Introduction

The Ocean Cleanup, brainchild of Dutch inventor Boyan Slat, was in the news again this past week after announcing that in addition to the fact that their system is unable to collect plastic as intended, it suffered a mechanical failure. “Wilson” is currently being towed to Hawaii, where it will undergo repairs and upgrades, presumably to be towed back out to the garbage patch for a second trial.

I am not a mechanical engineer, so I don’t intend to comment on the details of their mechanical failure. I am, however, a sea-going oceanographer. Which means that I am used to the sorts of situations with scientific research equipment that was so succinctly paraphrased by Dr. Miriam Goldstein:

Designing for physics

But beyond the engineering, there are the questions of what the *physics* are that TOC are relying on for their system to be successful. Some of you may recall that the original design was to moor (i.e. *anchor*) their device in 6000m (20000 feet) of water, and let existing ocean currents sweep garbage into the U-shaped structure. Thankfully, they realized the challenges associated with deep-ocean moorings, and abandoned that idea.

The latest design iteration (misleadingly called “System 001”, as though they haven’t built and tested any other previous to it), is to have a freely-drifting system, avoiding the use of anchors. TOC claim that under the influence of current, wind, and waves, their design will drift *faster* than the plastic — causing it to accumulate in the U, making for easy pickup. They summarize the concept with a little explainer video on their website, with a representative screen shot below:

Based on a quick Twitter rant that I had after thinking about all this for a few minutes (see here), I wanted to explain out the various points that have either a) been missed by TOC design team, or b) deliberately excluded from their rosy assessment of how they expect their system to actually collect garbage. What follows is a “first stab” at a physical oceanographic assessment of the basic idea behind “System001”, and what TOC would need to address to convince the community (i.e. scientists, conservationists, etc) that their system is actually worth the millions of dollars going into development and testing.

The premise

As outlined in the video, the premise of System001 as a garbage collection system is that through the combined action of wind, waves, and currents, the U-shaped boom will travel faster through the water than the floating plastic, thereby collecting and concentrating it for eventual removal. This appears to be based on the idea that while both the boom and the plastic will drift with the current, because the boom protrudes from the water (like a sail), it will actually move faster than the surface water by catching wind.

There are some issues with this premise. Or, at least, there are some real aspects of oceanography that have either been ignored or missed in thinking that such a system will behave in the predictable way described by TOC. I’ll try and outline them here.

Stokes drift

Any of you who may have had an introduction to ocean waves may have heard that during the passage of a wave, the water particles move in little circles (often called wave orbital motion). While not a bad “first-order” description, it turns out that for real ocean waves there is also some drift in the direction of wave propagation. This drift is named after Gabriel Stokes, who first described it mathematically in 1847 (see wikipedia article here).

The amount of drift depends nonlinearly on both the amplitude and the wavelength of the wave. For example, for a 0.5m amplitude wave with a wavelength of 10m and period of 10s (something like typical ocean swell), the drift velocity is about 10 cm/s right at the surface.

Of course, the Stokes’ solution describes the motion of the water parcels being moved by the wave. For those water parcels to then have an effect on anything in the water, one would need to consider the various components of force/impulse/momentum (i.e. our buddy Sir Isaac Newton). Needless to say, it seems obvious that a smallish piece of neutrally buoyant plastic will respond to the Stokes drift much more readily than a 600m long floating cylinder with a large mass (and therefore large inertia).

This alone could be enough to quash the idea of a passive propagating collection system. Mr Slat?

Ekman currents

While we’re talking about long-dead European fluid mechanics pioneers, any study of the effect of winds and currents wouldn’t be complete without a foray into the theories proposed by Swedish oceanographer Vagn Walfrid Ekman in 1905. What Ekman found was that when the wind blew over the surface of the ocean, the resulting current (forced by friction between the air and the water) didn’t actually move in the same direction as the wind. The reason for this is because of the so-called “Coriolis effect”, whereby objects moving on the surface of the Earth experience an “acceleration” orthogonal to their direction of motion that appears to make them follow a curved path (for those who want to go down the rabbit hole, the Coriolis acceleration is essentially a “fix” for the fact that the surface of the Earth is non-inertial reference frame, and therefore doesn’t satisfy the conditions for Newton’s laws to apply without modification).

Anyway — the consequence is that in an ideal ocean, with a steady wind blowing over the surface, the surface currents actually move at an angle of 45 degrees to the wind direction! Whether it’s to the left or right of the wind depends on which hemisphere you are in — I’ll leave it as an exercise to determine which is which. And what’s cooler, is that the surface current then acts like a frictional layer to the water just below it, causing it to move at an angle, and so on, with the effect being that the wind-forced flow actually makes a SPIRAL that gets smaller with depth. This is known as the Ekman spiral.

The actual depth that the spiral penetrates to depends on a mysterious ocean parameter called Az, which describes the vertical mixing of momentum between the layers — kind of like the friction between them. What is clear though, is that a small particle of plastic floating close to the surface and a 3m deep floating structure will likely not experience the same wind-forced current, and therefore won’t move in the same direction. Hmmm … that’s going to make it hard to pick up pieces of plastic.

What is a “Gyre” anyway?

The final point I wanted to make in this article (I have more, which I’ll summarize at the end for a possible future article), is to try and give a sense of what currents in the ocean (including in the “gyre” or in the region often referred to as the “Great Pacific Garbage Patch”) actually look like. The conception that there is a great swirling current 1000’s of km across is true only when the currents are averaged for a very long time. At any given instant, however, the ocean current field is a mess of flows at various space and time scales. An appropriate term for describing typical ocean flow fields is “turbulent”, as in an oft-viewed video made by NASA from satellite ocean current data.

To illustrate this, I took some screenshots of current conditions from the wonderful atmosphere/ocean visualization tool at earth.nullschool.net showing: ocean currents, surface waves, and wind.

These images illustrate the potential problem with TOC idea, by highlighting the fact that the wind, wave, and current fields of the ocean (including even in the “quiet” garbage patch) are highly variable spatially and temporally, and are almost never aligned at the same period in time. What’s more, is that the currents and waves at a given time and location are not always a result of the wind at that location. Eddies in the ocean are generated through all kinds of different processes, and can propagate across ocean basins before finally dissipating.

Similarly, surface waves have been measured to cross oceans (i.e. the famous “Waves across the Pacific” study pioneered by the transformative oceanographer Walter Munk).

Other issues

Following the “rule of three”, I tried to hit what I consider to be the biggest concerns with TOC system design and principle, from my perspective as a physical oceanographer. However, there are other issues that should be addressed, if the system as designed is really believed by the TOC team to be capable of doing what they say. And really, it seems like a crazy waste of time on behalf of everyone involved to have spent this much time on something if they aren’t sure it will even work theoretically … not to mention the money spent thus far. So, part of me *has* to believe that all the dozens of people involved care deeply about making something that might actually work, and they have studied and considered all the effects and potential issues I (and others) have raised.

Anyway, the other issues are:

• What is the actual response of the system to a rapid change in wind/wave direction? Wind can change direction pretty quickly, especially compared to ocean currents. What’s to prevent a bunch of accumulated plastic getting blown out the open end of the U after a 180 degree shift in wind but before the system can re-orient?
• What about wave reflection from the boom structure itself? It is a well-known fact that objects (even floating ones) can reflect and “scatter” waves (scattering is when the reflected waves have a shorter wavelength than the original ones), and it seems like this could create a wave field in the U that might actually causes drift *out* of the system.
• The idea that all wildlife can just “swim under” the skirt (because it’s impermeable) is not supported by anything that I consider to be rigorous fluid mechanics, aside from the fact that much of what actually lives in the open ocean are non-motile or “planktonic” species. There are a lot of communities in the open ocean that float and drift at the surface, and I see no way that if the System collects floating plastic as it is designed that it won’t just sweep up all those species too. The latest EIA brushed off the effect of the System on planktonic organisms by stating that they “are ubiquitous in the world’s oceans and any deaths that occur as a result of the plastic extraction process will not have any population level effects”. But that doesn’t take into account that the stated mission is to deploy 60 such systems, which are estimated to clean the garbage patch of surface material at a rate of 50% reduction every 5 years. It stands to reason that they would also clean the Pacific of its planktonic communities by the same amount.

The post The (ocean) physics of The Ocean Cleanup’s System 001 first appeared on Deep Sea News.

Réunion is practically on the the other side of the Indian basin from where investigators think the missing airplane may have gone down. So how did this chunk of airplane get all the way over there? Short answer: it was pushed by currents, winds and waves. From my physical oceanography perspective, I am going to discuss here what scientists and investigators thought the ocean would do to debris from a possible wreck, what the ocean actually did and what happened to the debris along the way.

What we thought the ocean would do.

Numerical models, also known as electronic oceans inside your computer, are used to predict where currents, winds and waves will push marine debris. In this case, a model run by Charitha Pattiaratchi from the University of Western Australia  was used to estimate the trajectories of crash debris as they were spread out by ocean currents and to figure out where they will end up. And that giant squiggle of red debris trajectories located just east of Madagascar are positioned right on top of Réunion! Of course, this is just a prediction and the timing is a little off since it’s only been 18 months since the crash. This mismatch probably occurred because the model was likely run with historical surface current data and idealized numerical debris, although I couldn’t find any details on the model itself (if anyone knows please send me a link in the comments!). And even though I think Prof. Pattiaratchi oversells his model by saying it “exactly predicted where the debris would go” (if it’s so accurate why hasn’t any debris been found on Australian and Tasmanian beaches?), there are enough realizations to show that debris from the crash would have likely ended up on the tiny isolated bump in the big blue sea called Réunion. So in some ways it’s not surprising that the flaperon washed up there and it’s also likely that more debris will too.

UPDATE: Another model!

And this one shows that the flaperon found on Réunion most likely came from the northern region of the search area. Hydrodynamic experts Maarten van Ormondt and Fedor Baart from Deltares used surface currents from the HYCOM model to track where marine debris might have been carried by currents in the 14 months since the crash. Particles released really far south never made it to Réunion in a year, while those released farther north did! This model more accurately tracks marine debris than the previous model because it incorporates real oceanographic data since March 2014 to estimate realistic surface currents, rather than making a prediction using historical data. That being said, predictive models are still really important! They help dictate where investigators should have searched before the debris were found, as was the case until last week.

What the ocean actually did.

Every news outlet seems to love posting the latest images from earth.nullschool.net to show the currents in the Indian Ocean. Why not? I love that site and the graphics are pretty! But the problem is it only shows a snapshot of the latest 5 days and is not at all indicative of the mean flow that pushed the debris across the Indian Ocean. To do that, we need to look at the average currents since the plane disappeared to get a better grasp on exactly what pushed debris to Réunion.

The most obvious feature in the graphic above are all the arrows pointing westward just south of the equator around 10-15° S. It’s called the South Equatorial Current (we oceanographers are very creative in our naming schemes). Debris from the aircraft got caught up in this flowing water and were likely pushed across the Indian Ocean smack dab onto Réunion.

But it’s a little more unclear in this image how the debris got north from the search area into the South Equatorial Current. The culprit? The West Australian Current that flows northward along Western Australia. You can’t see it too clearly here, because there is a lot of small scale eddies that mess with the averages. But if you look at a even longer term averages, it’s there. The debris probably just took a very squiggley northward path until it reached the South Equatorial Current.

I should also note that the debris was found about 4400 km away from where the plane might have gone down and it’s been about 505 days since the plane disappeared. Making a rough calculation with my TI-85, that means the drift speed of the debris needs to be about 0.1 m/s or ~5 miles a day to get to Réunion from the search area. That’s pretty close to the current speeds in the plot above so it’s totally plausible that this debris is from the crash.

What happened to the debris as it drifted.

Anything that has been in the ocean for more than a year will have some sort of sea life clinging to it, and this piece of wing is no exception. Look at all those gooseneck barnacles! Resident DSN barnacle expert Miriam Goldstein has informed me that this amount of barnacles could easily grow on the flaperon in the 16 months it has probably been out at sea. I’m actually a little surprised more hasn’t grown on it. She also notes that they are from the Genus Lepas, although she can’t identify the species from the photo. The barnacles don’t seem to preferentially growing on one side, which also leads me think that this piece of debris was mostly submerged while drifting.

WHAT NOW?

The search for answers regarding the plane’s disappearance has been a long and difficult one. More debris from the wreck could end up on Réunion or at least near it in the future, if it has not already. Even though we found pieces of the plane, we can’t pinpoint exactly where the plane went down as suggested by some media outlets. But there might be clues in the debris itself to at least indicate what caused the plane to veer so very far off course and disappear. My hope is if investigators can find more debris, they can figure out what happened to MH370 in the first place and finally give the families of those onboard the tragic flight can find some answers and peace.

ADDITIONAL RESOURCES:

The map above was made using ESR’s OSCAR data product, which combines sea satellite data (altimetry, winds, sea surface temperature) and in situ observations (NOAA drifters, moorings) to create global maps of ocean currents every 5 days. http://podaac.jpl.nasa.gov/dataset/OSCAR_L4_OC_third-deg

Here is a good summary of MH370 take-off, disappearance and subsequent searches: http://www.cnn.com/2015/07/30/asia/mh370-maps-of-takeoff-disappearance-search/

The post How currents pushed debris from the missing Malaysian Air flight across the Indian Ocean to Réunion first appeared on Deep Sea News.

I’m just going to come out and say it, any project that touts itself as the “World’s first realistic Ocean Clean-up Concept” is just asking to be torn apart.

“The Ocean Cleanup” is the brainchild of a 19-year old Boyan Slat. He proposes using the oceans themselves to clean up plastic. By setting up a line of giant sifting booms across the major ocean gyres, ocean currents will push plastic into these giant traps to be collected and reused for profit.  He plans to set up an array of 24 of these sifters and calculates they will clean the ocean in 5 years.

Before I add my two cents, here’s what Miriam had to say about the idea at the Marine Debris Listserv:

Dear all,

I’ve tried to stop fact-checking to every cleanup scheme, but I guess it’s an addiction at this point. Also, I feel that as a community we cannot move forward with practical solutions to marine debris until we lay some of these common misconceptions to rest. These points respond Boyan Slat’s TEDx talk, but you can also see photos of his proposal here: http://www.boyanslat.com/plastic5/ and http://www.boyanslat.com/in-depth/.

• Most zooplankton don’t survive being caught in a standard manta net, never mind being spun in a centrifuge. They might still be twitching, but they have lost a lot of their important parts, like antennae and feeding apparatus. When we want to capture live zooplankton, we use special live-collection nets and are very, very careful. For gelatinous zooplankton like salps, the only way to bring them up in good condition is to individually capture them in glass jars on SCUBA. I am highly skeptical that any significant proportion of zooplankton are viable after caught in a net and spun at 50 RPM. (though I realize that he’s not proposing to do this on a large scale.) 
• Mooring fixed “ships” in the open ocean (avg depth 4000 meters) is highly improbable for a lot of reasons. Just to pick one: I could not find data on the absolute deepest mooring in the world, but this implies that it is approximately 2,000 meters. http://www.offshore-technology.com/projects/atlantisplatform/. So these ships would have to be moored at twice the depth of one of the deepest moorings that existed ~2007. 
• Having seen no data, I can’t really speak to the efficacy of floating booms in removing microplastic. However, Giora Proskurowski & colleagues have shown that microplastic get mixed down below the surface in fairly moderate winds. These booms would be unlikely to function in any significant wind and wave action. And the mixed layer in the open ocean can get quite deep, around 100-150 meters in the winter with storms. 
• Speaking of wind and wave actions, ships on fixed moorings and thousands of miles of booms (because the scale of this is also improbable) have the potential to create a lot more marine debris, and seem particularly hazardous to entanglement-prone marine life. 
• This isn’t even getting into issues of scale (the California Current alone is ~300 miles across), maintenance and fouling…

I realize that Mr. Slat is a student, and have no doubt that he, and the inventors of countless other plastic cleanup schemes, have only the best of intentions. I am hoping we can work together as marine debris professionals to channel their energies into more productive directions.

Regards,

Miriam Goldstein

While I can’t speak to what these booms will and will not pick up, I completely agree with her I am highly skeptical whether the design is even feasible from an ocean engineering standpoint. Here are some of the major unanswered technical questions:

1) How does the sifter work?  To be honest, I am not completely sure. The website and TED talk are completely devoid of technical details. But from what I can gather from the concept art and the talk, I think the booms have large nets underneath them that gather plastic into what I think is a oversized swimming pool leaf trap shaped like a manta ray. UPDATE: I misinterpreted the images on the website. The design as it stands now has no nets, only the initial tests had nets. Now I have to ask, what is that sheet hanging down from the booms?

2) The booms.  The claim is that only 24 sifters are need to clean the ocean and span the gyre radius, which means the booms have to be huge. Possibly 100’s of kilometer wide. Are they rigid or flexible? Are they the manta rays? How will they be kept in formation?

3) Anchoring something that large.  I am going to assume that the booms need to stay relatively taut to retain their shape and the most obvious way to do this will be with multiple anchor lines. The water depths are deep (>3000 m), horizontal surface motions needs to be small and then there is all that water pushing on what is essentially a giant paddle. That means a fairly sophisticated plan for anchoring the array will have to be developed. Having seen how large anchors are for low-tension subsurface moorings (>1000 kg), I can’t even begin to imagine what they are going to use or how that is going to be set up.

4) Biofouling. I forsee two major biofouling issues. The first is biological growth, which can be particularly bad because all the major mechanical parts are near the surface. There is going to be growth on the mesh, on the booms, on everything submerged which can make the booms and nets heavy, dragging them underwater. The second is fishbite. Did you know that fish attack underwater moorings like crazed rabid zombie munchers?  Now I don’t know if fish would actually chew on the mesh, but previous experience indicates they are not picky about what pieces of underwater line they snack on. So what will happen if fish gnaw holes through the collection nets?

5) The assumption of low current speeds. This is a bad assumption. While the array may not be placed in the most energetic current regime, storms and eddies can briefly induce large currents which could place a lot of stress and shear on such a large array.

6) Zero bycatch by net avoidance .  Sorry, I couldn’t resist.

The Ocean Cleanup project is still in the planning stage, so all these problems have the potential to be solved. But I think it is highly unlikely that an array of this size and magnitude will ever be feasible.

UPDATE: Another good roundup by Micheal Cote of potential issues with the Ocean Cleanup design can be found here. http://climateadaptation.tumblr.com/post/46515698066/this-invention-keeps-popping-up-in-my-daily

UPDATE 2:  Some more critiques brought to my attention in the comments

Those Crazy Plastic Cleaning Machines

http://inhabitat.com/the-fallacy-of-cleaning-the-gyres-of-plastic-with-a-floating-ocean-cleanup-array/

The post The Ocean Cleanup. The newest of the new plans to remove marine plastic. first appeared on Deep Sea News.

Five days into the cruise and we have now chucked a veritable zoo of instruments into the ocean to study the currents in the Chukchi Sea.  While it may seem crazy to throw so much stuff overboard, each instrument allows us to see the ocean in a different way. Combining the data from these various instruments we hope to not only understand where water is carried northward into the Arctic Ocean by these currents, but also how the currents themselves evolve over time.  In this post I’ll be introducing a couple of species in our instrument zoo and talk about some of the behaviors of the currents we are studying.

Before heading out to sea, we had a pretty good idea of what the surface currents off Barrow looked like because we have a land-based high frequency (HF) radar system that measures them.  The system is pretty slick, it works by bouncing high-frequency radar off waves on the sea surface to measure the speed and direction of the surface currents.  But the system does have its limitations, especially in the Arctic.  HF radar tends not to work so well when there is ice or even when the Aurora is really strong.  In addition, the HF radar only measures currents at the surface, so we don’t exactly know what is going on below.  Deeper water could be travelling with the surface currents, or they could be going in the complete opposite direction.

So we pulled out some other instruments from our toolkit to verify which way the current were headed. The first were surface drifters. These basketball-sized globes float and drift with the surface currents.  Each drifter is deployed with a “drogue”, a glorified umbrella that hangs beneath the surface float and forces the drifter to be dragged with the ocean current rather than be pushed by the wind.  What still blows my mind is that we track these little gizmos with satellites. Satellites that are in SPACE!  Every hour, they send out their coordinates while we sit comfortably with coffee in our lab watching them float away on Google maps.  You can check out their sometimes squiggly tracks here 

But we still didn’t know what was going on below the surface.  So we decided to send out underwater robots to follow the drifters.  You heard me right, we frickin’ deployed oceanographic robots! Our robots are formally known as gliders because of the way they fly through the water, but ocean robots just sounds so much more badass. The robots fly up and down in a zigzag pattern by changing their buoyancy, measuring different sea water properties. While piloted by other scientists onshore, our robots are fairly smart and can navigate themselves independently to a specified location.  But sometimes they can run into trouble if caught in an eddy or strong current and need little corrective nudges from the pilots onshore.

And what happened after we deployed the drifters was wild. Normally, because of the winds and waves, drifters tend to disperse or spread out from each other.  But when we went to recover them 3 days later, we found out them in exactly the same size cluster as when we had deployed them.  Not what we expected at all.

This may be because we deployed them in the Alaska Coastal current, which was screaming that day. Speeds over 4 knots were observed! In ocean terms, this was incredibly fast. Faster than even the Gulf Stream off the eastern shore of the US.  But then again, the other group of drifters we deployed (and did not pick up) in the ‘slower’ region also remained in a clump.

We are still trying to sort out exactly why the drifters moved the way they did.  But since we picked up the drifters (which isn’t normally done but we did it because they were so conveniently clumped), we are just dumping them back in the water.  And we will get a rare chance to repeat our initial experiment. Awesome.

The post Notes from the field: Find the currents, deploy the ROBOTS! first appeared on Deep Sea News.

While the Census of Marine Life may be the most recent call to survey the ocean, deep-sea exploration has a rich, paradigm-shifting history. It has all the makings of a Hollywood blockbuster: colorful characters, high seas action, the drama of antagonistic actions between “men of honor”, you name it! Probably has a bit of romance in there too, but that tends to get left out of the scientific literature. Examining the history of deep-sea exploration is an excellent case study in how technological advances continue to yield new insights and increase our ability to ask better questions. This section of Deep Sea 101 will be composed of 4 parts.

Nearly 2400 years ago Socrates (via Plato) posited of the deep sea:

“… everything is corroded by the brine, and there is no vegetation worth mentioning, and scarcely any degree of perfect formation, but only caverns and sand and measureless mud, and tracts of slime wherever there is earth as well, and nothing is in the worthy to be judged beautiful by our standards.”

Such a damning indictment from such a classical thinker indeed! Curiously, the Greeks and other civilizations during this time were in no position to make such bold statements having an inability to visit and sample past a mere tens of a meters.

It was not until Aristotle we could really call anyone a marine biologist. He dedicated much of his life to describing the life on the Aegean coasts, describing 180 marine species nearly 1700 years before Linneaus. Aristotle was the first person to study form, function, ecology and behavior and developed a classification system based on multiple traits. He is perhaps most famous for describing the mouth parts of sea urchins, which is named in his honor (called Aristotle’s Lantern, at right).

But his enthusiasm for the ocean did not catch on in the ancient period and an attitude of complacency persisted all the way towards the Victorian Era when deep-sea exploration really took off, chiefly out of economic and imperial interests. Echoing the contented ignorance of the time, or perhaps a fear of the unknown, noted historian Pliny the Elder wrote in 40 AD:

“By Hercules, in the sea and in the ocean, vast as it is, there exists nothing that is unknown to us, and a truly marvelous fact, it is with those things that that has concealed in the deep that we are best acquainted!”

It took until the 17th century, during the tail end of the Renaissance, for these unfounded assertions to be even questioned, and by none other than Robert Hooke who stated:

“Animals and Vegetables cannot be rationally supposed to live and grow under so great a Pressure, so great a Cold, and at so great a Distance from the Air, as many Parts at the Bottom of very deep Seas are liable and subject to…

We have had instances enough of the Fallaciousness of such immature and hasty Conclusions…” (emphasis mine)

What Hooke has done was twofold. He first provided a set of testable hypotheses for absence of life in the deep sea disguised as common sense. Then, he made a statement hinting that perhaps we ought to actually test this because if past experience shows, common sense might not always be correct.

For the next 100-200 years the deep sea was considered lifeless based on 4 criteria: temperature, light, pressure and stagnancy of the environment (i.e. it was all uniform). The first three of these criteria were well-reasoned, though no one knew what the true depths of the deep sea were. But it was well-known that light was refracted by water and the visible spectrum gets filtered out, the deeper you go the more pressure an organism must bear – this is easily calculated estimating forces, and without the energy from the sun’s rays warming the deep waters it could be reasoned that it must be cold down there. In fact, some early scientists thought the bottom of the sea must be ice.

The last criteria of stagnancy is an interesting one that I am not entirely sure how it came about since they had no direct knowledge of deep-sea life until the mid-1800s. It may have been derived from calculation of the current speeds. Surface currents are wind-driven and any given body of water tends to be stratified, or composed of different layers. The top layer of the water moves at a given speed but experiences drag from rubbing against the layer of water below it. This causes the lower layer to move with the upper layer but at a slower speed because there is energy loss from friction against the seafloor (see figure at right). Because of this, water currents near the seafloor are typically much slower than surface currents. Therefore one can posit that at some depth water speed eventually just stops. This has important implications because animals down there would need fresh, constant input of dissolved nutrients (nitrogen, oxygen, etc.). Stop the flow, there’s no grow!

During the golden age of deep-sea exploration in the 1800s the Azoic Hypothesis of the deep-sea was largely championed by Edward Forbes and was based on his observations in the Aegean Sea, between Greece and modern-day Turkey (map below). I’ll refer to the Azoic Hypothesis as tied specifically to Forbes, but recognize that it had much earlier roots. Forbes was merely the first to study it scientifically. As I’ll go on to explain though, the Azoic Hypothesis was largely the result of common sense thinking, an unfortunate study area, inadequate sampling gear and ignoring previous results.

Though not known at the time, the Aegean Sea was a poor choice for a study site. First of all, it is not very deep and we now know it is not a very productive area away from coastal areas. The map above (at right) shows the concentration of chlorophyll – the pigment used by phytoplankton to capture solar energy to use in photosynthesis – in the Aegean Sea based on satellite measurements. Green to red signify higher concentrations of phytoplankton, and hence higher amounts of surface primary production. Blue is low productivity and you’ll notice that in the center of the Aegean Sea where the deep water is it’s mostly blue, or unproductive. Oligotrophic waters (meaning with few nutrients) are defined as containing less that 80 grams of carbon per square meter. The central Aegean Sea has about 30 grams of carbon per square meter. With very few plankton at the surface, there is very little food that falls down to support the denizens of the deep.

Had Forbes actually been in a productive area he still might not have found much life in the deep because he was using a dredge that was modified from ones used by oystermen. It was inadequately designed to sample the muddy deep seafloor. The mouth of the dredge was narrow and the bag was small. Note the design of the canvas bag (below, at right), there are vents only in middle of the sides. The dredge would immediately fill up with mud and thereafter become a wrecking ball let loose upon the seafloor until it was brought up. Curiously though, and deceptively, Forbes made an illustration of sea creatures all-too-happy to enter into his dredge for his book Natural History of the European Seas (his initials are under the dredge). But the difference between the dredge he used versus this idealized dredge that he illustrated is actually quite important. The illustrated dredge would have been ideal to use since it has a wide mouth and plenty of vents to discharge mud.

Throughout his sampling, he failed to document life in the deep, but did document very thoroughly patterns of animal abundance with depth. In general, the deeper his dredge dove the fewer organisms he found. He wrote in 1859, the same year as Darwin published On the Origin of Species:

“As we descend deeper and deeper in this region, its inhabitants become more and more modified, and fewer and fewer, indicating our approach towards an abyss where life is either extinguished, or exhibits but a few sparks to mark its lingering presence.”

By means of extrapolation he asserted that life ceased to exist beyond 550 meters. Which probably seemed pretty reasonable to Forbes’ contemporaries. Forbes had went out to sea, carried out an extensive sampling program and had based his conclusions on data. In hindsight we can say that Edward Forbes was ill-prepared to adequately sample the deep sea and had a poor choice of study area, but was his extrapolation just the result of bad luck, or is there more to the story?

Find out in the next installment of Deep Sea 101 as we continue to examine the early evidence for life in the depths during Forbes’ time and enter whom I refer to as the father of modern oceanography, Sir Wyville Thomson!

The post Deep Sea 101: Early Paradigms and Exploration first appeared on Deep Sea News.

One of the denizens of the deep is the 30cm long tripod fish, Bathypterois grallator.  This unusual fish is typically found anywhere between 1-5km deep in the Atlantic, eastern Pacific, and western Indian, although future exploration wil likely reveal that is global.  First described over a century ago in 1886, the common name comes from the modified pelvic and lower caudal fins that are elongated.  Although rigid on the seafloor, the video above demonstrates these fins can be quite flexible while swimming.  The scientific name comes from the Greek bathus meaning deep, Greek pterois meaning feathery referring to the spines of a fish, and the Latin grallator, one who walks on stilts.

The video also shows that on these modified finds the tripod fish can stand on the seafloor.  The tripod allows the fish actually to place itself up off the bottom.  The need for this stems from the fact that currents centimeters near the bottom are slow to nonexistent.  This layer called the benthic boundary layer is not the ideal place to wait for food.  This is the same reason why you often see filter feeding seastars, basketstars, and brittlestar, among many other organisms, climb high on corals and sponges.

It is hypothesized that the fish uses the elongated pectoral fins, seen in the below video extending above the head, to detect small crustaceans coming in as the fish faces into the current. These elongated pectoral fins also are thought to direct the crustaceans toward the mouth.

Another interesting tidbit about this species…its a simultaneous hermaphrodite.

The post The Fish That Walks on Stilts first appeared on Deep Sea News.