What do we measure and how?
Zoe using the Filtration Rig
Ross using the fluorometer
There are thousands of species of marine phytoplankton. They contribute to around half the oxygen generation on the planet (the rest comes from land-based plants and trees) and, by fixing carbon dioxide into their tissues, then sinking to the deep ocean, they contribute to the reduction of anthropogenic carbon dioxide in our atmosphere. For these and many other reasons, we’re interested in understanding as much as we can about what controls phytoplankton dynamics. Submesoscale processes are often illustrated in satellite ocean colour imagery, where phytoplankton serve as passive tracers – one of the questions on the SMILES cruise is whether these physical processes have a significant impact on phytoplankton populations, for instance by supplying nutrients to the surface waters.
We’re measuring chlorophyll-a as an indicator of phytoplankton biomass. Water samples are being collected from the ship’s ‘uncontaminated seawater supply’, which carries water through clean piping from about 6 metres below the water surface via an inlet in the bow. This water passes through two types of fluorometer before it gets to us – these instruments fire flashlets of intense blue light, and measure the fluorescence (a fraction of the light that the phytoplankton were not able to use for photochemistry) – this serves as one indicator for chlorophyll-a concentrations, but the signal always depends on what kind of phytoplankton are present, and what shape they’re in.
Similar instruments are also being towed on the MVP and SeaSoar. To calibrate these signals, we filter our water samples through glass fibre filters – for each sample, we’re filtering about a litre of water. The filter is then put into a small vial with some acetone and silicon-zirconia beads, and pulverised in a ‘Beadbeater’ (this particular Beadbeater, from Biospec Instruments, was purchased from a lab in the USA, via Ebay!). Finally, we filter the mushed up residue through very fine membrane filters and measure the fluorescence signal using a calibrated fluorometer (on loan from project partners Plymouth Marine Laboratory – with thanks). By extracting the chlorophyll pigment from the cells, we remove the bias that affects the underway fluorometers.
To augment our chlorophyll-a datasets, we’re also estimating the absorption and fluorescence spectra of the particulates in the water (that’s mostly phytoplankton – we try to pick off the cheeky zooplankton that are munching away at our signal). We preserve small quantities of water and also filtered water samples for analysis of the phytoplankton population and the nutrient levels back home.