ABOARD THE RVIB N. B. PALMER, ON THE SOUTHERN OCEAN– Not only do we want to know about what type of phytoplankton grow close to icebergs but we also want to know how well they grow. Primary production, or the rate of inorganic carbon taken up by cells is one of the methods used on this cruise to determine productivity. Diane Chakos takes the water collected by the Niskin bottles in CTD rosette (see previous dispatch) and incubates them for 24 hours under sunlight to estimate daily organic carbon production.

Diane Chakos in the lab preparing samples for a 24-hour incubation under sunlight.

Based on underwater light levels we sample water from surface and at depth corresponding to 50%, 25%, 10%, 5% and 1% of surface light. Within the layer defined by 100% and 1% surface light most of the primary production occurs. Biomass, light intensity and abundance of nutrients, including inorganic carbon, all contribute to production. During austral fall in Antarctic waters we are experiencing only 12-h day light, plenty of nutrients and phytoplankton biomass equivalent to 0.5 milligrams per cubic meter results in about 5-10 milligrams carbon produced per cubic meter per day.

Karie Sines filtering cultures to estimate phytoplankton abundance in productivity experiments by chlorophyll concentration.

ABOARD THE RVIB N. B. PALMER, ON THE SOUTHERN OCEAN– Within 40 nautical miles southeast of C18A iceberg, we found an area known as the Iceberg Alley: a large concentration of icebergs in western Weddell Sea, moving in a north-northeast direction following the clockwise circulation around the Weddell Sea gyre. Hundreds of icebergs, medium and small, bergy bits and growlers can be seen all the way to the horizon. Our question is: Are phytoplankton here similar to what we found close to the large icebergs? Can we see similar iceberg effect?

An iceberg in the Iceberg Alley.

More icebergs in the Iceberg Alley.

A striped iceberg in the Iceberg Alley.

The number and variety of icebergs is incredible. We sample from surface to 500m with a CTD rosette (Conductivity-Temperature-Depth sensors mounted on a stainless steel frame with twenty-four 8-liter bottles). Phytoplankton concentrate on the surface, where there is plenty of light. Our sampling is designed to see plant abundance and composition and to capture any vertical structure in relation to the chemical and physical properties of surface ocean waters.

CTD rosette: Conductivity-Temperature-Depth sensors mounted on a stainless steel frame with twenty-four 8-liter bottles.

If icebergs change the physical and chemical structure, we expect phytoplankton to show parallel changes. With the release of the micronutrient iron from the ice, do phytoplankton change their concentration? Do we find more large cells, as expected from relief of iron limitation? Or is the mixing of the upper 200 meters pronounced and we see less stratification in the Iceberg Alley when compared to non-iceberg impacted waters? Analysis of cell number, microscopic determination of species and nutrient concentration at different depth will give us answers to these questions? Unfortunately we need to wait until we are back in our home institutions before analysis. The ship motion precludes any detailed analysis under the microscope.

The ARIB Nathaniel B. Palmer’s shadow seen on an iceberg during a clear evening at the Iceberg Alley.