What Makes Plankton Migrate?

Every day, zooplankton make their way to deep water in the morning and rise as the sun sets. This process, known as diel vertical migration, is carried out all over the world by marine and freshwater plankton alike. The reason for this has long been attributed to the trade-off between obtaining tasty morsels in the surface ocean and avoiding becoming a tasty morsel for predators while they're there.

Zooplankton feed on phytoplankton. Since phytoplankton rely on sunlight to photosynthesise, they are abundant in the surface ocean. Being somewhere where there's more light makes you easier for a predator to spot though, so zooplankton are thought to feed at the surface when darkness falls and move to deeper, darker water during the day to avoid predators. This means they can make the most of the available noms without making themselves an easy meal.

In the subtropics, light 100 m below sea level is 1% of that at the surface, but not all colours make it this far. Different wavelengths of light can travel to different depths in the water column. Longer wavelengths, such as red and yellow attenuate the fastest, which is why these shades are lost as you go deeper. Blue light can penetrate the furthest, which is why everything appears blue-black when you're diving deep in the ocean. But even the furthest penetrating light is over 100 million times weaker than it is at the surface when you’re 600 metres underwater!

But if it's so dark in the deep ocean, why do plankton here migrate vertically too?

There is a growing body of evidence for diel vertical migration in deep sea plankton. Like plankton at the surface, those at depth migrate up and down in the water column to the rhythm of the sun and their movements follow the variation in day length with latitude, despite the absence of light at depth.

So what are the causes of this strange behaviour? One possibility is that the noise generated by thousands of wriggly plankton at the surface is picked up by plankton further down and triggers their descent. This works for the migration to greater depths at the start of the day, but in the evening deep sea plankton are the early risers. Without a change in light level, what could trigger them to move towards the surface?

There is another possibility: that just as birds have a precise internal clock that tells them when and where to migrate, zooplankton could have an internal clock that does the same. One way of setting these internal clocks is learning during early life stages, when plankton could be exposed to light higher in the water column. Alternatively, they could contain a biochemical oscillator, which acts like a chemical metronome, maintaining a continuous rhythm over days, months, or even years. This is what many migratory animals have, including birds, mammals and even some marine species.

Whether or not zooplankton have internal clocks is still uncertain. If you wanted to see if internal clocks were behind their migration, how would you study them?

References

Berge, J., et al. Diel vertical migration of Arctic zooplankton during the polar night. Biol. Lett. 5 69–72 (2009)

van Haren, H. Monthly periodicity in acoustic reflections and vertical motions in the deep ocean. Geophys. Res. Lett. 34 (2007)

van Haren H. and Compton T. J. Diel Vertical Migration in Deep Sea Plankton Is Finely Tuned to Latitudinal and Seasonal Day Length. PLoS ONE 8 (2013)

Mihalcescu, I., Hsing, W. and Leibler, S. Resilient circadian oscillator revealed in individual cyanobacteria. Nature 430 81–85 (2004)

Roenneberg T. and Merrow M. Circadian clocks – the fall and rise of physiology. Nature Reviews 6 965–971 (2005)

Images

1) Just some of the many zooplankton that make the diel (twice daily) journey up and down in the ocean. Up top we have krill and at the bottom, a larval shrimp. In the middle (from left to right) a juvenile gastropod, what looks like a crab larvae, and two copepods (these crustaceans dominate the zooplankton). Credit: Matt Wilson/Jay Clark, NOAA NMFS AFSC via Wikimedia Commons.

2) The attenuation of light in the ocean. Blue light penetrates the furthest, which is why everything appears blue/black at depth, unless you put a spotlight on it. Credit: Tom Morris, Fullerton College via Science Education Through Earth Observation for High Schools.