ANT XVIII/5b: Krill biology
Southern Ocean GLOBEC

Weekly report 2, 24.04.2001

At the onset of Antarctic winter, biomass of phytoplankton (the minute drifting algae in the ocean) and zooplankton (their consumers) have reached annual maxima. During autumn the light regime shifts from near total daylight to near darkness, a situation which organisms will face for the following 6 months. Species have adapted to this pattern during evolution, either by hibernation or finding other modes of over-wintering in the dark, deep-water layers. Most of the phytoplankton algae disappear from the upper ocean layers by sinking out, or being eaten by zooplankton, both processes being observed during our expedition.

Sinking of phytoplankton in the Ocean can be measured directly by deploying sediment traps. These funnel shaped collectors concentrate and eventually preserve sinking material in sampling cups which rotate under the funnel. A microcomputer assures that this rotation is precisely timed, in our case every 48 hrs. On Wednesday 18 April, we deployed an array of two such sediment traps in the ocean at 66° 37.30 S 71° 45.04 W, west of Adelaide Island in 840 m water depth. The physical oceanographers have attached current meters to the mooring line, which is kept straight by 3 train wheels on the ground and 21 glass floats in the water, to measure the current field along the shelf slope. This is a programme co-ordinated with our American colleagues, who have deployed three additional moorings on a line towards the coast to cover the flow field on the continental shelf in this part of Antarctica.

Sediment traps also collect zooplankton faecal pellets that are excreted after feeding on phytoplankton or on the microscopic animals. We have encountered phytoplankton blooms in concentration above 2.5 µg /l. These high concentrations are rarely found in Antarctic waters in autumn. Dominating species of these blooms are diatoms, monocellular phytoplankton protected by silica shells. The species names are as bizarre as their shape: Corethron, Chaetoceros, Thalassiosira, and Coscinodiscus.

Large-scale distributions of phytoplankton in the Southern Ocean are provided by SEAWIFS satellite images. Optical sensors on board such satellites are designed to "see" discrete wavelength emissions from the ocean which are characteristic for the various algal pigments. To quantify the concentrations of phytoplankton from these images, or even calculate the phytoplankton primary production, additional information about the absorption of light between the algae and the satellite has to be known. Clouds, dust and atmosphere absorb light of different wavelengths, as does suspended matter and the various plankton particles in the ocean, amongst other things. The more of these optical properties are measured to provide ground truthing to the satellite data during our expedition the better we are able to do our calculations. Thus, we deploy various light sensors to detect water column properties and do photo-physiological experiments with algae on board ship in the laboratory containers.

Life cycles of zooplankton species are closely adapted to Antarctic conditions. Some groups, such as salps and certain copepods, leave surface layers to over-winter at great oceanic depths, and so we catch them rarely. Over the slope and the rather broad shelf west of the Antarctic Peninsula krill larvae were found in very high numbers but juveniles and adults are rare so far. With the krill "babies" comes their predators: myctophids (10 cm small midwater fish) and medusas of 25 cm diameter. Most of the zooplankton species are examined physiologically and bio-chemically. How much and which lipids had they accumulated over summer? Which kind of food do they prefer? How much do they excrete and respire? Is there egg production and if so how high is it? How much plankton does zooplankton need as food and how much do they egest? Most answers to these questions will be given only after final examination in the home laboratories.

POLARSTERN is equipped with modern acoustic instrumentation to detect zooplankton and krill down to several hundred meters while cruising. Sound pulses are sent directly to the ocean floor. Particles including plankton and krill echo back to the ship. Each group of zooplankton have characteristic vertical migration patterns which depends on time of day and season, as well as on biological features like mating behaviour, escape reactions and feeding patterns. If one knows the species specific acoustic characteristics of each zooplankton species and krill developmental stage, one can calculate total biomass of each species in the ocean.

Our international programmes of whale and bird research partially depend on the information about these migration patterns of the food for top predators. Also of importance is how the onset of winter changes top predators feeding conditions as the food might migrate to other areas (deep ocean, sea-ice etc.). Unfortunately German environmental regulations do not allow German researchers to fully conduct the research as planned. One day before departure from Punta Arenas, we received research allowance but with very hard limitations. We practically only can conduct acoustic research during the short daylight hours under calm weather conditions, a combination of factors hardly fulfilled during our cruise so far.

Sea-ice west of the Antarctic Peninsula is forming rather late this year. Only south of Alexander Island remote-sensing information reveals fields of newly formed ice and dense sea ice fields. The group of ice physicists and ice biologist eagerly await their first big sea-ice sampling station. Hopefully we can serve these needs very soon.

On Thursday a deep depression with winds of Beautfort 7 and wave heights above 5 m hit POLARSTERN. Not only that we had to collect the dishes from the floor and to clear our cabins; we also had to stop working with gear outboard. Fortunately no major damage occurred on the highly stressed material. At moment we have snow showers with the temperature at about minus 10° C, but it will get colder soon.

We all are healthy and with good spirit for the work to come and wish all the best to the beloved which had to stay home.

Uli Bathmann (chief scientist)