Abstract

Zooplankton undertake vertical migration to feed near the surface and excrete in deeper water, which contributes significantly to CO2 drawdown. The classic behaviour is diel vertical migration (DVM), such that the animals move upward to feed during the hours of darkness and downward during daylight to escape predation. This synchronised movement of animals is driven by the light/dark cycle characteristic of spring and autumn at high latitudes. Such behaviour is thus generally believed to cease during the periods of midnight sun and polar night, and is also expected to decrease in the presence of sea ice due to light attenuation. However, the absence of a synchronised migration pattern does not necessarily presume total absence of animal migration. Here, we present temporally coincident data from acoustic Doppler current profilers (ADCP) moored in two Svalbard fjords. This allows a direct comparison of zooplankton vertical migration in an ice-free and a seasonally ice-covered fjord over almost two years (2006-08), making this the first study to quantify the annual variation in migratory behaviour in the Arctic environment and its impact on carbon flux. DVM is clearly observed at both sites during autumn and spring, even in the presence of low light levels and significant sea ice cover. The largest contrasts between the two sites are observed during the midnight sun, where weak synchronised vertical motions occur over much of the summer at the ice-free site, whereas the ice-covered station is characterised by asynchronous movement. Calculations of carbon flux suggest that DVM leads to reduced CO2 drawdown compared with unsynchronised vertical migration, so future reduction in Arctic sea ice cover may lead to an increase in DVM and a decrease in CO2 flux.