Impact of Arctic sea-ice retreat on zooplankton foraging behaviour and vertical carbon flux

Diel vertical migration (DVM) by zooplankton is a prominent feature of many marine ecosystems. 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. Herbivorous zooplankton move upwards to feed at night under the cover of darkness in the near-surface where primary production occurs but where the risk from visual predators is greatest. This upward/downward migration redistributes C fixed by photosynthesis near the surface to deeper waters, and may reduce the rate of atmospheric CO2 accumulation. In the open sea DVM periodicity varies with latitude and season as day length varies. The occurrence and amplitude may also vary as a function of light intensity, resulting in unusual patterns at high latitudes during continuous daylight. Sea ice cover impacts underlying waters physically (eg shading) and biologically (eg reducing photosynthesis and excluding surface diving predator) and so may impact zooplankton abundance/behaviour. However, due to major logistic difficulties associated with sampling under ice, little is known about the behaviour of plankton there. Sea ice in the Arctic is already reducing, and the rate of loss 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. The loss of ice may well change the behaviour of plankton and impact significantly on C cycling in the region. Furthermore, most ice will be lost initially around the Arctic rim, over the shallow coastal seas where fisheries production is greatest: understanding consequences to zooplankton will also be vital if predictions on the effect of plankton-dependent fish species are to be made. We propose taking advantage of moorings (scientific instruments suspended in mid water on floats anchored to the seabed) already in place (supported by 'Oceans 2025' and the Norwegian Research Council) in ice-free and seasonally ice-covered fjords at Svalbard (c. 80 deg N in the Atlantic) to explore zooplankton behaviour year-round in these contrasting environments. The moorings are equipped with acoustic Doppler current profilers (ADCPs) that can track plankton migrations, sediment traps that collect plankton and their fecal matter, and temperature and salinity probes to monitor watercolumn physical properties. Any apparent differences between sites will give useful insights into how ice loss may affect the marine ecology of this sensitive region, and will help to predict future changes in presently ice-covered locations following ice retreat. This work fits with Oceans 2025 Strategic Objective 13 'Arctic Shelf Time Series'. Data will be explored in a model framework that will tell us how much fecal pellet production by zooplankton is exported to depth through the process of satiation sinking i.e. feeding at the surface and sinking into the ocean interior to digest, which can occur many times over the day and night. When this behaviour is absent most fecal pellets will remain in the surface layers and be recycled. Where zooplankton perform satiation sinking however, the amount of C particles sequestered in the ocean interior may be increased by 10-25%. The presence of ice and also of continuous light is likely to affect satiation sinking, and we will gauge what effect these factors have on carbon sequestration. We have considerable experience working on plankton migrations and on sea-ice systems. Our collaborative efforts here have the potential to provide much added value to already-funded mooring deployments, and to lead to an improved capacity to predict ecosystem consequences of change in the Arctic. The project will strengthen collaborations between UK institutions and other European/Scandinavian organisations working in Svalbard and in the wider Arctic.