Fair winds and following seas

Today is the day! My fourth research cruise and my first to the Bermuda Atlantic Time Series. After a quick dip in the harbour to make the most of our last opportunity to be in the water for the next week, I packed myself up and boarded the R/V (Research Vessel) Atlantic explorer. After a few last-minute checks and a little unpacking, I headed out to the middeck to wave goodbye to our colleagues on the shore. At 1200 hours we departed, heading out through ferry reach to the wide sargasso sea. The weather was glorious and we kept our eyes on the horizon for whales, though no joy today we did see a number of Bermudian longtails, a sign that summer is on its way!

One of the big differences between this and previous research cruises I’ve been on is that typically I’m used to having at least a day if not two or three after setting sail to reach a sample station, before the science begins. But today we reached Hydrostation S in the early afternoon and were straight into our first CTD cast. This was a deep one – down to almost 4 kilometres, and we sat in the bridge, watching the traces of fluorescence, oxygen, temperature and salinity as the CTD descended.

I was watching because it’s exciting, yes, but there was also a scientific reason behind my interest. One of the most important decisions I have to make on this cruise is from where, vertically through the water column, I want to collect samples. And this isn’t as simple as it sounds. The ocean is constantly moving, which means levels of light and nutrients are moving with it. And these things control the distribution of phytoplankton through the water column (and vice versa, but let’s not get into that right now).

One term you’ll hear me use a lot is ‘deep chlorophyll maximum’, and it’s a really important concept, so let’s break it down. The deep chlorophyll maximum, or DCM, is a layer of the ocean just below the surface (let’s say somewhere around 100 metres, though it can vary a lot depending on location, seasonality and more), where the concentration of chlorophyll, that green pigment in phytoplankton, is the highest. The open ocean, like where we are now, is what we call ‘stratified’, meaning layered. In its simplest form we can think of it as having a warm, sunlit, surface layer, where phytoplankton can grow (because photosynthesis requires light) but nutrients are low, and a cooler, darker, lower layer, where phytoplankton don’t grow so much, but nutrients are high. And the DCM is something of a ‘goldilocks’ zone between these two; there’s enough light for the phytoplankton to photosynthesise, and they have access to nutrients coming up from those deeper layers, providing the molecules they require to grow.

Now, I’m interested in the phytoplankton in and around this DCM. But the DCM isn’t always at the same depth, even in the same location, because it’s dependent on many physical, chemical and biological factors. So, if I want to compare samples between the same ‘depth’, what I really want sample is the same depth relative to the DCM, not the same number of metres from the ocean’s surface. So I need to watch these parameters we get from the CTD, looking at the light, the fluorescence, temperature and oxygen, to deduce where this DCM is, and therefore, where my sample depths should be, relative to it.

With that all sorted out, the CTD came back from its 4,000m jaunt just as the sun was setting, which meant all hands on deck (literally). The BATS team were sampling for oxygen and temperature to calibrate the sensors in built to the CTD, which they will then validate using chemical assays with the seawater they collect. I collected a pilot carboy full of water to test out my filtering rig – speed, pressure, volume and time are all factors which can change depending on the set up, tubing, and how much phytoplankton is in the water. All went according to plan, and I’m ready to sample my first midnight CTD!