I’m posting this for students we’ll be meeting next month at the Victorian College of the Arts (VCA). It’s a story by scientist Rob King that he wrote as inspiration and information for us to animate.
From: Rob King <Rob.King@aad.gov.au>
To: Lisa Roberts lisa@lisaroberts.com.au
CC: “lisaroberts49@gmail.com” lisaroberts49@gmail.com, So Kawaguchi So.Kawaguchi@aad.gov.au
Subject: RE: krill impacts [SEC=UNCLASSIFIED]
Date: Thu, 18 Jan 2018 03:02:21 +0000
Hi Lisa, sorry for the slow reply, it has been flat out here with Krill coming back from Antarctica and the overflow of stock from last year being accommodated in a new 10,000 litre acoustic tank facility which is working so well that I am flat out keeping it up and running.
http://www.abc.net.au/news/2018-01-16/echo-sounding-of-krill-aims-to-estimate-populations/9333248
But below is my summary of what the Meyer paper tells us and how it might best be depicted. So has provided comments and edits to what I’ve written and we feel it’s a good balance between detail and simplicity, but it is of course up to you how much of this detail you can show without losing the main message.
The key finding is that the krill larvae like living in the complex rafted sea ice during the day and then drop down dispersing into the water column at sunset. Then at sunrise they move back up to the sea ice again. When in the sea ice they are constantly scraping away at the ice with their front legs to gather any food material, whatever is available on the surface of the ice. Food that was frozen into the ice when it was formed is largely inaccessible especially in the middle of the winter because it is embedded within the ice. During that period, ice surfaces that are facing up on the concaved terraces becomes a good place for larvae to feed since any materials that are released from the ice ceilings above will settle and be trapped there and larvae can then feed on them. However this is still less than ideal because it will still only provide just enough for them to survive the winter.
We have shown that there is very little food within the ice but nevertheless the larvae persist in scraping away at the surface as there is quite literally nothing else to do. What’s important here is that it’s the complex ice that has ledges and shelves that they favour. This is thought to be for predator avoidance as well as entrapment of food released from the ice above. Big stretches of dead flat sea ice usually had very low larvae numbers, the high concentrations we observed were in the shelves and caves in the complex rafted ice. (rafted ice is ice that has broken up and then the wind and waves have pushed layers up on top of each other and then its frozen together creating a complex 3D habitat)
The other key thing that’s happening is that when the larvae migrate down into the water column at night they are being exposed to the currents under the ice which can be in a different direction to the movement of the ice which is under more local wind influence. This has the effect of immediately separating them from the piece of ice that they descended from that evening, at sunrise they will come back up under a different piece of ice, potentially some that has not yet been heavily grazed, therefore providing more food for them.
Our models show that their movement being carried along on the ice flows during the day together with their movement when in the water currents under the ice at night overall adds to propelling them towards the marginal ice zone (MIZ). This zone is much more productive than the pack ice zone with more chlorophyll. It’s a predation risk to get into the water column to gather the food and be propelled along but by doing it at night when they are less likely to be seen by a predator, they increase their chance of success. As the season approaches late winter or early spring, due to increasing ice temperature, the surface of the sea ice begins to melt and it starts to release materials that were locked in the ice during the period when the ice froze and this allows the larvae access to this material. Ice algae also start to grow as the light level increases.
So I would see the animation looking something like larvae tucked away on a shelf under the ice, grazing away like a flock of sheep, with a sun high in the sky above them. The sun then sets and exactly at the time the sun sets the larvae all herd off the ice shelf like lemmings and swim down into the upper 20 metres of the water. As soon as they get just below the sea ice you show the larvae being carried along below the ice by the currents. Tricky to show here that the ice is also moving but the current is in a slightly different direction and/or speed. Importantly the larvae are plankton, they can swim up and down effectively but can’t compete with horizontal ocean currents, they go where this current goes once they descend into it. (Whereas adult krill are truly pelagic, they can swim both vertically and horizontally.) So for the entire time the sun is down you show the pattern of ice flows above the larvae changing. While at the same time the larvae are filtering away with their front legs and swimming about with their hind legs but importantly they are not propelling themselves horizontally under the ice, this is happening due to the current. I would show them in a sort of looping swimming behaviour to suggest they are swimming, but swimming all over the place, rather than them swimming in the direction they are being carried so it is clear that it’s the current that’s carrying them along.
The night progresses and when the sun comes up the larvae all charge up into their ice shelves within the pack ice again and begin scraping away at the ice like a heard of sheep again. You could even show some larvae unluckily coming up under dead flat sea ice and being eaten by a squid or fish or something whereas the ones just next door in an ice shelf are protected. I would then accelerate the sequence sort of time lapse style to represent many days and nights and as this is flicking through you can start to decrease the density of the pack ice to indicate arrival in the MIZ. Where the animation begins it will be 10/10ths pack ice, this means all ice, no open water. It will be like this for a long way and then will start to decrease density 9/10th 8/10th etc. all the way to open water which is 6 to 9 months later when the larvae is now bigger and looks pretty much like a juvenile krill at the ice edge munching away on phytoplankton. You could increase the learning potential of the animation by showing that the growth of the larvae doesn’t really take off until they hit the MIZ. This is where it is really productive and they really take off and grow. The journey to the MIZ is all about just surviving the ride, like inverted hot air balloonists, flying by night and staying landed during the day. But by both day and night they are moving in s helpful direction to end up in the MIZ.
If you really want to go to town on this you could then proceed to show that as juveniles in open water, the vertical migration behaviour now reverses because juvenile and adult krill will dive deep during the day (200 metres) to get into darker water for predator avoidance, forming schools also as a predator defence mechanism. Then at night the school swims up and disperses at the surface to feed on the phytoplankton which is growing strongly in the waters at the top which received lots of sun during the previous day. Then at sunrise they dive down into their schools again. This is the typical ice free open water behaviour we see for juvenile and adult schools of krill.
You could also extend the other end of the animation by showing the spawning in summer open water and the egg sink / hatch and the growth of the ice sheet over the larvae as autumn progresses to winter. But this is becoming a very long story.
Food for thought?
The DVD of the voyage was not acted on when I first requested it, it is just being copied now so I will get that to you ASAP. Cheers, Rob