The diatom Chaetoceros dichaeta under the microscope taken from a plankton net in the Southern Ocean. Photo: Bente Edvardsen
One aim of this cruise is to obtain information on the abundance of phytoplankton available as food for higher trophic levels, such as krill and salps, in different regions and by depth, and which species occurring in these areas. We also want to increase the understanding of which environmental factors that determine algal growth and species composition.
Water samples are collected from 1500 to 5 m depth with water bottles attached to a rosette with electronic sensors (CTD). The water is tapped on flasks for later analyses of water chemistry, chlorophyll a (an estimate for algal biomass), and phytoplankton species composition and abundance. Sampling takes place night and day. We also take vertical phytoplankton net haul samples from 100 m depth through the water column up to the surface to collect and concentrate the plankton larger than 10 µm. The plankton is collected in a container and then transferred to flasks added a fixative to preserve the form.
CTD sonde with bottles for water sampling ready for action in the deep. A plankton net is seen on the right side of the picture. Photo: Espen Bagøien
Diatoms dominate in the net haul samples. Several of the diatom genera present in Norwegian waters are also found here (e.g. Chaetoceros, Dactyliosolen, Fragilariopsis, Pseudo-nitzschia, Proboscia and Rhizosolenia), but the species are usually different. Diatoms are surrounded by a protective siliceous cell wall and many cells are often joined into long chains. The genus Chaetoceros, characterised by cells in chains and with long setae, embraces several hundred species of which many are present in Norwegian waters.C haetoceros dichaeta is however restricted to the Southern Ocean and is common in our net haul samples (Figure 2). Another frequent diatom in our samples (net haul, water and krill stomachs) is the chain forming Fragilariopsis kerguelensis (Figure 3). This species has its main distribution in the Southern Ocean.
The chain forming diatom Fragilariopsis kerguelensis under the microscope. Photo: Bente Edvardsen
To determine the abundance of each species the number in a known volume of water sample is counted under the microscope. The tiniest plankton organisms, the so called pico- and nano-plankton (including flagellates), and small forms of diatoms are now dominating in numbers. This is a typical summer situation both here and in temperate waters and may be correlated to low nutrient levels. Knowledge on the diversity of pico- and nanoplankton in the Southern Ocean is limited. On the G.O.Sars expedition we collect samples of these organisms that will be analysed in the electron microscope and by molecular methods in the laboratory at the University of Oslo.
On board are also illuminated temperature controlled rooms where we can grow algae and perform growth experiments. In flasks with growth medium we culture algae that later can be used for detailed studies of their morphology, genetics, biochemistry and physiology.
Written byBente Edvardsen, Univ. of Oslo