Yesterday started with another of our pre-dawn set of measurements. Fundamental biological measurements we need from these pre-dawn CTDs are how fast the microbial plants (the phytoplankton) are absorbing and using carbon and nutrients, and how fast the bacteria are growing by using the organic matter available in the water. Think of these as the two ends of a food chain, with the phytoplankton converting the inorganic elements into organic material, and the bacteria breaking down the organic material back into the inorganic. Between them we have the zooplankton, and other marine animals, eating the organic material provided by the phytoplankton, and in turn providing waste material that the bacteria use.
Measuring uptake of elements by phytoplankton and bacteria requires very careful laboratory work. The method involves using tiny quantities of radioisotopes of the elements we are interested in (carbon, nitrogen, phosphate, silicate) and incubating samples of seawater that have been treated with these isotopes. After a set period of time the sample is filtered to collect the phytoplankton or bacteria, and the activity of the samples counted to tell us how much of the element the organisms used. We have two laboratories dedicated to this work on the ship. Alex Poulton (National Oceanography Centre, Southampton) and Kyle Mayers (University of Southampton) are working in one to measure the phytoplankton rates. Sharon McNeill from the Scottish Association for Marine Science in Oban is dealing with the bacteria rates.
We steamed quickly over to the deep ocean side of the shelf edge yesterday afternoon, and at about 8 pm we started the second of our line of sample stations to measure iron in the seawater. This line started in a deep canyon, and we are working up the wall of the canyon back towards the continental shelf.