AMT Blog

Bio-Argo floats: Robots to monitor ocean biology and biogeochemistry

Hi there, my name is Giorgio Dall’Olmo and I am a scientist at the Plymouth Marine Laboratory.  I want to tell you about a small revolution that is taking place in the field of biological and chemical oceanography.

Figure 1: Bio-Argo float being deployed in the blue waters of the sub-tropical gyre. The white bottom contains the inflatable bladder used to regulate the buoyancy of the float (photo by Virginie van Dongen-Vogels).

By now you should have understood that scientists go on research cruises to gather information about the ocean, its organisms, its chemistry, and its physics.  Satellites orbiting around the Earth provide us with a second important means to observe the ocean.

Both these observing systems, however, have important limitations.  Research cruises can help determining the horizontal, temporal, and vertical distribution of various ocean properties, but are extremely expensive and thus limited in space and time.  In contrast, satellite sensors scan the ocean at the global scale with a relatively high temporal and spatial resolution, but can only detect a limited set of parameters near the surface and under cloud-free conditions (some sensors can “see” through clouds).

Most biological, chemical, and physical processes develop in a three-dimensional ocean that continuously changes over time.  So, what are we missing with the current observation methods?

The truth is that we do not know for sure.

Our understanding of ocean processes can be further improved by a third observational method: Argo floats (Figure 1).  These are autonomous robotic platforms that carry scientific instruments and that can regulate their buoyancy.  Once deployed, they spend most of the following 3-5 years at a “parking” depth of 1000 m in a dormant state.  Every ten days or so, they turn on, sink to 2000 m, turn on their scientific instruments and collect data as they rise to the surface.  From there, they transmit the data to land via a satellite link and then sink back to the parking depth where the cycle restarts.  About 3500 Argo floats are currently recording temperature and salinity profiles that are fundamental for understanding ocean physics (see Argo web site).  But what about biology and biogeochemistry?

Well, that is where the revolution is happening!

Argo floats have recently been equipped with sensors that can measure biological and chemical properties. We call these “Bio-Argo floats” and the data they are collecting are opening exciting new opportunities to further our understanding of ocean biology and biogeochemistry.

Figure 2: Giorgio waiting for a Bio-Argo float to be ready for deployment. (1) antenna; (2) conductivity, temperature and pressure sensors; (3) dissolved oxygen sensor; (4) chlorophyll and coloured dissolved organic matter fluorometers, and optical backscattering sensor; (5) transmissometer; (6) ballast to support the biological and biochemical sensors; (7) nitrate sensor (photo by Virginie van Dongen- Vogels).

During AMT22, besides six “traditional” Argo floats, we also deployed eight Bio-Argo floats: four in the northern and four in the southern sub-tropical gyres.  They will contribute data to a larger project lead by Dr. Herve’ Claustre at the Laboratoire d’Oceanographie de Villefrance-sur-mer (France, http://www.OAO.obs-vlfr.fr/).

Besides temperature, salinity and depth, the instruments on these floats also measure chlorophyll fluorescence (a proxy of phytoplankton pigment concentration), optical scattering (a proxy of particle concentration), dissolved oxygen (produced by photosynthesis and consumed by living organisms), nutrients (needed by all ocean organisms), as well as light (the main ocean fuel, Figure 2).

Importantly, these floats will monitor the gyres after the passage of the cruise.  We will thus be able to better understand how the ship-based “snapshot view” of these ecosystem will evolve with time.

The science on board may have stopped (for the most part) but we are still a busy bunch and blogging is part of that, this post is from Sara Creegen from NOC.

We are rapidly approaching the end of the 22nd Atlantic Meridional Transect Cruise and as science operations have officially ended, we are all starting to think about the multitude of data and samples that we have been collecting in the last month at sea. My samples consist of vials filled with preserved zooplankton samples that I will analyse back in the lab. I am a second year PhD student at the National Oceanography Centre in Southampton and my project is looking at the bacterial communities (microflora) living inside copepods, a group of planktonic crustaceans. One of the first questions that come to mind is probably: “Why is this important?”. Well, the environment I am interested in is the open ocean or pelagic environment and this is generally quite nutrient poor (compared to nutrient rich costal areas or shelf seas). Comparatively, the inside of an animal’s intestine is very nutrient rich and also the supply is more or less constant. Many animals (terrestrial and aquatic) have been shown to have a microflora in their gut and that the relationship is beneficial. Bacteria are a very important and abundant part of the open ocean environment, both as nutrient cyclers and also food for certain organisms. Two broad groups found in the ocean are free-living and particle-attached bacteria and both are well studied. However, not many studies have looked at the bacteria living inside zooplankton and for these reasons it is interesting to know what the relationship between copepods and the potential bacterial microflora inside their gut is. The reason we chose copepods, as our target group is that they are very abundant among zooplankton and they have a wide global distribution. This enables us to study them across a large area and several different pelagic environments, something that the AMT Cruise provides. We hope that results form this study will add another piece into the jig-saw puzzle of the global carbon cycle, which affects all environments and the organisms in them. The samples I have collected will be analysed back in Southampton using molecular approaches that enable the visualisation and quantification of bacteria. I have been collecting copepods all along the transect using a size-fractioning zooplankton net (see photo). By separating different size fractions I could target specific size groups of animals, e.g. adults and larvae. So, although the past month has been very busy, most of the work to answer my research questions will be done back on dry land.

Guest post galore on the blog recently. This time my fellow UEA PhD student Natalie Wager explains her science. Over to Nat:

My name is Natalie Wager and I’m a second year PhD student, studying at the University of East Anglia, Norwich.  My PhD project looking at the air-sea exchange of the long lived climatically active gases carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) and involves measuring both atmospheric and sea surface dry mixing ratios and calculating the air-sea exchange for each gas.  I am also measuring carbon monoxide (CO), which although is a short lived greenhouse gas, it is important as it indirectly influences the atmospheric residence times of other climatically active gases, such as CH₄.

Measurements of the dry mixing ratios for each gas are made using two Los Gatos ICOS analysers, one measuring CO₂ and CH₄ and the other N₂O and CO. These analysers take readings approximately every second during the AMT transect, providing us with very high-resolution data in comparison to other methods.  Sea surface measurements are made from an uncontaminated seawater source (from approximately 5m down in the water column).  The water runs through an equilibrator (see image) which has a very high surface area, created by hundreds of small glass beads within its interior.  The high surface area allows the small amount of air at the top of the equilibrator to equilibrate to the gas concentration in the seawater, which runs continuously through the equilibrator. Measurements from the sea surface water are taken continuously except when either when atmospheric readings are made or when the daily calibration is taking place.  Atmospheric measurements are made directly from the bow of the ship from an installed airline placed as far as possible from the ship’s stack to avoid contamination.  These are taken for approximately five minutes every four hours. Atmospheric concentrations in the ocean environment are much more consistent then those made in the sea surface waters and are therefore not required as often.

Another guest post today, this time by Sina Hackenberg. Sina is an atmospheric chemist from the Universty of York. Sina spends a huge amount of time tending to her GC-MS.

Many instruments used in analytical chemistry are fragile, expensive and like being kept in clean, dry, stable conditions – and even then, they can often be temperamental. Luckily, labs tend to have spares of most instrument parts in someone’s drawer, are built on solid ground and an engineer is usually only a phone call (and maybe a few days’ wait) away. So where does an atmospheric chemist decide to take their lovely instrument? On a ship, of course. And put it in a lab next to the CTD hangar, with all occupants of the lab sampling water in one way or another and increasing the potential for flooding (me included!).

From the lab, an air sampling line goes to as close to the bow of the ship as we could get and draws in humid marine air. We have also added a Purge-and-Trap setup in order to analyse dissolved gases in the water. This involves bubbling a clean gas such as zero nitrogen through a water sample and, as you can imagine, results in more humid gas entering the instrument. It probably shouldn’t come as a big surprise that I have had to spend the majority of my time on this cruise looking after it (with help from back home via email), talking to it (not always in the nicest tones…) and finally coaxing it back to a vaguely life-like state when it decided it had had enough (it didn’t make a full recovery). It could have gone better, but we have now almost made it to the end of the cruise without any completely unfixable breakdowns in an essentially hostile environment, so I can’t really complain.

Like Ming, I am looking at sea-air gas exchange of trace level volatile organic compounds in the remote marine atmosphere, but we are interested in different compounds. “My” compounds are small hydrocarbons produced by phytoplankton and emitted into the atmosphere where they react. It is thought that they can end up forming aerosols and hence contributing to cloud formation and changes in cloud properties, which in turn influences the clouds’ climate cooling effects. The main problem currently is that there are very few measurements so far that could help with calculations of how important the contribution actually is. The technique I am using is coupled Thermal Desorption-Gas Chromatography-Mass Spectrometry, which is capable of separating out and detecting very low levels of compounds in gas phase samples.

So if you are still asking yourself why anyone would do this – it’s trying something new and going into the field to measure and find out more about something that no one really knows about, which is really exciting and actually fun, even if my instrument doesn’t always agree!

As if storms weren’t cool enough anyway, here is Ming explaining how they can affect atmospheric chemistry.

“Too thick to breath; too thin to swim in”, such is a description of the amorphous, third layer that forms between the atmosphere and the surface ocean under extremely windy conditions. (Sadly the person whom the quote is credited to has escaped my mind.) It fitting describes the deviation away from the simplified ‘two-layer’ picture of the air-sea interface. One may even ask, does the air-sea interface still exist then?

We have just endured a sizable storm in the South Atlantic, east of Brazil. Fifty knot winds and significant wave height of several meters. Conditions not yet extreme enough to form the aforementioned third layer, but nevertheless stirred up a boiling pot. Not just whitecaps, but streaks of bubbles plumes and spray blown across the sea surface. Waves stepping up and breaking. The sound of winds whizzing by, the creaking noise of the ship, the thunderous explosion of water as the ship’s bow slams down, after a moment of near zero-gravity. (many of us are familiar with the image of breaking waves, but only from the beach, where the shallowness of water forces waves to tumble and break; in comparison, in the deep waters of the open ocean, it takes a lot of energy for large waves to break). The albatrosses and shearwaters seem enthralled by the stormy sea though, gliding and weaving around the peaks and troughs. I am as well.

If you have caught my previous entry to the blog earlier during the cruise, you might have remembered that I hoped for higher winds and rougher seas. This is my wish becoming true. Looking at the seas yesterday, it seemed a different world than only two days before, when the water surface resembles a slowly undulating piece of silk. One of my primary research interests entails physical processes governing air-sea gas exchange, which you can imagine differ dramatically between a flat surface and rough seas.

Understandably making measurements of the rates of air-sea gas exchange is extremely difficult and risky under these stormy conditions, as evident by my motion sensor blacking out a couple of times yesterday because the acceleration exceeded the maximum range of 3 g. Actual observations of gas exchange are very rare, and with large uncertainties under these conditions. Yet gas exchange in strong winds is likely very important for climate and elemental cycling (e.g. the oceanic uptake of CO2) because of the expected high rates. Thus encountering such a storm is the fantasy of many researchers in this area. So I say, let it blow!

A squid:

Beautiful, isn’t it? I am very lucky that I get to see these wonderful things first thing in the morning. It is a contender for this weeks beast of the week (post tomorrow).

The storm in the title refers to the ‘bad’ weather we have had the past 24 hours. High winds and high waves (up to 7 metres this morning) have meant that there hasn’t been much science on board today, there was no predawn CTD and no noon CTD either, the sea was just too rough. This is unfortunate, however, there have been upsides, mainly that this type of weather is exciting. Or at least I think so. It was exhilarating to stand on deck and watch the ocean and feel the movement of the ship. Plus, the weather has brought with it a lot of bird life, there were three albatross today (photos tomorrow when I get them off the camera that took them). It was a joy to watch them fly over the waves.

So, despite there being no CTDs, today was a good day for recharging the batteries and getting excited about being at sea and doing science out here. Several people, me included, remarked at just how lucky we are that we are able to do this.

Station cancelled. A bit windy… Force 8!

12th of November, 9 pm and most of us a are going to bed for some rest before another 3.30 am start. The sun rises earlier now, so we have to have our station earlier in order to get our experiments set up in time for sunrise.

Earlier this morning, as we sat in glorious sunshine, someone remarked that this time last year we missed a station due to the weather. As if on cue, the pressure started dropping, the wind picked up and by 5pm, the sky had turned steel-grey, the decks were awash and off limits, hatches closed. Welcome to the South Atlantic! Now it’s blowing 30+ knots, a force 7 and only going one way.

On the edge of a storm

An earlier satellite image shows we’re just on the edge of the storm and heading straight into it (see image; we’re roughly where the red dot is going in the direction of the arrow towards the swirling clouds). Everyone’s double-checked their equipment, making sure that nothing can move. The forecast is marginal for tomorrow’s station, but we’ll still be there, ready to go. In many ways a cancelled station is quite disruptive to our routines, so I’m hoping it goes ahead. At the moment we have the wind on the starboard-beam (that’s the right side as you face the bow), so we’re rolling  along. She’s a fine ship though and seems to handle it well. My bunk lies in a bow to stern direction, so expect I’ll be moving a bit tonight. Luckily I have a bulk-head (wall) on one side and a parapet on the other, so I won’t be falling out of bed! Not a problem. It’s all good. Goodnight.

Time for another guest post, this time from Dr Gavin Tilstone from PML, giving a nice summary of the Atlantic gyres.

The Atlantic Gyres

Close to the equator we saw flying fish bouncing like skimmers over the surface of the ocean, periodically diving below the surface to feed on the micro-life that live just below. Either side of the equator there is a vast deep blue slab of ocean, which is barren and seemingly devoid of life. These open ocean regions are so deep (>4.5km) that nutrient rich waters that lie at depth, seldom reach the surface and the sunlit, upper ocean is fuelled by re-mineralized nutrients from grazing and breakdown of phytoplankton cells by zooplankton, bacteria and marine viruses.

After having spent the first 2 weeks in the North Atlantic Gyre we are now 1 week in to the South Atlantic Gyre. So what is a Gyre? It is a swirling vortex, which in the ocean is created by wind or currents. There are two main gyres in the Atlantic Ocean, which are created by currents; the northern Gyre which circulates clockwise and is created by the North Equatorial and North Atlantic currents and the southern Gyre which swirls anti-clockwise, created by the South Equatorial and Antarctic Circumpolar currents. These areas are the ocean deserts, which occupy 70% of the world’s oceans, are inhabited by the smallest of the marine phytoplankton, the cyanobacteria which live deep in the water column in the twilight zone where light and nutrients are just high enough to sustain their existence. So why are we here and why is it important to study these deserts? The Gyre ecosystems are a delicate balance between phytoplankton growth, zooplankton grazing and the release and re-mineralisation of nutrients. Any adverse affects on this tightly coupled food web, can have major consequences higher up the food chain on the fish and whales that graze either the phyto- or zoo-plankton.

The Cyanobacteria that inhabit the Gyres are evolutionary relics of the earliest plants. It is thought that the chloroplast of higher plants formed from an endosymbiotic relationship with cyanobacteria. The ability of this group of organisms to perform oxygenic photosynthesis is thought to have converted our atmosphere into a place habitable by respiring organisms (including man). Cyanobacteria live in the harshest environments and can be found in habitats as diverse as polar ice to desert rocks; from freshwater mud to the twilight zone of the deep ocean. They are arguably the most important contributors to global carbon and nitrogen budgets and account for 30% of the global carbon fixation. The Atlantic Meridional Transect therefore offers an unbroken long term time series of measurements to monitor changes in these delicately balanced Gyres and the cyanobacteria communities that dwell within them. In the past we found that the productivity of the Northern Gyre had decreased as a consequence of warmer, more stratified conditions and a decrease in the photosynthetic activity of the cyanobacteria. This year we have observed that the Northern Gyre is more productive than we have previously recorded due to increases in cyanobacteria productivity, which signifies a re-balancing of this ecosystem, which is good news for the higher forms of life that survive in these blue deserts.

Photo: CTD coming out of blue water by Rob Thomas, BODC, UK.