Filtering day and night
Alongside the gliders, CTDs, floats, drifters, and krill nets, Jill Peloquin and I have been tirelessly extracting filtered samples of the microscopic biology present throughout the top 200-meter water layer of the Southern Ocean. It is fascinating to see a piece of white filter paper turn a greenish-yellow after passing through it just half-a-liter of pure, clear seawater. That greenish-yellow tint reveals the oft-hidden presence of the oceans’ true mega producers: phytoplankton! Phytoplankton are microscopic plants that float freely in the water column; you can think of these organisms like the wheat fields of the sea. With each day split into two halves, Jill and I alternate shifts of filtering, compiling a collection of petri dishes, cuvettes and microscope slides that will be later analyzed in a more fully equipped lab ashore at the Virginia Institute of Marine Science in the US.
Danny hard at work measuring chlorophyll after sampling the eddy last night
So, why would a glider-based research cruise need a bunch of biologists on board? Well, two of the gliders that have been deployed have fluorometers on them. These special instruments can estimate the amount of phytoplankton in the water column and will gather these data with great sampling density. In order to know whether or not these data are accurate, direct measurements also have to be taken where the gliders are deployed and along their sampling transects. This is where Danny and I come in! One of the core measurements that we are making is chlorophyll a, and will inform the glider folks if their measurements of phytoplankton biomass are reliable. We also make a suite of extra measurements that will tell us more about the phytoplankton composition, but chlorophyll is our main gig here.
The anatomy of a filtration rig.
Today, we just finished the last sampling station (I think!) that we’ll do on this cruise. It’s unbelievable how quickly this cruise has gone. The ship steamed and sampled through an eddy that is more or less constant in this area to better investigate its unique physical and biological properties. After completing this last station, we (are you ready for it?), filtered approximately 700 liters of seawater. For those of you in the US, this is approximately equal to 184 gallons of milk, or (for my nieces) this is about 5 bathtubs full. Phew, is our pump tired!
Our filters, made up of a matrix of extremely small glass fibers, look like the one on the left after we pull them off the rig:
That golden color is the phytoplankton collected onto the filter, they are likely diatoms in these waters – phytoplankton that have a special silica shell around them. They really are among the most beautiful phytoplankton in the world and make for an excellent food source for krill.
For us, clean up, packing, and cruise reports show up next on our to-do lists as the rest of the group does some final sampling before our steam North on Tuesday.
Back to the journals.