Deep-sea diving and noisy neighbours

05 December 2017

Question

How deep do you have to go under the sea to be in complete darkness?

Answer

Chris Smith asked marine biologist Kate Feller, from the University of Cambridge, to dive into an answer to this question from Liz. Plus, Kate told everyone in the studio about a new citizen science project which has taken to the water recently... 

Kate - I wrote down the numbers because I am bad with numbers if we didn’t get that from the quiz! But 1000 metres is the official cutoff point when it’s called the ‘aphotic zone,’ so anything deeper than 1000 metres no sunlight will be coming from the surface. However, you will not be in total darkness because animals make their own light. There’s loads of bioluminescent organisms found at these kind of depths so, while the background that you’re looking at this animals at will be totally dark as far as there’s no sunlight, you will see the bioluminescence.

Chris - Matt?

Matt - I think, correct me if I’m wrong, is that connected to the idea of an optical depth down into the water? Because it’s the same principle as when you’re driving along through fog, you only ever see to what we call one optical depth, which is the point at which half of your photons get scattered. So you’re only seeing to one optical depth when you’re driving through fog and I’m guessing maybe it’s a similar thing when you have the water.

Kate - That’s one of the reasons why studying animals’ vision underwater is really great because you have this highly predicted spectrum of light from the surface to depth, based on the physics of how light interacts with water molecules and the stuff that’s dissolved in water.

Chris - So animals- fish, marine species - that live close to the surface, they’re seeing the full spectrum of light because it hasn’t been soaked up yet, but the deeper you go the less and less red light there is in the water, and once you get really, really deep there’s almost no light? So does that mean then that the animals that make light are making any colours of light or do they tend to make light of certain colours and they can see it, but the other animals can’t?

Kate - I’d say most of the bioluminescence that you have down in the open ocean at depth is going to be on the blue/green end of the spectrum. But there are a few, particularly the dragon fish…

Chris - Stomid fish?

Kate - Yeah. They actually have bioluminescent patches underneath their eyes - that are red. Because there's no red light at that kind of depth no-one can see it, basically, except these guys can.

Chris - So it’s like being spotlighted?

Kate - Yeah. They’ve got these like stealth scopes and they just snoop around in the dark.

Chris - They can use it to talk to each other and they can use it to see the things that they want to eat, but the thing that they’re after can’t see it being spotlit?

Kate - Yeah.

Chris - Isn’t that incredible.

Kate - There’s two ways you can make bioluminescence. The animal can either synthesise the enzymes and all the molecules themselves, or they can have an organ that cultures a bacteria that does it for them. Animals that live closer to the shore where the light environment, I guess, at shallower depths where you have more of those organics I mentioned earlier, the gelbstoff, they will actually have red shifted bioluminescence.

Chris - Isn’t the marine realm an amazing place. Now, there’s a really interesting story this week, Kate, for people who are keen on kayaking. You tell the story because it was a fascinating read this…

Kate - Yeah. There’s this amazing movement that’s happening right now where there’s all of this open sourced technology that’s available to just normal people who are just really keen. It’s not all isolated to people with a lot of money or who are working in labs and they’re experts.

What this research group did is they developed a data tracking system that you can mount onto a kayak. Specifically it’s what they were targeting it on, but you can also put it on a canoe, or a paddle board, or really any kind of pleasure water craft that is paddle powered. It turns information from the environment into an auditory signal or sound so you can basically play music based on changes in temperature. It will play a higher note from a digital thermometer so it converts the information from the thermometer into a digital tone that changes in its frequency depending on if it is warmer or colder, so higher temperatures, higher note; lower temperatures, lower note. You can be paddling along and go err-urr-err-urr if you’re going through different temperatures.

Chris - People go on these sorts of pursuits to get away from it all. It sounds like the reverse!

Kate - It has a very artistic element to it. If you are interested in creating auditory soundscapes and just enjoying something like that it has that kind of element to it. But what is more important is that it’s actual data that this open source thing you can build is collecting.

What they talk about that’s most important is that there aren’t a lot of very large data sets for changes in temperature or changes in the soundscapes of harder to reach waterways like estuaries, for instance, where a lot of people would be using these boats because, as you’re going, the little computer called the Raspberry Pie that’s the heart of this is uploading your deta as well as your GPS coordinates.

So it can generate these maps that go to public databases that scientists can access and so science with related to climate change and how things are changing in aquatic systems. It’s really cool and I really want to build one with my dad.

Chris - Well next time you’re on the programme Kate, you can tell us how you got on.

Kate - Okay.

Chris - But maybe wait for the weather to be nice, which might take a while in this country?

Kate - Yeah.

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