Let’s talk about the triangle of filtering. Just to make sure we’re all on the same page, when we talk about ‘filtering’, we are talking about pushing litres of seawater through a very fine mesh. And what’s key here is that, at least in my case, what I’m interested in is what stays on the filter. The water produced just runs back into the sea, but what doesn’t fit through the pores is what we’re here for: tiny, microscopic phytoplankton that live in seawater (about a million in a teaspoon!). Which all sounds like fun and games, but can be a bit more complex than first meets the eye. What’s really important is that you need to balance the volume you can filter, with the time you’re filtering it in, and the pressure you’re filtering it at. Don’t filter enough? You won’t be able to extract sufficient biomass. Filter it too slowly and all your RNA will have degraded by the time you preserve your sample (RNA is incredibly volatile – it’s only designed to last in a cell for long enough to be read like a set of instructions to produce DNA). Filter at too high a pressure and you break open your phytoplankton, losing all that precious RNA along the way. My peristaltic pump filtering set up, pump courtesy of Woods Hole Oceanographic Institution. It’s generally accepted that, in oligotrophic (low phytoplankton biomass) regions like we’re in here, you want somewhere around 10 litres of water to get enough plankton to sequence their RNA. And, as a rule, you want to keep the time from water coming on deck to samples in the freezer under an hour. So really, the only metric you can play with is the pressure you’re filtering at. We (as a scientific community) have also come up with a number for that, which is under 200 mmHg (or 7.8 inHg, for our imperial unit-using friends), as a good point of reference. Therefore, the question is less, ‘ how much  should I filter, how fast  and for how long’ , and more ‘ can I  filter this much, this fast, under this pressure?’. The fourth and final factor that we can control (at least now I’m on board with the equipment I have available), is the type of filtration rig used. Here you have two main options; essentially what this comes down to is whether you’re pulling water or pushing water. The most commonly used filtration rig in oceanography uses vacuum – pulling water through a filter using negative pressure. The other option, which I’ve used prior to now, is a peristaltic pump. This uses a peristaltic ‘head’ to push water through lines of tubing, forcing it out through a filter at the other end. BATS vacuum filtering rig aboard the R/V Atlantic Explorer, Bermuda Institute of Ocean Science. Pros of peristalsis: it can in theory filter more water, quicker, which gets us a tick on both volume and time. Cons are that the rig set up is fiddly – lots of individual tubes running through individual heads of the machine which can easily become twisted and dislodged (and do, regularly). Pros of vacuum: the rig, particularly the one the BATS people are using out here, is more sophisticated, has less moving parts, and, importantly, you can read the pressure you’re filtering at, which gets you a tick on the pressure front. So, we shall see. My first CTD is on deck at 0200 tomorrow morning, so it’s time to do some soul searching and come up with an answer before then!

Today brought with it our first CTD! Through we weren’t looking to sample our full smorgasboard of filtering delights, it was very exciting to see that depth profile for the first time on this cruise, and allowed us to do a number of tests through our respective rigs to get a feel for filtering speed and biomass. ‘Filtering’ is not actually as straightforward as it sounds – there are a number of factors at play and it’s important to find a happy medium to get high quality samples. In my case, I need to sample enough volume to collect a high enough biomass to enable me to do genetics analysis, but I also need to make sure this happens quickly enough that the DNA and RNA doesn’t degrade. With that in mind, it takes time to push my samples through the filters – and this time increases with the amount of plankton in the water. So I need to have the pressure high enough that I can filter enough volume in a short enough time, without causing excessive shear stress that either dismantles a) my filtering rig, or b) the plankton I’m collecting! There aren’t really hard and fast rules about any of these things, it mostly comes with experience, which is why having days like this is so useful, as it enables us to trial run all these things. In the afternoon it was time to put the first set of moorings in. These are a really awesome deployment – essentially a 4.3 km stretch of wire lined with various instruments such as temperature loggers and current meters. We lay them out over about four hours, horizontally on the surface so you can see them stretching behind us for miles. At the same time the ship’s swath bathymetry system continually maps the seabed to assess the best place to drop the mooring. Left: sunset over the South Atlantic. Middle: deployment of moorings from the aft deck. Left: CTD deployment just after dawn. Once the whole length is laid out, the bottom (ie the end still on the ship) is attached to an old anchor chain, and on the order this is dropped to the bottom of the ocean. As this happens the whole 4.3 km of mooring essentially moves towards you as it follows the anchor down to the bottom of the ocean, and is now totally vertical in the water. Overall this system measures temperature and currents in the water, and will be recovered in a couple of weeks, along with its data.

Did you know, as we float atop the waters of the Subtropical North Atlantic, we also sit on layers and layers of ocean from all over the globe, and that these can be decades old? The ocean here is around 4 kilometres deep, with different layers having originated in different places and made their way here by deep convection – the process by which mixed surface layers of the ocean travel to it’s interior. In this way, a water mass’ ‘age’ refers to the last time it was in contact with the air, and we can divide the Atlantic into roughly four vertical layers to help us characterise them. The first couple of hundred metres we call the upper thermocline, the region where our phytoplankton are living and interfacing with the atmosphere, and where my samples are coming from. Below us, between 200-500 metres, lies North Atlantic Subtropical Mode Water, which is formed 300 miles north of us every winter, and subducted here through deep water currents. This water, once it reaches us here, hasn’t been in contact with the surface for anywhere between 6 months and 5 years. Below the upper layer, we have an intermediate layer (~500-1000 m) which consists of Antarctic Intermediate Water (AAIW), Subarctic Intermediate Water (SAIW) and Mediterranean Water (MW). Then we have the deep and overflow layer, which predominantly consists of North Atlantic Deep Water (NADW), originating from Labrador Sea Water (LSW), Iceland-Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW). This water can be 10 - 20 years old! Finally, we have Antarctic Bottom Water (AABW), which here in the North Atlantic we call Northeast Atlantic Bottom Water (NEABW) [Liu and Tanhua (2021)]. The calm waters of the subtropical North Atlantic. So, how do we know what these waters are and trace where they came from? Well, first we need to know the properties of the ‘source water type’, defined as ‘the original properties of water masses in their formation areas’, in order to characterise a ‘water mass’, ie ‘a body of water with a common formation history, having its origin in a particular region of the ocean’ [Tomczak (1999)]. The difference between these two things is that a water mass refers to a specific ‘package’ of water, whereas a source water type defines only mathematical properties of seawater. In order to characterise source water types and define water masses, we use 6 properties of water, delineated into two types – conservative and non-conservative. Conservative properties are those which can only be affected by abiotic (non-biological) factors, such as temperature  and salinity . Non-conservative properties are influenced by the ocean’s physics and biogeochemistry; oxygen  concentration and nutrients such as silicate , phosphate  and nitrate . By identifying specifics of these 6 properties, we can essentially ‘fingerprint’ different water masses, tracing them back to their source water even if they’ve travelled hundreds of miles away over many decades. It’s crazy to think, whilst we’re sailing over this one point in the North Atlantic, we’re also sitting on all those layers of water, from such different places, that have travelled so far so be here. Isn’t the ocean amazing?

For me, the most exciting moment of a cruise is the moment the first CTD hits the surface of the water. The second, a few hours later (if you’re lucky!), is the moment the first samples hit the -80 degree Celsius freezer. This means my sampling is well and truly underway, and we’re on our way to answering some of the big questions we have about the ocean and the Earth as a whole. This happened today at 0100 hours. We typically meet in the ship’s main lab about half an hour before the CTD is due to go in, confirm how deep we're going, what everyone needs from it, and sort out any last minute sample collection prep. Then, once we see it go over the side we head up to the bridge, where the winch operator and CTD computers are so we can watch the traces of our important parameters as the CTD descends through the water. Left: the CTD ready to go into the water. Middle: what we observe on the computer as the CTD goes down. Right: sampling from the CTD Niskins. Speaking of important parameters, let’s have a quick CTD 101. We use the term ‘CTD’ to refer to the rosette of Niskin bottles we use to collect seawater from up to thousands of metres deep in the ocean. But, technically, ‘CTD’ refers to the sensors attached to the frame of the rosette, which measure (amongst other things), Conductivity , Temperature and Depth . Conductivity tells us about the salinity of the water (how salty it is), temperature is self-explanatory, and we calculate depth from a pressure sensor. In addition to this core CTD, we also have a PAR sensor for Photosynthetically Available Radiation (ie light level), an oxygen sensor, a fluorescence sensor which detects chlorophyll, ie how much phytoplankton are in the water, and measures of beam attenuation, which tells us how much particulate matter (ie carbon) is in the water. So, we watch these traces as the CTD descends, and then decide, based on these parameters, which depths we want to take water samples at. As we lower the CTD, all its bottles are cocked open, allowing water to flow freely through them. When we reach a depth we want to grab a sample from, we press a button to ‘fire’ a bottle, which snaps closed a quick release wire and encapsulates 12 litres of water at that precise depth, which we can empty at the surface for whatever analysis we please. In my case, I’m sampling for the DNA and RNA in the water. Which brings me to the question almost everyone on this ship has asked me: why on earth am I sampling in the middle of the night?! Well, there are a couple of reasons for this. The main one is that I’m sampling communities from the deep water – up to 200 metres. As we’ve just learned, there are different light levels through the water column. Since I’m looking not just at which genes are present, but also which genes are  switched on , if I were to bring those communities up into the bright daylight, they’d become extremely stressed (the same way you or I would if someone switched all the lights on in the middle of the night!) and I’d get a completely messy signal. The second reason is because plankton exhibit diel cycling – undergoing physiological and ecological changes on a 24-hour cycle, and I want to be sure I’m sampling at the same point in this cycle each time, so I don’t introduce additional variables into my analysis. The CTD came over the water around 2 am, and thus ensued a flurry of sampling, filtering and fixing. The chief scientist and one of the BATS techs very kindly helped me to collect my water from the CTD, and then it was party for one in the aft lab until just after 4 am, whereby all samples were in the liquid nitrogen, in the -80 degree freezer, or safely tucked into the fridge. Left: these are my filters, the end product of my filtering. Each one has had 10 litres of seawater filtered through it, and contains millions of precious phytoplankton. Right: the sunset over the North Atlantic. My midnight sampling schedule means I head down for another few hours of rest until about 10am, at which point I'm up with the rest of the scientists for other general time series sampling, which today included the recovery of a 12-hour production array. This entry has already rambled on enough, so I'll save details of that for another day. In the evening there was another very exciting first, as you might have guessed from the title, I saw my first green flash!

Come along with us on our trip from Rio to Namibia via the mid-Atlantic ridge looking at the role microscopic photosynthesisers play in absorbing carbon from our atmosphere and storing it away in the ocean depths. Hello! If you're new here, I'm Ari, a  PhD student  at the University of Oxford, an avid  science communicator   and video reporter/producer for  New Scientist . I'm about to embark on my second research cruise, for the expedition that will provide me with the bulk of field-data for my DPhil (that's the Oxford term for PhD). Before joining the ship, I spent an incredible week in the Amazon Rainforest - I couldn’t miss the opportunity whilst in Brazil! But I'm now on my way to join the ship and my team in Rio de Janeiro, from where we will sail to Walvis Bay, Nambia, over 47 days at sea. T he cruise, aboard the Royal Research Ship James Cook, is sailing under project  CarTRidge . The overarching project is looking into the role the mid-Atlantic ridge plays in carbon export. I'm part of Team Plankton, looking at the role these microscopic photosynthesisers play in absorbing carbon from our atmosphere and storing it away in the ocean depths. We are a team of four: Professor Alex Poulton (Herriot-Watt University), Dr Ben Fisher (Herriot-Watt University) and Frieda Schlegel (The Marine Biological Association/Southampton University) ... and me! I 'm going to be sharing about my own experience of being a PhD student at sea - the ups and downs, night shifts and hours at the filtering rig. If you're interested in more information about the cruise at large, our chief scientist Professor Jonathan Sharples is keeping a  blog  I'd highly recommend a read.

Today started as any other; a 2300 alarm, an apple from the galley, and a rendezvous in the main lab to prep our CTD cast (if you missed it, here’s a CTD 101). But today was an exciting CTD as I fired my own bottles! By which I mean, I pressed the button at the correct time at the correct depth 24 times, but it was a big moment for me. I again took all the samples I need - another 50 litres (10 litres per depth from 5 different depths) filtered through 0.2 micron mesh for genetics analysis, and flow cytometry samples taken for cell enumeration, fluorescence and size analysis back on dry land. The process in all, from preparing the CTD to the last samples in the freezer takes about 4-5 hours, after which nothing looks more appealing than a ship bunk bed! After my sleep, I woke just in time for lunch (which is, in my defence, at 11:30), which was, as always, delicious. The afternoon’s operation was recovering sediment traps which we’d deployed a few days ago and had left sitting, gathering samples in the water column ever since. A sediment trap is essentially a way we can measure actual sinking of organic matter out of the water column. This is one of the most fundamental metrics we need in order to relate ocean observations back to practical change in the capacity of our ocean to store carbon. It’s all very well knowing which and how much phytoplankton are in the water, because they are the ones who carry out biological transformation of dissolved carbon dioxide to solid forms, but those solid forms can only store carbon in a useful way if they can somehow get out of the upper layer of the ocean, to store in the deep ocean. Left: the traps are left out in the ocean for a few days, attached to the mooring. The officer on watch then locates the mooring by the 'high flyer', the buoy with the flag attached, which has an array of communication devices to ping its location back to us. Right: these are the sediment traps, which are here being recovered after sitting at 150-300 m for a few days. Our current definition for considering carbon ‘stored’, is 100 years below 1,000 metres. Much of the organic carbon that is produced in the surface ocean never makes it out of the euphotic zone due to what we term ‘remineralisation’, the process by which sinking organic matter is turned back into dissolved organic carbon by microbes. Some good ways for carbon to escape this cycle and make it to the deep ocean are a) simple sinking (the slowest option) b) being eaten by zooplankton and excreted in larger, heavier particles, c) forming aggregates which are faster sinking than the sum of their parts, and d) calcifying – becoming part of inorganic carbon ‘shells’ which sink when the organisms that make them die. Together, all these forms of sinking organic detritus are termed ‘marine snow’ – a rather romantic term for what is mostly dead cells and plankton poo! But tracking the sinking of these particles is essential if we want to understand how much of the carbon in the surface is actually stored. Which is where sediment traps come in. Left: once recovered, the top layer of seawater is siphoned off, so the brine solution can be filtered. Right: particulate organic matter on the filter, which will be analysed for its organic matter composition. Sediment traps are open-topped columns filled with a very dense brine solution (much denser than seawater), which are attached in arrays to racks and lowered into the ocean, in situ. We put them at different depths, from 150 to 300 metres, and leave them out there for a few days, attached to a simple mooring. Particles that sink into the brine are therefore trapped there and stored, ready for us to sample when we recover the traps. After a few days we pull the traps back out, siphon off the seawater that has settled on top of the brine, and then pull the brine down through a fine mesh filter. The biomass collected on that filter, then, can be weighed and analysed for organic and inorganic carbon, phosphorus and nitrogen. And finally, this can be scaled up to calculate rates of carbon storage, per unit area, per unit time, for this part of the ocean at least.

Well hello there! You’ve stumbled across a special entry to my DPhil (PhD) diary, which is the first time I write to you from the Bermuda Institute of Ocean Sciences , a marine research station nestled in the subtropical North Atlantic, in the middle of the Sargasso Sea, on a little island called Bermuda. If you’re here deliberately, I hope this blog, and subsequent entries, is everything you dreamed of (hi mum!!). If you’re here by accident, then welcome to a glimpse inside the life of a third year PhD student living the dream of every young marine biologist and oceanographer. Bermuda Institute of Ocean Sciences Why is this the dream, you ask? Well, apart from its gorgeous climate, pink sand beaches, and sunsets and rises to boot, Bermuda and its Institute of Ocean Sciences is absolutely steeped in marine science history, and, in being here, I’m becoming a piece of that history, leaving a little bit of my scientific footprint behind. Bermuda Institute of Ocean Sciences (BIOS) has been here since 1903, when it was founded as the Bermuda Biological Station for Research. It has its own research vessel, the R/V Atlantic Explorer (which I can literally see from my bedroom window!), and is perfectly and uniquely placed in the middle of the subtropical North Atlantic ocean. The really exciting part for me, that I’d heard of and used data from long before I set foot here, is the institute’s sustained ocean observations, Hydrostation ‘S’ and the Bermuda Atlantic Time-series Study , known as BATS. Hydrostation S is the longest-running oceanographic time series in the world , and has been measuring the ocean's properties at that same spot every 2 weeks since 1954! And a little further away from Bermuda, the BATS site has been sampled every month since 1988. This kind of sustained long term ocean observation is absolutely key for our understanding of long-term trends in ocean biogeochemistry. This is more important now than ever, as we track climate change and its effects on our marine ecosystems, and the impact this will have on our planet as a whole. So, what’s a PhD student from Oxford, UK, doing in such a place as this? Well, I’m extraordinarily fortunate to be the recipient of a UK Associates of BIOS scholarship, allowing me to spend three glorious months here, gathering data for my PhD in marine biochemistry and biological oceanography. During my time here I’m working with the Microbial Ecology Laboratory alongside an ongoing project called BIOS-SCOPE , which is a program studying the ocean’s smallest life forms, the roles they play in global cycling of elements such as carbon, and how they influence the ability of the ocean to support life on earth. Whilst I’m here, I’ll hopefully catch what we call the ‘spring bloom’, which is pretty much what it says on the tin – the blooming of the ocean’s tiniest biological organisms, phytoplankton, as the weather begins to warm up in the spring. The spring bloom isn’t like the ‘harmful algal blooms’ you might have heard of, this is a totally natural phenomenon that happens most years as a result of increasing light, temperature and nutrients, which create an environment that’s perfect for phytoplankton to survive and thrive. I’ll be going out on the R/V Atlantic Explorer to the BATS and Hydrostation S sites regularly, so I can track exactly what’s happening within the microbial community through this bloom: who’s blooming when, what that means for nutrient (particularly carbon) cycling, and which genes and genetic pathways are involved in this happening each year. A spring bloom in the Barents Sea (in the Arctic). Image by NASA.

In case you couldn’t guess, if there’s one place I love to be, it’s in the water. And this evening I had the opportunity to witness and experience the underwater world in a totally different way: at night. Now whilst night dives are common, night snorkels are less so. But there’s something to be said about looking down at an absolute smorgasbord of life under the waves, and looking up to see stars, planets, and a blood moon rising (it was a full moon night!). Our evening started with a sunset wander along Bermuda’s railway trail, just a few minutes walk from BIOS. We watched the sun go down over the north Atlantic, eyes peeled for a ‘green flash’ (no joy today). Our first port of call was a very special event happening 56 minutes after sunset, in the shallow waters beneath a nearby bridge: glowworms. These critters have a unique lifestyle, reaching sexual maturity on just a few days of each month, those after the full moon, and it’s during their monthly spawning event they can be seen best; twinkling beneath the water’s surface. March is a little early for them typically, so we tempered our hopes, but were rewarded with the few faintest glimmers beneath the waves. By this point it was completely dark, so we headed onwards to Whale Bone Bay, our snorkel spot for tonight. Wading out into the (not-so-warm) ocean, we left behind us a wake of blinking, bioluminescence. This is caused by tiny single-celled phytoplankton that emit light when they become agitated. They do this through a chemical reaction between a molecule called luciferin and its enzyme luciferase, causing them to bioluminesce bluish-green. We bravely dunked our heads in the water and, dive torches in hand, began to drift out towards the reef. The first life that greeted us was Sea Hares (a type of mollusc, of the order Aplysiida), large aquatic slug-like organisms, in so many beautiful colours and patterns it was hard to keep count. Also scuttling along the sandy bottom were huuge lobsters – much bigger than I would have expected – and the way they move is more like a spider than a crab. They had these log antenna-like appendages that were at least thrice the width of their bodies. As we got closer to the reef we spotted our first sleeping parrotfish! The parrotfish is a beautiful iridescent green fish, about the length of your forearm, and they’re protected in Bermuda. But what’s awesome is that when they sleep they blow a mucus bubble out of their mouths, which fills with water to allow them to breathe, but be protected from parasites and such. Whale Bone Bay, Bermuda One of my favourite spots, though perhaps not the most coveted, was the pufferfish. Unpuffed, they have the dearest little faces and look really as if they are smiling at you. We also saw a couple of octopus, one was huge—bigger than my head!—and pastel pink and green in colour, the other was much smaller and hiding in the rockery. On the outer rocks of the bay we also came across eel and squid, the latter of which put on a real show for us, letting out a little bit of ink when we swam past! And we were constantly surrounded by shoals of needlefish, small thin fish which swim right at the water’s surface, so you don’t see them when you are looking down, but look up and one might (and did!) swim right into your goggles. After an hour and a half in the water we began to shiver so begrudgingly I dragged myself out and walked the few miles back to the research station under the light of the full moon. It was an absolutely magical evening.

With all this snorkelling, boat days and sunset cruises, you must be wondering, when am I actually going to do some work? But fear not, for I have (believe it or not) been working away during the trusty 9-5 to ready everything for our first expedition. This cruise is different from others I’ve been on since I’m going ‘alone’ (ie, not with anybody working on my same project), it’s testing my organisational and experiment planning skills to get everything ready. Doing science at sea is very different from on land – if you don’t have something you need, even if that something is as small as pipette tips, or sample tubes, or gloves, it could put a stop to the whole shebang. There’s no going downstairs to stores, or into the next door lab, or looking in the back of a cupboard. Everything you could possibly need, you have to pack. So, with that in mind, I’ve spent the last week gathering everything I need, from sample collection containers, to storage, pipettes, tips, chemicals, tubing, boxes, tape, gloves, wipes and much more, to make sure I’ll be able to do all the work I want to whilst I’m at sea this time. On this cruise, we’ll be going out to the Hydrostation S and BATS sites (for more information see this blog), the former of which is the longest running oceanographic time series in the world. In total, whilst I’m here in Bermuda I’ll go out on four research cruises to hopefully capture precisely how the phytoplankton community in the surface to the ‘deep chlorophyll maximum’ is behaving over the spring bloom, the blooming of microscopic life in response to increased nutrient supply in the early part of the year. In order to do this, I have 2 main types of samples I’ll be collecting: genetic samples, for DNA and RNA analysis, and flow cytometry, for cell counts of different types of plankton. The latter will give me my fundamental understanding of what’s happening to the numbers of the smallest plankton in the ocean – our cyanobacteria and pico/nanoeukaryotes. These bugs are small in size but so numerous that they have a huge impact on global biogeochemical cycles, and the ability of the ocean to store carbon. So, it’s important to know how their community structure and distribution is changing through the vertical water column over time. My second type of sample is for genetic analysis. This will allow us to ask two very important questions: who is there, and what are they doing? The who is there can tell us about all the different types of plankton, regardless of size (because we sequence the DNA of the whole community), which matters because different plankton play different roles in the ocean, so knowing who’s there impacts what sorts of processes they can carry out. And the what are they doing can tell us which genes and gene pathways different plankton are using in response to different environmental stimuli, like light, nutrient supply and mixing of the water column. We care about this because it helps us to understand not just what the plankton might be doing based on all their genes, but which ones are actually active and carrying out those processes. The rest of this week will be spent in meetings and the lab, preparing to sail on my first cruise this weekend.

This week has been exceptionally sunny and calm for a March in Bermuda and so, determined not to let it go to waste, we decided to head out on BIOS’s dive boat, the Rumline, to do a little pilot sampling to test methods ahead of my first cruise aboard the R/V Atlantic Explorer later this weekend. The boat was heading out anyway with a group of undergraduates here for a ‘spring semester’, so another member of Team Microbe and I tagged along, carboys and cool boxes (for sample collection, of course) in hand. Postdoctoral Researcher Dr Brett Jameson teaching the students some oceanography 101. We first headed out to hourglass reef, so called for its unique shape when viewed from above, so that the students might do some sampling for their Coral Reef Ecology course. They were laying transects (with a tape measure) and imaging the corals along it. This meant a late morning of snorkelling for the rest of us, and we even caught dinner! Lionfish are an invasive fish species in Bermuda, so hunting them is encouraged, and helpful for the local ecosystem. They’re caught using a special three-prong paralyser spear tip, and are, unluckily for them (but luckily for us), delicious on a barbeque. Brett with our supper! After we’d dried off, we headed out to deeper water to do some oceanographic sampling. We targeted 4 sites at different distances out from BIOS, to see a gradient in ocean conditions as we sampled further and further from shore. The students learned how to do a simple surface zooplankton tow using a net, and how to prep, deploy, and sample a Niskin bottle for seawater for nutrients. I really loved watching them be exposed to these sorts of techniques at this stage in their career, it’s something I missed out on having not done a degree in marine biology or oceanography, so it was nice to see it through their eyes. We finished the day with a spot of whale watching and were duly rewarded with a sighting of a pod of migratory humpbacks. They breached and put their flukes up for us. It was an exceptional end to a lovely day on the water.

Saturday morning rolled around, bringing with it a change of scenery as today was the first day I stepped aboard the R/V (Research Vessel) Atlantic Explorer, BIOS’s research ship. I met Rod, chief scientist of this cruise and PI of the Bermuda Atlantic Time Series on board in the morning, and had the pleasure of meeting many of the BATS and AE techs and scientists over a delicious lunch prepared by the ship’s chef, Dexter. Alongside the regular BATS sampling and my own sampling, we have a team on board led by Kristen Buck, OSU, looking at iron in the ocean. Rod gave us an orientation of the ship and then passed us over to the first officer, Jack, for a safety briefing. Having prepared my boxes the week before, once the safety briefing was up I set about unpacking into the lab that will be my home for the next 7 days. My main job for the day was to set up my peristaltic pump and filter rig, which I’ll use to process my samples for genetic analysis of phytoplankton in the ocean. This is the same set-up I’ve used on both my previous cruises, so it’s becoming a pretty slick operation, but I am borrowing a peristaltic pump from Woods Hole Oceanographic Institute, so I had to spend a little time getting to know it. I’ll also be taking samples for flow cytometry, a technique that allows us to identify different groups of phytoplankton based on their size and chlorophyll fluorescence. Chlorophyll is the pigment which makes plants and phytoplankton green, and allows them to photosynthesise – turning carbon dioxide into oxygen for us to breathe. It also fluoresces when you shine a blue or red laser at it, and we can detect that fluorescence to detect the presence of chlorophyll-containing phytoplankton in our samples. We don’t have the machine we need for this on the ship, so we have to ‘fix’ the samples – freezing them in time at the moment we collect them. For this we use a chemical called glutaraldehyde. Glutaraldehyde is really horrible because, for the same reason we use it as a fixative – it binds and immobilises nucleic acids – it will do the exact same thing to a human should it accidentally come into contact (we very much need our nucleic acids to be alive and well!). So, to minimise my contact with it in its stock concentration, I pre-prepared my sample tubes with a little in the bottom of each, so when I get the samples on deck, I can simply add them into the pre-‘spiked’ vials. After mobilisation was done, everything set out and tied down, we headed out to Nonsuch Island for a snorkel and a spot of whale watching. We snorkelled around the reef, seeing parrotfish, butterfly fish and sheepshead fish, and were even treated to another  sighting of a pod of whales. The weather was absolutely beautiful so we headed out to the northeast of the island to catch the sunset, where we bumped into some pals from the research station and rafted the boats together to watch it go down. Then it was home for some much needed rest before the big day tomorrow!

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!