Pan-Arctic Acoustic Archives: Quantifying zooplankton behaviours in a changing Arctic

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The Arctic is changing rapidly. One of the clearest changes is a reduction in the extent and thickness of summer sea ice. The loss of ice is predicted to increase in the coming years as a consequence of climatic warming. There may be no summer sea ice in the Arctic by 2030. Critically, the ice acts as a shade to sunlight and as it retreats it exposes open water to illumination causing a rapid increase in the growth of marine plants (phytoplankton). These plants use up carbon dioxide (CO2) from the atmosphere and are therefore an important component of Earth's climate system. Once formed, the phytoplankton become food for herbivorous zooplankton who are able to transport this source of carbon to deeper waters where it is excreted and buried in the sediments. This process, called the 'biological pump', transfers carbon from the atmosphere and locks it away. It is important that we understand the relationships between ice, phytoplankton, zooplankton and carbon and these relationships can be simulated in models of biogeochemical cycles. The critical link in this chain is the herbivorous zooplankton. They have a particular behaviour called 'diel vertical migration' (DVM) which is a prominent feature of many marine ecosystems. The animals move quickly tens to hundreds of meters vertically around dawn and dusk in migrations that comprise the most massive periodic shifts in biomass on Earth. The classical view is that DVM occurs as a trade off by individuals between food acquisition and predator avoidance. Zooplankton move upwards to feed at night into the nearsurface where primary production occurs. Here, under the cover of darkness, the risk from visual predators is minimised. This upward/downward migration redistributes carbon fixed by photosynthesis near the surface to deeper waters, and may remove larger quantities of CO2 from the atmosphere than would otherwise be the case, reducing the rate of CO2 accumulation in the atmosphere. Studying zooplankton in the Arctic year round is difficult because of access and ice cover. One successful technique for recording DVM behaviour uses an instrument called an acoustic Doppler current profiler (ADCP). Many ADCPs have been deployed in the Arctic over the last decade to measure currents but the acoustic signals also record zooplankton migrations. Usually these data are only analysed to understand the ocean currents within the localised region where the instrument was deployed. We are at a critical time in Arctic research where we must take a wider, 'pan-Arctic' view of marine processes. We propose to work with international groups to collate, process and archive the ADCP data, creating a unique resource for studying DVM. The regular, rhythmic behaviour means that we can use numerical techniques (circadian rhythm analysis) to quantify how strong and regular the migration behaviour is and relate this to the biological communities that are present, the level of illumination and the amount of sea ice cover. We will use this knowledge to improve models of how zooplankton transport carbon, through their faecal material, to depth. Understanding zooplankton DVM is important for many reasons. Quantifying DVM behaviour will allow us to improve our ability to predict how changes in sea ice might alter changes in the way carbon is captured and stored in the productive Arctic seas. It will give us a greater insight into how and why animals undertake such regular migrations and how the timing of these migrations is controlled. By relating the acoustic data with species data we will be able to understand the role of zooplankton in Arctic ecosystems and this is of particular importance if predictions on the effect of plankton-dependent fish species are to be made.