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Entries in plankton (7)


Fertilise our way to a cooler planet? Five questions with Pete Strutton

My fellow Aussie and all-round good mate Pete Strutton is a marine biologist of a very different flavour to me.  Whereas I work on bigger critters like whale sharks and (formerly) coral reefs, lobsters and fish parasites, Pete studies big-time plankton and nutrient cycle stuff in the open ocean.  He also edited a book about marine ecology, which you can get on Amazon (even in Kindle format!).  Pete’s at the University of Tasmania these days but formerly of Oregon State, Stony Brook University’s School of Marine Science and the Monterey Bay Aquarium Research Institute.  I caught up with Pete recently and asked him some questions about his work.

AD: Hey Pete, can you tell us a bit about your favourite areas of research?

PS:  In general my work concerns the intersection of biological and physical oceanography. In other words, I’m basically a biologist who knows enough physics to be dangerous. What this means in practice is that I try to investigate what causes variability in the productivity of the surface ocean. To do that I need to understand physical processes such as mixing and upwelling, that vary a lot in the ocean and deliver nutrients to the surface where they are consumed by phytoplankton for photosynthesis. This is important because the oceans are responsible for about half the photosynthesis that happens globally. Or as I’ve heard many times recently, every second breath we take is thanks to phytoplankton.

So like I said, most of the work I do is trying to understand how physics impacts biology, but recently I’ve become excited about the reciprocal process: Biological influences on physics. One cool example of this that I don’t work on is the contribution of ocean animals to mixing [of ocean water]. John Dabiri at CalTech was recently awarded a MacArthur Fellowship for his work in this area. What I have done some work on is how phytoplankton influence ocean warming. As we all know, phytoplankton absorb solar radiation to carry out photosynthesis. They actually re-radiate a lot of this energy back into the water as heat and fluorescence. So when the concentration of phytoplankton in the surface ocean increases, this means that there’s greater potential for trapping heat near the surface, rather than it penetrating more deeply into the ocean’s interior.

I’ve looked at two scenarios for the variability of phytoplankton (as measured by chlorophyll concentration) and the impact this has on ocean warming. The first is natural variability, and the case I looked at was the 1997-98 El Nino event in the tropical Pacific [Journal of Climate, 2004 17: 1097-1109]. More recently I’ve become very interested in the impact that geo-engineering scale blooms might have on upper ocean warming, particularly because the goal of these blooms is to ‘cool the planet’, but I think you’re going to ask me about that next.
AD: I want to ask you about ocean fertilization.  I gather the basic idea is that plankton production (or algae growth) in the oceans is limited by one or more nutrients that are in short supply, so if you add that nutrient back in, you can encourage huge increases in productivity.  This growth in plankton sucks carbon dioxide out of the atmosphere and it becomes “fixed” (turned into animal tissue) and enters the food chain or, ultimately, sinks to the bottom of the ocean where it remains trapped for extremely long periods.  This idea has been termed “carbon sequestration” and proposed as a way to offset or even reduce atmospheric carbon dioxide and thus ameliorate global warming.  Where did this idea come from and who were the key players in its genesis?

PS:  You’ve described the idea very well. The most famous and relevant example is that of iron limitation. For several decades, biological oceanographers wondered why chlorophyll concentrations (phytoplankton populations) were not higher in vast areas of the ocean, in particular the North Pacific, equatorial Pacific and Southern Ocean. When they did cruises to these areas, there seemed to be plenty of nitrogen, phosphorous and silicon available. These elements are all important building blocks for phytoplankton (by the way, carbon is never a limiting nutrient in the ocean). What was stopping phytoplankton from taking up these nutrients?

An important breakthrough came in the 80s by way of technological and analytical developments. John Martin’s group at Moss Landing Marine Labs in California developed careful techniques to accurately measure trace metal in the ocean. Iron, for example, is present in seawater at extremely low concentrations – it would take about 500 olympic swimming pools of seawater to make one 5 gram nail. Most of the sampling equipment we use in oceanography, including the ships themselves, are made of iron, so contamination was difficult to avoid. To cut a long story short, the Moss Landing group developed techniques to measure iron at parts per trillion concentrations. When they made uncontaminated measurements, it became clear that dissolved iron in the ocean was extremely low, particularly in the places I mentioned above. Phytoplankton were using up all the iron (in enzymes for photosynthesis among other things) before they ran out of nitrogen and the other nutrients. That’s why these other nutrients were sitting around unused.

There was a vigorous debate in the community as to whether low light, or consumption by higher trophic levels could be limiting nutrient uptake, but in the end, for the most part, the iron idea won out. John Martin further suggested that long term (tens of thousands of years) variability in dust inputs to the ocean could be regulating Earth’s climate (glacial cycles). He is (in)famous for saying ‘give me half a tanker of iron and I’ll give you an ice age’. He tells the story of this quote in a newsletter in 1990: ‘I first said this more or less facetiously at a Journal Club lecture at Woods Hole Oceanographic Institution in July 1988. I estimated that, with 300,000 tons of Fe, the Southern Ocean phytoplankton could bloom and remove two billion tons of carbon dioxide. Putting on my best Dr. Strangelove accent, I suggested that with half a ship load of Fe … I could give you an ice age. After which we all had a beer on the lawn outside the Redfield Laboratory’ (see picture [at top] of me having a beer on the lawn outside the Redfield lab 21 years later). I often use this quote in my lectures, although I sometimes get blank stares at the mention of Dr Strangelove.

Pete and colleagues loading iron into dispersal tanks during the SOFeX cruiseIn the 1990s, somewhat cautiously, oceanographers started testing this idea at sea, first in the equatorial Pacific (1993 and 1995), then in the other regions of interest: North Pacific and Southern Ocean.
AD:  Can you tell us a bit about the SOFeX experiment and the SOFeX cruise in 2002?

PS:  So yes, in 2002 I was part of a US cruise to the Southern Ocean to perform a relatively large scale iron fertilization experiment. We left out of New Zealand and headed southeast towards the Ross Sea. We fertilized two patches about 15km x 15km. The two study areas were more than 1000km apart – our goal was to test the response of the phytoplankton in two parts of the ocean with different combinations of dissolved nitrogen and silicon.
AD:  So now, eight years later, where is this “carbon sequestration” idea headed?  Isn’t it just delaying the inevitable? Is it a viable option, or still controversial?

The “fish”: a device used to distribute the iron at a constant 10m depth. It was covered in rust, depsite being made of plastic!In general, in all of the experiments conducted so far, and there have been more than a dozen of them, blooms have been generated but the amount of ‘fixed’ carbon that ends up raining out of the upper ocean, let alone getting stored in sediments, is considerably less than hypothesized by Martin. There are theories as to why this might be, one of them is that the patches we’ve made to date have been too small, leading to dilutionOne of the plankton blooms produced during the SOFeX cruise, as seen from MODIS satellite. The red patch indicates higher productivity at their boundaries. Some are advocating that we perform even larger experiments, say 100km x 100km. My feeling, and I’m sure I’m not alone, is that iron fertilization is not the silver bullet that will save us from CO2-induced climate change. Nonetheless it is still talked about as potentially part of the solution, and it is also being considered by some in the context of carbon trading. That is, ‘I’ll go fertilize a part of the ocean with iron and suck up 10 tonnes of CO2, then sell that credit to you, Mr Coal-fired Power Plant Owner’.

AD:  SOFeX and some of your other cruises are definitely Science Writ Large.  What’s it like to work on a UNOLS vessel and how do you balance the research interests of individual PI’s against the collective goals of the cruise?  Is it fun or just a grind?

 R/V Revelle, one of the two UNOLS vessels involved in the SOFeX experimenPS:  Good question, particularly with regard to SOFeX. That cruise was very challenging. Even though we had two large ships in the end, there was still strong competition for space on the ship and this translated into competition for time to do science. We wanted to do lots of different tasks, like scan the region as quickly as possible to map the evolution of the patch as seen in surface properties, compared with detailed station measurements that required us to stay in one location for up to 6 hours. These types of sampling were often in competition with each other which makes it particularly challenging for the chief scientist (who constantly has people lobbying for time). To further complicate matters, I was running a drifter that was supposed to mark the center of the [fertilised] patch as it was moved around by the currents. We had real-time radio communication with this drifter, but on a couple of occasions, the GPS dropped out. So although we were getting updates from it, we had no idea where it was. We ended up spending way too much valuable sampling time searching for lost drifters.

On a totally non-scientific note, the other challenge of SOFeX was the food. There was a breakdown in communication re the ordering of supplies prior to sailing and we ran out of a bunch of staples: Bread, eggs, milk, cheese. You’d reckon they could increase our beer allowance (1 per day) to compensate, but no. Oh well, we still managed to have the occasional ‘safety meeting’ in an undisclosed location…

 [AD: If you have questions about geo-engineering or other parts of Pete’s research, post them in the comments and I’ll see if we can get some answer for you]


Everything you wanted to know about how whale sharks feed

I’m really excited about a new paper that’s finally out about how whale sharks feed, from the way their filter pads are built to what they eat and how much.  I’m not an author on the paper but I’ve been a witness to a lot of the work and its terrific to see it come to fruition.  So who’s it by and what’s it about?

Phil Motta is the senior author, with 11 co-authors from Georgia Aquarium, Mote marine lab, Project DOMINO and the University of California.  Eleven seems like a lot of co-authors, but it’s a very comprehensive and very broad ranging look at feeding in the worlds largest fish.

First the what.  Many folks are aware that whale sharks are filter feeders, meaning that they swim the worlds oceans sieving tiny food particles from the water.  That much was fairly obvious from their enormous mouths and 20 filter pads that are visible inside.  What wasn’t known was exactly what they eat and how much, especially relative to how much energy they spend, a balancing act we can call the energy budget.  For the first time, Phil and his colleagues were able to measure the size of the whale sharks mouth, how much time they spend with it open and the speed at which they swim, and from that the amount of water that they filter in an average day.  By combining that with measurements of plankton density in the coastal waters of Mexico where whale sharks gather and nutritional analyses of samples taken there, they worked out how much whale sharks eat in that natural setting.  The answer: between 1.5 and 2.7 kg (3-6lbs) an hour, scaling up to between 15 and 30,000 kilocalories a day (up to 125,000 kilojoules).  Not surprisingly, the amount of plankton in the water was higher where whale sharks were eating than where they were not, mostly due to calanoid copepods and sergestid shrimps (one of which, with the cool genus name of Lucifer, is illustrated below).  That could mean whale sharks really like those items, or just that they really like dense patches of food, and those ones just happened to be shrimps and copepods.  Or it could be both.

Some of the coolest stuff in the paper, though, is about HOW whale sharks feed.  They filter, yes, but not like baleen whales and not like other filter-feeding fishes.  A baleen whale takes a huge mouthful of water and then squeezes it out through their baleen combs, which trap the food items, like pasta gets caught in a colander.  Thats a perpendicular or dead-head filter, and the problem with those is that they have to be backflushed from time to time to blow the particles off the screen (left panel below).  In whale sharks, on the other hand (right panel below), water flows mostly parallel to the filter surface, only deviating slightly to dip across the filter surface and siphon out through the gills.  Food particles, which have more momentum, don’t get trapped on the filter but carry on to the back of the mouth, forming an ever more concentrated ball of food.  This is the same principle behind plankton nets and its very efficient because the filter doesn’t clog up with particles the way a baleen plate (or standard kitchen colander) would, and it rarely needs backflushing.

Its an ingenious system, illustrated nicely in the figure above from Elizabeth Brainerd’s 2001 paper in Nature. 

I could go on all day about whale sharks and their feeding, or you can skip the middle man and go get the PDF of Phil’s paper here.  Its well worth a read; there are some great images and a far more interesting and detailed discussion than the precis I have here.  Check it out.  You can learn more about Georgia Aquarium whale shark research from the tag list on the left, or by going here.

Motta, P., Maslanka, M., Hueter, R., Davis, R., de la Parra, R., Mulvany, S., Habegger, M., Strother, J., Mara, K., & Gardiner, J. (2010). Feeding anatomy, filter-feeding rate, and diet of whale sharks Rhincodon typus during surface ram filter feeding off the Yucatan Peninsula, Mexico Zoology DOI: 10.1016/j.zool.2009.12.001

Brainerd, E. (2001). Caught in the Crossflow Nature, 412 (6845), 387-388 DOI: 10.1038/35086666


It seemed like a good idea at the time...

Human-powered plankton tow, in an attempt to catch larval fish under sargassum without catching the sargassum itself. 

Note to self: towing 15 feet of 100 micron mesh through the water is HARD WORK



Take a Levy walk on the wild side

ResearchBlogging.orgI've mentioned before that this summer I’ll be part of some whale shark field work studies in Mexico. Some of it will focus on how these amazing animals find patches of their planktonic food in the ocean. There’s a pretty good likelihood that they have an incredibly sensitive sense of smell and can detect food from miles away. They’re a bit different than toothy sharks though, because they aren’t detecting “blood in the water” as such; rather, they need to be able to distinguish patches of ocean where plankton is denser from places where its less dense. How do they do that, and what chemicals are they smelling exactly? These are among the questions we will be trying to answer.

In reading up for this work, I came across the idea of Levy Walks. This is not a walk in the sense of your evening constitutional down to the Piggly Wiggly for a 6-pack and some Slim Jims. No, it really is just the name for a certain pattern of animal movement (shown at the right), one in which animals make several short “legs” of directed motion, usually in bunches, separated by longer legs with major reorientations. Its not random motion, but neither is it all that predictable, except that the pattern exists at all scales: its fractal. In other words, if we sketched the motion of an animal on paper, and drew it to scale, it would look similar if we zoomed out to the range of kilometers instead of meters and drew the pattern again. It turns out that moving by way of Levy walks increases your chances of running into patches of food, or the trails of scent they leave behind. At that point, more directed motion takes over and the animal zig zags towards the source of that delicious scent (whereupon it becomes not too different from homing in on the Slim Jims at the Piggly Wiggly after all). Sims et al. show that Levy walks are almost ubiquitous among animals that seek mobile prey; they conclude that its a sort of biological rule for finding food that has a patchy distribution.

It’s a fascinating idea; I wonder if you could apply a deliberate Levy walk pattern if you were looking for your sunglasses, trying to find Waldo, or trying to find an empty patch of beach to put your towel on. People might look at you a bit funny, but who’d have the last laugh?

Sims, D., Southall, E., Humphries, N., Hays, G., Bradshaw, C., Pitchford, J., James, A., Ahmed, M., Brierley, A., Hindell, M., Morritt, D., Musyl, M., Righton, D., Shepard, E., Wearmouth, V., Wilson, R., Witt, M., & Metcalfe, J. (2008). Scaling laws of marine predator search behaviour Nature, 451 (7182), 1098-1102 DOI: 10.1038/nature06518


More on the geo-hacking idea

Not so long ago I posted about the idea of capturing all the extra atmospheric CO2 into the worlds oceans by fertilising them and thereby creating enormous plantkon blooms that would convert all the CO2 to plant tissues, which would then sink to the bottom and be buried in the ocean depths.  This new scientist article probes a different angle that I didn't think of, which is Who decides what we will or won't do to change these things?  The author Jim Giles refers not just to ocean fertilising, but engineering the whole planet to combat climate change - what has become popularly known as "geo-hacking" - including sensible concepts like reforestation and cloud seeding, as well as the more absurd notions such as building giant reflectors to bounce the sunlight away.  Its a thought provoking question, so who do you think should decide these issues?  The US? UN? UNESCO?  Perhaps we need a new body with that as its sole charter?


2 of the 10 worst jobs in science

Popular Science has just published its annual "Ten worst jobs in science" issue, and two of them are in marine science!  How is this possible?  Marine science is clearly the best job since, well, ever.  Hmmmm, lets take a closer look.

1. Oceanic Snot Diver.  The name sounds gross enough, but what does it mean?  Well, it turns out that they are talking about collecting "sea snot" true enough, but to call it nasty is a bit of a beat-up, IMHO.  Scientists call this stuff "aggregates", and its an incredibly important part of the nutrient cycle in the ocean.  Really, sea snot is just the secreted mucus and fecal casts of hordes of plankton.  Wait a sec, when you write it like that, it does sound gross!  Its biggest role is in "exporting" nutrients from the surface layers of the ocean, where the sun sponsors all that plankton growth, to the dark depths, where sunlight never penetrates, but life nonetheless thrives.  Not only do some animals down there eat the stuff (ew), but at those crushing depths, some of the snot also dissolves under the immense pressure of all the water above it, much like snow melting before it reaches the ground.  In this way, the snot plays a very important part in taking nutrients produced at the surface, and dissolving them in the water at great depths.  Maybe not the most attractive concept, but pretty important in the grand scheme of things.  Like Tom Cruise says in The Firm: "Its not sexy, but its got teeth". 

2. Whale slasher.  OK, I have to concede that one.  I've seen a few stranded whales being cut up on the beach (this is called a necropsy, not an autopsy, which is reserved for people only), and it pongs.  I'm not talking sweat sock pong, or even doggy-breath-after-eating-goose-poo pong, but serious, invasive, gets-into-your-hair, throw-your-clothes-away stink.  While the cause of death is always interesting, wading through week-old whale giblets that have been baking on the beach?  Not so fresh...


If you can catch lightning in a jar, why not gas in a puddle?

Imagine if you could take all the greenhouse gases and somehow keep them away from the atmosphere, where they would otherwise contribute to global climate change.  Well that's kind of the idea behind SOFEX, a huge experiment done by marine scientists a few years back (my buddy and fellow Aussie Pete Strutton was involved).  The idea stemmed from an observation that the growth of plankton (which absorb carbon dioxide as they grow and multiply) in the oceans is limited by some nutrients, especially iron.  So, if we fertilise the oceans with iron, perhaps we can get the plankton to "bloom", suck up all the carbon and then sink to the bottom, taking the greenhouse gases with them.  The colour picture hereabouts shows a satellite view of an artificial bloom created by adding iron to the ocean.  It was actually a neat idea, except I could never shake off the feeling that the stuff would resurface one day and that it was just delaying the inevitable; it depends to some degree on whether the sunken material gets buried on the bottom or not, I guess.

Well, the idea recently received another blow; a new paper in PNAS reports that the sort of plankton that bloom after iron fertlisation are the same ones (Pseudonitzschia ) that produce domoic acid, a nasty toxin that causes horrible problems as it accumulates higher up the food chain, especially in sea lions and other marine mammals.

Marine mammals are kind of a sacred cow in biology, so my guess is that that will be that for iron fertilisation.  Ironically enough, the whole problem with domoic acid in the oceans, which is a relatively new phenomenon, may have climate change as its root cause anyway - blooms of Pseudonitzschia are supposed to have increased in frequency and intensity because of environmental changes.  You can't win, sometimes.