Clark while she was studying for her PhD at the University of St Andrews.
'We can now see the whole thing as being controlled by the animals' fat reserves,' he adds. 'We used to think these reserves were like the petrol tank in a car - you just fill it up and off you go until it's empty. But now we're realising these animals are very selective about which kinds of fat they store, and which kinds they use.'
The researchers focused on the copepods of the species Calanus finmarchicus in Loch Etive on Scotland's west coast, sampling them throughout the year and analysing how much of various kinds of fat their bodies contained.
This population is unusual in that the loch is much shallower than the open-ocean habitats they're used to. This means that during diapause, they can't go nearly as deep - the loch's deepest point is 140m, whereas in the ocean copepods typically diapause below 500m.
The scientists found that these copepods burn up a huge chunk of their energy reserves during diapause.
Pond's earlier research had already shown that copepods use their fat reserves to control their buoyancy, so they can stay deep underwater without having to waste energy swimming.
This is possible because these oily liquids react to pressure by solidifying, which makes them denser. So when the copepods swim down to this critical depth, water pressure turns a proportion of their stored fat solid, weighing them down so no swimming is needed to stay in position.
This phase change happens around 500m beneath the surface, so the Loch Etive copepods never get deep enough to trigger it. This makes diapause an energy-intensive business, as they have to swim to stay deep.
This in turn means they burn stored energy quickly, eating through half their fat reserves during one diapause period. Pond says that in nearby open ocean, between 5 and 10 per cent is more a typical figure, so there's a definite cost to the animals of living in such shallow waters although this may be partly counterbalanced by the fact the loch's waters are particularly rich in edible algae.
'The copepods can certainly thrive here, but their evolutionary stronghold - the habitat in which they're at the biggest advantage over other species - is the open ocean, and that's where the diapause gives the most benefits,' Pond says.
This latest paper completes the circle, showing that not only do fat reserves control buoyancy during diapause; they play a crucial role in both starting it and in bringing it to an end.
Pond explains that the unsaturated fats that are crucial for buoyancy control are found mostly in plankton algae known as diatoms, so having these to feed on is crucial for copepods' ability to diapause successfully. He now plans to look at whether similar mechanisms help control the movements of other animal plankton, such as krill.
The paper appears in the journal Limnology and Oceanography.
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Clark KAJ, Brierley AS, Pond DW. Composition of wax esters is linked to diapause behaviour of Calanus finmarchicus in a sea loch environment. Limnology and Oceanography. 57: 65-75.