was filtered using gauze of

6. Biology

formaldehyde). At the home laboratory, the animals will be cut, and the gut content will be analysed using raster electron microscopy (REM).

Other groups took cores for the determination of temperature, salinity, texture, nutrients, DMS/DMSP content and abundance of CaCO3-Crystals at the same location (within 2 m²).

Live counts of sea ice meiofauna from ice-core bottom sections (0 – 5 cm) revealed higher abundances than it had been expected on the basis of earlier studies of sea ice organisms during winter time. Live counts and non-quantitative analyses also revealed a surprisingly high metazoan diversity. In accordance with earlier studies conducted during austral winter, white turbellarians, harpacticoid copepods and copepod nauplii were again found to dominate the metazoan community. However also organisms of other taxa were found, including the calanoid copepods Stephos longipes and Paralabidocera antarctica as well as some species, which have not been described for sea ice so far. Small individuals of the ctenophore Calianira antarctica, juveniles and adults of the nudibranch Tergipes antarcticus, eggs and larvae of gastropods (assumably of Tergipes antarcticus). Furthermore big, red turbellarians of an undetermined species were found in the ice at several stations and partly with considerably high abundances. At one station, very delicate, worm-shaped organisms of bright red colour were found.

Detailed taxonomic analyses, based on morphological studies and DNA analyses, and further countings will be conducted at the home laboratory. They shall provide further information on the community structure of sympagic meiofauna, on vertical distribution within the ice and on possible regional trends.

6.2 Life cycle strategy of pelagic and sympagic zooplankton

6.2.2 Experimental studies