Monday, 22 September 2008

Fun with pressure

Two things about the sea. It's very big. And it's very deep.

The expanse of the sea isn't something I'm going to talk about in this post. This post is all about depth!

On average, the sea is about 2km deep. That's 2,000m of water. On land, or in the air, 2,000m doesn't seem very far, or cause much of a change. But water is heavy. For each 10 meters you go down, the pressure increases by the earth's atmosphere again. So, at 2,000m you have (a bit more than, because seawater weighs more than 1kg per litre because of the salt in it) 200 atmospheres of pressure.

We survey as deeply as we can with the CTD - we're often in water over 4,000m deep, but we only have about 3,000m of wire on our CTD winch, which limits the depth to which we can send the CTD.

Yesterday night, I thought I'd try a fairly classic sort of experiment with the CTD that would illustrate the amounts of pressure we're talking about here. As one of the CTD casts during my shift was due to reach 3,000m, I thought I should find something and put it on the CTD and see what happens. Polystyrene cups are full of air - air that high pressure can squeeze out.

A friend of mine wondered what would happen to an apple- she thought that apples were kind of "foamy", but I suspected they were really pretty much solid and full of fluid; solids and fluids are more or less incompressible (at least compared to gases). So I attached an apple to the CTD too.
The enquiring mind of a scientist is unhappy to rely on conjecture when there is a chance to get some empirical evidence!

Henning made a cunning sample vessel out of a 2l plastic jar and tied it on, once he saw the ziplock bag I was planning to use; he said I'd get better results with a jar. The apple went in my ziplock contraption.

The apple made it back, but sadly, the first trial with the cups went missing... I guess the knots on the jar didn't hold!

So, we tried again, this time with more rope, more holes and a more cunning way of fastening things. Henning even made sure that the lid would stay on by threading string through there too.

Second time lucky!

Anyway, here are some fairly dramatic before and after shots:

To the left, you can see my neatest effort in terms of hand-writing (still terrible), and what things looked like before going to 3,000m.

Alongside the polystyrene cup is a small piece of flat polystyrene I found lying around. I wrote "Pressure" on it.

I thought the calipers looked sufficiently "scientific" and they also give you a scale to refer to...!

To the left, you can see what happened to them after briefly visiting 3,000m - and an un-treated cup to compare it with! Honey, I shrunk the polystyrene cup!

I tried several cups, as you can see on the left (compare with the un-treated cup also in the picture)

The flat piece of polystyrene doesn't look so impressive in the pictures above, but it got seriously squashed. Here is me holding it sideways with an unshrunken piece of polystyrene from the same place.

The one that went to 3,000m is the one at the bottom.

So, what about the apple? No real change, although it did feel slightly more squashy. (The other change was me getting more cunning with flash placement).

< Before
After >

So, there you have it. The sea is deep. And it's pretty squishy down there if you contain compressible airspaces!

I thought I'd add something of educational interest to the post. So...

Is this just an amusing quirk? What's the relevance to anything?

Well, think for a moment about fish. Particularly fish that might migrate up and down in the water column (something a lot of them do). As you may or may not know, many fishes have something called a "swimbladder" they use to achieve neutral buoyancy; by filling it with more or less air, they can displace an amount of water roughly equal to their mass, and so become neutrally buoyant (float in the water column, neither going up or sinking) and hover, without expending energy on swimming, much like divers can with their BCDs. There are two basic types of swimbladder, one with a tube connected to the outside [physostomous], and one without [physoclistous]. The latter have to excrete gas into this closed sack, using a structure called the rete mirabile, and remove gases using a similar structure, the oval window. The gas moved is primarily oxygen.

There are two potential problems you then run into. The first is going up too fast; as there is no easy way to "dump" large amounts of the gas inside (at least not in physoclistous species). This means that fish swimming at great speed towards the surface (or getting dragged that way by a trawl) risk severe internal injury as the gas bladder expands far beyond its normal size. Fishes with large swimbladders tend not to change depth particularly fast - and fish that change depth quickly tend not to have large swimbladders, or none at all.

The second problem is that of partial pressure. As you get deeper and deeper, you have to develop a higher and higher partial pressure of oxygen in the rete mirabile in order to "pump" more oxygen into the swimbladder in order to inflate it. There is a physiological limit to how high this pressure can be, so fishes that live deep in the ocean have evolved other ways of maintaining neutral buoyancy, like having very reduced skeletons (bone and cartilage are heavy), light tissues (not much heavy muscle) and lots of oils (which are less dense than water and float) stored in various tissues.

Anything with an airspace inside needs to worry about big pressure changes like this; if you've ever felt uncomfortable in an airplane or car as you go up or down (that "stuffy' feeling in your ears) - that comes from just a fraction of a change of one atmosphere. Those cups were subjected to a change many hundreds of times bigger than this!

Incidentally, if you didn't know, the biggest migration on the planet is not the wildebeest in East Africa, which you've probably seen on TV. It actually happens every single night in the ocean, when literally billions of tons of organisms rise from the depths up into the shallows to feed. This community can clearly be seen on sonar and this is termed by those operating sonar as the "deep scattering layer". During the day, it sits at around 400m or deeper, but moves up into the surface waters at night. This article on lanternfishes, a major constituent of the deep scattering layer, will let you know more about this "diel vertical migration" (and a lot about myctophids) if you are interested!

Oh, and the CTD station where I did this was about here.

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